Antibiotics & Weight Gain: Are They Causing Obesity?

Antibiotics are potent medications that are most commonly used to treat (or in some cases prevent) serious bacterial infections that would otherwise cripple our body’s defenses and prove fatal.

One potential issue associated with using antibiotics, particularly if used for a long-term, is significant weight gain.

It is known that antibiotics have been used as “growth promoters” in livestock for over 70 years – and this effect occurs from their impact on gut microbiota & downstream effects. (R)

As someone who once needed 3 antibiotics (ethambutol, rifampicin, azithromycin) at high doses for ~18 months to treat a mycobacterial lung infection – I was curious as to whether the antibiotics may have caused weight gain.

Do antibiotics cause weight gain in humans?

It may depend upon the angle of analysis:

  1. Weight gain from eliminating the infection (with antibiotics)
  2. Weight gain directly from the antibiotics (as a side effect)
  3. Weight gain as a post-exposure phenomenon (post-antibiotics)

Antibiotics can cause significant weight gain by directly eliminating a bacterial infection that had caused significant weight loss (prior to treatment).

Example: Tuberculosis causes you to drop 30 lbs. below your normal baseline weight. You eradicate the TB infection with antibiotics and regain 30 lbs. to pre-infection weight.

Most people wouldn’t say that this “weight gain” is problematic – as it’s mostly a byproduct of the body reverting to homeostasis after the infection was eliminated.

There’s no strong evidence to suggest that antibiotics cause weight gain directly as a side effect (though this may occur in some cases with select antibiotics at certain dosages).

Okay, but what about weight gain independent of infection eradication after completing an antibiotic regimen?

Antibiotics may cause significant weight gain for some (post-treatment), but causality cannot be proven – such that there’s no conclusive evidence that antibiotics definitively cause weight gain in humans.

Likelihood of weight gain from antibiotics probably varies based on factors such as:

  • Specific antibiotic
  • Duration of antibiotic usage
  • Dosage of antibiotic
  • Genetics of the user
  • Diet & exercise habits

Also, we should be cognizant of the fact that many antibiotic users: remain skinny, stay at healthy body weights, or lose weight – so if weight gain occurs, it’s not for all antibiotic users.

Antibiotics may cause weight gain in susceptible individuals, but there are far too many limitations & confounds to prove causality (i.e. antibiotic use causes weight gain).

Example: One large study found strong associations between weight gain and bacterial infections – completely independent of antibiotic use.

It may be that bacterial infections have lasting effects on physiology (post-eradication) to induce weight gain – and antibiotics have no significant effect on weight.

If you suspect that you gained weight from antibiotic use, you may be correct – but it’s impossible to know with certainty… probably smartest to focus on solutions rather than dwelling on the weight gain.

Antibiotics & weight gain (Research)

I searched for “antibiotics” & “weight gain” in various scientific journals in attempt to understand whether there was a strong relationship between antibiotic usage and weight increases.

There are likely many more studies examining the relationship between antibiotics & weight gain – than those listed below… my goal here was to highlight some of the most recent (as these tend to provide an overview of earlier research).

I grouped the studies under “YES” (weight gain) and “NO” (no weight gain).

Yes or maybe (weight gain from antibiotics)

Li et al. (2022): Early-life antibiotic exposure was positively associated with acceleration of childhood overweightedness and obesity from 1 to 2.5 years. (R)

This study involved 2,140 mother-infant pairs in China and used an open-ended questionnaire & anthropometric parameters as measures.

Compositions of gut bacteria were analyzed via 16S rRNA high throughput sequencing.

Disruption of Faecalibacterium, Agathobacter, Klebsiella, and Bifidobacterium at 2.5 years were associated with antibiotic exposure & weight gain.

Zhuang et al. (2021): Use of antibiotics during pregnancy is positively correlated with risk of obesity in 5 year old children. (R)

A total of 132 children born 2014-2016 age 5 were included in this study.

Researchers adjusted for a variety of variables (pregnant BMI, diabetes in mother, maternal smoking, mode of delivery, weight gain during pregnancy, etc.).

Vallianou et al. (2021): The loss of diversity of the gut microbiome may have long-term detrimental effects on the gut microbiome & metabolic health – and ultimately lead to weight gain. (R)

Researchers summarized current evidence on the association between antibiotics and the subsequent development of obesity via alteration of the gut microbiome.

Most animal studies & meta-analyses of human studies examining the association between antibiotics & obesity are suggestive of a link (ABs may cause obesity).

Administration of antibiotics early in life (during the first 6 months), repeated exposure to antibiotics (>3 courses), broad spectrum antibiotics, and male gender – are associated with increased risk of obesity.

Wilkins & Reimer (2021): Studies suggest that antibiotic use has potential to cause weight gain and obesity – in both humans and animals. (R)

Researchers reviewed a variety of human studies & animal studies examining links between antibiotic use and obesity risk.

Park et al. (2021): The risk of obesity is higher in subjects who took systemic antibiotics more frequently – and more prominent when taking antibiotics targeting Gram-negative bacteria. (R)

This study examined the association between antibiotic use, obesity, and metabolic syndrome in Korean adults (N=266,447).

Duration of systemic antibiotic use in the previous 10 years was longer in the obesity group vs. the non-obesity group.

Antibiotics targeting Gram-negative bacteria had the strongest association with obesity – perhaps because Gram-negative bacteria release LPS (lipopolysaccharide) into the bloodstream when eradicated with antibiotics.

High levels of LPS in the bloodstream are associated with diseases like obesity, diabetes, and cardiovascular diseases.

Chen et al. (2020): Infant antibiotic exposure was associated with disruption of gut microbiota and higher risks of childhood obesity & increased adiposity. (R)

Researchers examined associations between infant antibiotic exposure and childhood adiposity (body fat) in 1,247 mother-offspring pairs in Singapore.

Antibiotic exposure occurred in 38% of study infants – and higher odds of obesity from 15-60 months of age were observed for any antibiotic exposure & >3 courses.

CAG “Eubacterium hallii” was negatively correlated with higher child adiposity whereas CAG “Tyzzerella 4” was positively correlated with repeated use of antibiotics and childhood adiposity.

Chelimo et al. (2020): Children repeatedly exposed to antibiotics had significantly higher BMI than those not exposed. (R)

A study with a cohort of 5,128 children examined the relationship between repeated antibiotic exposure (before 48 months) and BMI at 4.5 years of age.

Repeated antibiotic exposure was associated with significantly higher BMI vs. non-exposure – indicating this is a potentially modifiable risk factor for childhood obesity.

Interestingly, first exposure to antibiotics after the first year of life was not associated with higher BMI.

Furlong et al. (2019): Chronic use of certain antibiotics (bactericidal & penicillin) during adulthood may have lasting effects on BMI. (R)

NIEHS Sister Study (longitudinal cohort of sisters of women with breast cancer) examined associations between chronic antibiotic use (>3 months) during the 4th decade of life (30-39 years) and obesity at enrollment (mean age: 55).

Also examined associations between chronic antibiotic use in the 5 years & 12 months prior to enrollment and weight gain post-enrollment.

Researchers adjusted for: race/ethnicity, education, urban/rural status, age, and smoking.

Adjusted analyses (N=50,237) revealed chronic penicillin use during the 4th decade of life was associated with obesity at enrollment (OR = 2.00, 95% CI) & use in 5 years prior to enrollment was associated with increased BMI change after enrollment.

Use of bactericidal antibiotics during the 4th decade of life was also associated with obesity at enrollment.

Association between penicillin and bactericidal antibiotics were consistent across all indications.

Dawson-Hahn & Rhee (2019): Children with 1-2 antibiotic exposures had a higher rate of BMI change than children without antibiotics – and this was more pronounced in children with 3+ antibiotic exposures. (R)

Data were examined from 586 children in a 6-year follow-up study.

Antibiotic exposures, weight/height, sociodemographic factors, smoking during pregnancy, maternal diabetes, breastfeeding, etc. were all documented – and adjustments were made for confounds.

Growth trajectory for those with 3+ antibiotic exposures was significantly higher than those without antibiotic exposures.

Aghaali & Hashemi-Nazari (2019): Antibiotic exposure early in life significantly increases the risk of childhood weight gain and obesity (OR: 1.05, 95% CI). (R)

A systematic review & meta-analysis was performed to determine the association between early antibiotic exposure and risk of childhood obesity.

Databases were searched for quality studies, data were gathered, and a random effects meta-analysis was performed.

19 studies with at least 671,681 participants were included in the analysis.

Del Fiol et al. (2018): The relationship between antibiotic use and the development of obesity has become increasingly evident and apparent in humans. (R)

Authors state that “the scientific community has reached a consensus on the relationship between antibiotics and obesity.”

Specifically, it was reported that antibiotics cause dysbiosis (to varying degrees) which can lead to increased energy intake from diet and weight gain.

The most susceptible to antibiotic-induced weight gain may be children under the age of 3 years.

Data were reviewed from a variety of human & animal studies – and a positive relationship was showcased between antibiotic usage per 1,000 pop. & obesity rate.

Shao et al. (2017): There was a dose-response relationship between antibiotic exposure early in life & childhood adiposity with a 7% increment in risk of overweight & 6% increase in risk of obesity for each additional course of antibiotics. (R)

Researchers stated that antibiotic exposure early in life significantly increases risk of overweightedness & obesity – and noted that the relationship was “obvious.”

Results were determined via a meta-analysis that included 15 cohort studies involving 445,880 participants

Edmonson & Eickhoff (2017): There was no evidence that prolonged exposure to trimethoprim-sulfamethoxazole prophylaxis increases prevalence of overweight or obesity in healthy infants & young children. (R)

Trimethoprim-sulfamethoxazole (N=302) OR a placebo (N=305) was administered daily for 2 years in a 2-year RCT involving 607 children (median age: 12 mo.).

Anthropometric data were collected at a 24-month visit for 428 children and weight gain was similar between groups after adjustments & analyses.

Cox & Blaser (2015): When given early in life, agents that disrupt microbiota composition & its metabolic activity can influence body mass of the host – promoting either weight gain or stunting growth. (R)

Authors believe that microbiota disruptions early in life (such as from using antibiotics) could have long-lasting effects on body weight in adulthood.

It was also noted that medical advances such as C-section births & formula feeding may alter gut bacteria to increase risk of obesity – as well as consumption of animal products containing antibiotics.

Antibiotics added to the food & water of livestock (cows, pigs, sheep) promote growth – and a variety of agents have this effect regardless of drug class, chemical structure, mode of action, and activity spectrum.

Human epidemiological studies suggest that early exposure to antibiotics (particularly early in life & broad-spectrum) may increase risk of excess adiposity (body fat) later in life.

Administration of probiotics to human mothers in the last month of pregnancy (or early life in animal models) can affect growth of offspring (increasing or decreasing – depending on dose/strain of probiotics & host genetics/microbiota).

Angelakis et al. (2014): Abnormal weight gain is a side effect of long-term doxycycline & hydroxychloroquine treatment. (R)

Researchers assessed changes in BMI in Q fever endocarditis patients (N=48) treated with doxycycline and hydroxychloroquine – vs. healthy patients without antibiotic treatment (N=34).

Fecal samples were also examined using molecular assay to measure changes in specific species of gut bacteria.

Around 23% of treated patients exhibited abnormal weight gain – and antibiotics caused significant reductions in Bacteroidetes, Firmicutes, and total bacteria.

It was suspected that gut microbiota modifications at the phylum level may lead to weight gain following long-term AB treatment.

No evidence (antibiotic weight gain)

Yoshida et al. (2020): Oral antibiotic use did NOT alter body weight or metabolic status in obese adults without bacterial infection. (R)

Authors analyzed RCTs to determine whether antibiotics cause weight change and/or metabolic alterations in obese adults without bacterial infections.

A total of 2 studies (5 records) met inclusion criteria and data were synthesized for a meta-analysis.

Sejersen et al. (2019): Children exposed to antibiotics had similar BMI, BMI z-score, and body composition between 1 & 6 years of life vs. unexposed children. (R)

This study does NOT support the hypothesis that antibiotic exposure during infancy leads to development of obesity in the first 6 years of life.

This was a prospective cohort study with 600+ mother-child pairings in Denmark.

Information on antibiotic exposure, height/length & weight measures, and body composition (DEXA) were documented between ages 1-6.

Jess et al. (2019): Prenatal exposure to narrow-spectrum ABs is not associated with overweight in offspring. Some broad spectrum ABs may increase odds of overweight in early childhood, but the association doesn’t persist in later childhood. (R)

This study involved 43,365 mother-child dyads from a nationwide cohort of pregnant women and offspring in the Danish National Prescription Registry.

Linear and logistic regression was used to examine associations between prenatal antibiotic exposure and BMI z-score and overweight (incl. obesity) at age 7 & 11 years.

Furlong et al. (2019): Some evidence from a large analysis (N=50,237) suggests that certain antibiotics may be associated with BMI reduction, reduced odds of obesity, or have zero effect on weight. (R)

Bacteriostatic antibiotic use in the 5 years prior to enrollment was associated with a reduced BMI after enrollment & tetracycline use during the 4th decades of life was associated with reduced odds of obesity at enrollment.

Tetracycline antibiotic use during the 4th decade of life was associated with reduced odds of obesity at enrollment.

However, these inverse associations were only present for those using antibiotics for skin conditions.

Cephalosporins, macrolides, quinolones, and sulfonamides were NOT associated with BMI change over time.

Li et al. (2017): Infection, but not antibiotic use, during infancy is associated with risk of childhood obesity. (R)

Evidence indicates that treatment of common infections with antibiotics during infancy is unlikely a main contributor to childhood obesity.

Longitudinal birth cohort study (1997-2013) with 260,556 individuals – controlled for maternal age, race, pre-pregnancy BMI, infection without AB use, etc.

Dose-response relation observed between infection episodes and risk of childhood obesity.

Infants with AB-treated infections during infancy were not associated with childhood obesity.

Neither broad-spectrum nor narrow-spectrum antibiotics were associated with risk of childhood obesity.

Findings were supported by results of the twin set analysis.

Contopoulos-Ioannidis et al. (2016): Contrary to expectations, long-term antibiotic use in healthy adolescents with acne was not associated with weight gain. (R)

1,012 adolescents who received long-term antibiotics for acne were analyzed (median age at start of AB use = 16).

Majority of antibiotics were tetracyclines (82%) and median antibiotic course was 60 days.

Findings were robust and consistent across all analyses and did NOT support a growth-promoting effect of antibiotics in healthy adolescents.

Gerber et al. (2016): Exposure to antibiotics within the first 6 months of life is NOT associated with difference in weight gain through age 7 years old. (R)

This study examined 38,522 children & 92 twins (46 matched pairs) born between 2001-2011.

Researchers analyzed antibiotic use within the first 6 months of life (N=5,287 or 14%) and compared body weights/compositions to the unexposed at 7 years.

Researchers concluded that there are many reasons to limit antibiotic use in young & healthy children – but weight gain isn’t one of them.

Animal research

Chen et al. (2022): Exposure to very low doses of antibiotic residues, whether continuously or early in life, may exert long-lasting effects on host metabolism of mice by altering the gut microbiota and its metabolites – leading to obesity & metabolic dysfunction. (R)

Schulfer et al. (2019): Subtherapeutic antibiotic exposure to mice significantly increases weight – especially on a high-fat diet. (R)

Sidener et al. (2017): Rhesus macaques exposed to antibiotics (prior to age 6 mo.) had zero difference in growth curves relative to those not exposed. (R)

Carvalho et al. (2012): Modulation of gut microbiota with antibiotics in mice on high-fat diets can improve insulin signaling and glucose tolerance & reduce LPS/inflammation. (R)

Castanon (2007): Low doses (subtherapeutic) of antibiotics are put into animal feeds as “growth promoters.” (R)

Cromwell (2002): Use of antibiotics during all phases of animal growth benefit the rate & efficiency of body weight gain in pigs. (R)

How antibiotics cause weight gain (Mechanisms)

Understand that it is unclear as to whether antibiotics legitimately cause weight gain – as causality is difficult to prove.

If antibiotics cause weight gain, the mechanisms listed below may be how they do it.

Note: There may be many other mechanisms by which antibiotics cause weight gain – I’m just listing some possibilities below.

Dysbiosis (gut bacteria shifts)

If antibiotics cause weight gain, the primary mechanism probably involves perturbation of gut bacteria or “dysbiosis.”

Basically, the gist is as follows:

Antibiotics -> dysbiosis -> downstream physiological changes -> body fat & weight gain.

Loss of diversity of the gut microbiome (particularly early in life) may have long-term detrimental effects on the adult host gut microbiome & metabolic health. (R)

Early antibiotic exposure in animal models showcases reductions in the populations of specific microbiota (gut bacteria) such as:

  • Lactobacillus
  • Allobaculum
  • Rikenellaceae
  • Candidatus Arthromitus
  • Lachnospiraceae
  • Bifidobacterium
  • Firmicutes
  • Bacteroidetes

Basically antibiotics may have potential to alter gut bacteria for a long-term (some think permanently) – especially if used early in life and/or excessively.

This effect could be potentiated via exposure to low-dose antibiotics in the food & water supply. (R)

Some studies suggest that the proportion or ratios of Firmicutes to Bacteroidetes influence the degree of weight gain/loss. (R)

Specifically: Increase in Firmicutes relative to Bacteroidetes could increase calorie absorption from food, supply larger amounts of fat to the host, and cause gains in fat mass & weight.

Downstream to dysbiosis (antibiotic-mediated gut bacteria imbalance), the following mechanisms could contribute to weight gain on antibiotics:

Metabolic changes

Antibiotics have been shown to cause metabolic changes in animal models by modulating physiological signaling pathways that impact the metabolism of short-chain fatty acids (SCFAs) and bile acids. (R)

Antibiotics are also thought to increase gut permeability (disrupt integrity of gut lining & junctions) – causing bacteria & toxins to enter the blood stream which triggers inflammation and metabolic dysfunction to promote weight gain & obesity. (R)

Metabolic changes from antibiotic use in animals are thought to increase the rate & efficiency of fat storage (from dietary fats & carbohydrates) – and ultimately increase: body fat, bone mineral density, and body weight.

If similar metabolic changes occur in humans, it’s possible that they could end up converting a greater percentage of dietary calories to body fat – even if calories remain the same, leading to weight gain (relative to pre-antibiotics).

It’s possible that some antibiotics also alter metabolic rate by slowing it – such that the body burns fewer calories throughout the day than had it not been on antibiotics.

Metabolic changes from antibiotics may increase risk of metabolic disorder – which encompasses: diabetes, obesity, hypertension, hypercholesterolemia, hyperlipidemia, etc.

Gut-brain axis (Neurotransmitters & Vagus Nerve)

The gut-brain axis (GBA) refers to a multidirectional communication pathway between the gut and the brain/CNS – such that changes to the gut influence the brain, and vice-versa.

Antibiotic use can significantly alter gut-brain axis activation in a variety of ways that could: increase appetite, slow metabolic rate, modify fat storage, and cause weight gain.

  • Neurotransmitters (peripheral): Neurotransmitter levels in the gut change while on antibiotics, and this could influence brain activity & appetite – possibly affecting body weight.
  • Neurotransmitters (central): Antibiotics can directly alter concentrations of neurotransmitters in the CNS, which may influence appetite & energy levels and impact body weight.
  • Vagus nerve: Antibiotic use may alter activity of the vagus nerve, which could alter appetite & metabolism – ultimately leading to weight gain.

Neurotransmitters such as: serotonin (5-HT), GABA, dopamine, glutamate, etc. – are all susceptible to being influenced by antibiotics – and all of these can influence appetite, feeding preferences, and body weight. (R)

Iatrogenic effects (side effects & long-term effects)

Antibiotics can cause a variety of side effects, symptoms, or adverse long-term complications that arise directly from treatment.

Possible iatrogenic effects associated with antibiotics that might lead to weight gain:

  • Depression: Some people develop mental depression while on antibiotics – and depression may lead to food cravings, unhealthy food choices, binge eating, reduced physical activity, etc. – all of which cause weight gain. (R)
  • Fatigue: Some people develop extreme fatigue and/or chronic fatigue from antibiotics – and this fatigue makes it difficult to stay active and/or eat healthily, resulting in weight gain.
  • Anxiety: Some people experience anxiety from antibiotics – and this anxiety could lead to weight gain in a subset of people who cope by engaging in unhealthy behaviors (e.g. binge eating).
  • Appetite changes: Some people notice that antibiotics significantly increase their appetite or cause food cravings. This could be due to its mechanism of action and/or elimination of a bacterial infection – resulting in weight gain.
  • GI disturbances: Some people experience GI disturbances while on antibiotics and these may limit one’s ability to exercise and/or eat certain foods – which could increase OR decrease body weight.

Keep in mind that most people experience some side effects while on antibiotics, and when the course is finished the side effects dissipate – so iatrogenic reactions to antibiotics are probably not a common mechanism for weight gain.

Epigenetic switches

Antibiotics may alter DNA methylation & histone modifications – which can yield changes in gene expression patterns (epigenetics).

Any significant epigenetic alterations from antibiotic exposure could influence: metabolism, inflammation, and immune function – all of which have been associated with weight gain & obesity.

Mitochondrial dysfunction

There is some evidence to suggest that antibiotics can cause mitochondrial dysfunction. (R)

Mitochondrial dysfunction may contribute to weight gain and obesity – by decreasing energy production, increasing oxidative stress, increasing inflammation, etc. (R)

Immune activation

Antibiotics may affect activation and/or development (early in life) of the immune system. (R)

This could cause alterations in the balance of Th1/Th2 activation, Th17 activity, and the activity of immune cells (dendritic cells, neutrophils, monocytes, etc.).

Oxidative stress & inflammation

There are heaps of studies documenting “links” between oxidative stress, inflammation, and obesity and other metabolic disorders. (R1, R2)

It’s possible that certain antibiotics at certain doses can increase oxidative stress (e.g. ROS, NOS, etc.) and inflammatory mediators (e.g. IL-1B, IL-6, CRP) – which might modify appetite and increase body weight (especially body fat).

I’m skeptical that this would be an actual mechanism for antibiotic-related weight gain because many antibiotics exert an anti-inflammatory effect – and much of the oxidative stress & inflammation may from the bacterial infection being treated and/or an immune response.

Hormones & fat storage

Antibiotics can potentially affect the production and regulation of hormones downstream from altering the gut microbiome – which influences hormone synthesis & metabolism.

Hormones like Peptide YY (PYY), Pancreatic Polypeptide (PP), and Neuropeptide Y (NPY) are involved in regulating food intake & energy homeostasis.

Some speculate that antibiotic use, particularly if long-term, could alter the production & metabolism of these hormones and others – to increase appetite, food intake, fat gain, and weight gain.

I could not find a shred of evidence to support the idea that antibiotics significantly affect hormone production long-term – so this is a low probability mechanism. (R)

Others have implied that antibiotics might affect levels of estrogen, progesterone, cortisol, testosterone, etc. – which could affect body composition – but I couldn’t find any evidence that this actually happens. (R)

Interaction with diet types (macros)

Antibiotics have been shown to significantly increase body fat & cause weight gain in animals consuming specific types of diets (e.g. high-fat, high-carb, etc.).

Much of this is probably contingent upon the specific animal & specific antibiotic administered (including dosage), however, it’s possible that antibiotics (in general) are more likely to cause weight/fat gain via specific diet types in humans.

If evidence from animal studies extrapolates to humans, then high-fat diets may increase risk of weight gain while using antibiotics (relative to other types) – and lower-fat diets (with the exception of omega-3 fats) might help mitigate antibiotic-induced weight gain.

Right now there’s no strong evidence to support the idea that a specific macro split increases risk of weight gain on antibiotics in humans – relative to another split (e.g. high carb/low fat vs. low carb/high fat vs. mod carb/mod fat).

Note: Many of these mechanisms are interconnected and multifaceted.

Limitations of the research (antibiotics & weight gain)

There are some massive limitations attached to most studies investigating whether antibiotic use is likely to cause weight gain in humans.

Because of these limitations, we should avoid expressing certainty that antibiotics cause weight gain – as causality hasn’t been established.

  • Observational designs: Most studies exploring the relationship between antibiotic use and weight gain are observational – making it impossible to prove causality. Just “linkage.”
  • Confounding factors: Diet, genetics, age, BMI, breastfeeding, maternal smoking, presence of infection, lifestyle, physical activity, depression & anxiety (as a reaction to infection), non-adults (still growing – weight fluctuates), socioeconomic status, digestive changes on antibiotics, comorbid medical conditions, etc.
  • Infections causing weight gain (?): A large study (260K+ individuals) found a relationship between infection & risk of obesity during childhood and adolescence – independent of antibiotic treatment. Odds of obesity were higher in those with untreated infections vs. those treated. (R)
  • Resolution of infection: Many infections cause substantial weight loss. Resolution of the infection results in significant weight gain (back to baseline).
  • Antibiotic compliance: Data regarding antibiotic compliance have been fairly unreliable – as these are based upon recalls from parents that enrolled their children in studies.
  • Lack of RCTs: There aren’t any randomized controlled trials in which antibiotic usage is compared to a placebo over a long-term in a large sample of participants.
  • Antibiotic specifics: Specific antibiotic, class of antibiotic, dosage of antibiotic (high vs. low), spectrum of activity (e.g. broad vs. narrow vs. Gram positive/negative, etc.), monotherapy vs. multiple antibiotics, etc.
  • Calories not controlled: I’ve yet to find a study in which caloric intake is controlled while on antibiotics (such that it’s roughly the same as pre-antibiotics). It’s possible that some people are gaining weight while on antibiotics simply because they’ve decided to treat themselves & eat anything while on antibiotics because they’re sick and unable to cook healthy foods.
  • Sample specifics: The samples in studies examining the effect of antibiotics on weight differ in demographics. Some involve middle-aged women, others involve children, others involve only Koreans, most are in developed countries, etc. This could explain disparities in findings between studies.
  • Data collection & analysis: Some of the data may be inaccurate due to the fact that it was obtained via insurance databases, self-reports, medical records, etc. – and analyzed retrospectively.
  • On vs. off antibiotics: People may gain weight on antibiotics for a variety of reasons (infection fixed, metabolic changes, etc.). However, it’s possible that once antibiotic usage is stopped – bodyweight normalizes to homeostasis. (Example: Livestock gain weight on antibiotics, but weight may normalize if antibiotic use is discontinued).
  • Animal studies: Many animal studies that weight gain occurs with antibiotic usage, however, results cannot be extrapolated to humans due to physiological differences (humans vs. animals), antibiotic differences, dietary differences, etc.
  • Non gainers: Plenty of people treated with antibiotics don’t gain any significant weight – even after using broad-spectrum antibiotics for a long-term and/or at a young age. Why aren’t these people gaining weight if antibiotics are a cause?
  • Obesity increases infection risk (?): Obesity may increase susceptibility to various bacterial infections – thus explaining the correlation between antibiotics & obesity/overweightedness. (R)

Critical thinking (antibiotics & weight gain)…

Bacterial infections cause weight loss

The “significant weight gain” observed with long-term antibiotic use could be due to the fact that individuals had a serious bacterial infection that caused weight loss.

When a person successfully treats the bacterial infection with long-term antibiotics – they gain significant weight (making it appear as though the antibiotics caused weight gain).

For example: Someone loses a significant amount of weight from pulmonary tuberculosis (relative to pre-infection), treats the infection with long-term antibiotics, and gains “significant weight” thereafter.

In this case, although significant weight gain occurred, much of the weight gain was probably healthy – as tuberculosis caused an unnatural weight reduction via: immune activation, appetite reduction, nausea/vomiting, etc.

Unhealthy lifestyle = more antibiotics (?)

It’s possible that unhealthy lifestyle factors are associated with extreme weights (e.g. underweightedness & overweightedness/obesity) – such that risk of contracting a bacterial infection increases.

If this is the case, then it’s possible more: underweight & overweight individuals get bacterial infections and end up on more courses of antibiotics than those at normal healthy weights. (R)

The data may show dramatic weight gain in underweight individuals following antibiotics (due to the fact that the infection scared them into make lifestyle changes to improve body composition & overall health).

Additionally, antibiotics may be more used in overweight/obese persons – so high body weight may correlate with their use, giving the impression that they might cause weight gain.

Depression & anxiety from infection and/or being sick

It’s possible that depression & anxiety resulting from: severe bacterial infections, being sick (cognitive & physical limitations), fear of antibiotic side effects, and/or surviving a severe infection (post-treatment mental status) – could cause weight gain.

Depression and anxiety can lead to hormone changes, sleep disturbances, reductions in physical activity, and/or emotional eating (unhealthy foods & binge eating) – resulting in weight gain.

Additionally, treating depression & anxiety with conventional psychiatric medications (e.g. SSRIs) can result in weight gain.

Risk factors for weight gain from antibiotics

Included below are some variables that may increase risk of weight gain from antibiotics.

These risk factors were mentioned in studies that discovered significant links between antibiotic use and weight gain.

  • Antibiotics while pregnant: Fetal exposure to antibiotics may increase risk of weight gain & obesity. (R)
  • Early life antibiotics: Exposure to antibiotics in infancy or childhood may increase risk of weight gain & obesity. (R)
  • Repeated antibiotic exposure: Repeated or chronic exposure to antibiotics (3+ courses) may increase risk of weight gain & obesity – particularly if over a narrow timeframe. (R)
  • Broad spectrum antibiotics: Use of broad spectrum antibiotics (targeting both gram-negative & gram-positive bacteria) may increase risk of weight gain & obesity (relative to narrow-spectrum antibiotics). (R)
  • Gram-negative antibiotics: Gram-negative antibiotics may increase risk of weight gain & obesity relative to gram-positive antibiotics. (R)  This could be due to the fact that gram-negative bacteria release LPS (lipopolysaccharide) into the bloodstream, and high LPS levels are associated with obesity, diabetes, and cardiovascular disease.
  • Male sex: Male sex is associated with increased risk of weight gain & obesity from antibiotics relative to female sex. (R)

Note: There may be additional risk factors that I missed associated with weight gain & obesity from antibiotics. (If you know of others – leave a comment and cite a source).

Strategies to minimize weight gain from antibiotics

Included below are strategies that may help reduce risk of weight gain from antibiotics and/or promote healthy body weight post-antibiotics.

Most of these ideas focus on: managing calorie intake, physical activity, and modifying gut bacteria (primary driver of AB-related weight gain if ABs cause weight gain).

Before using these strategies, consult a medical doctor to ensure that they won’t cause you harm.

Calorie restriction

An effective way to manage body weight involves diligent calorie tracking – and it’s not as hard as you’d think.

Buy a cheap food scale, use a free app like MyFitnessPal – and track as precisely as possible without cheating. Limit your calories.

Calorie restriction prevents weight gain & favorably improves gut composition. (R)

Low fat diet

Some research suggests that antibiotics may interact with high-fat diets to increase body fat & cause weight gain.

This effect may be offset by following a lower-fat (or low-fat) diet during & after antibiotic use. Low fat diets can favorably affect gut microbiome composition as well. (R)

Omega-3 fatty acids

Wilkins & Reimer suggest that omega-3 fatty acids have a positive impact on the gut microbiota in mice exposed to antibiotics.

Elevated omega-3 fatty acids counteract antibiotic-induced dysbiosis in mice and reduce body weight & insulin resistance severity.

These may counteract high LPS levels & inflammation driving weight gain. (R)

Might be worth trying high-potency fish oil (affiliate link) for extra omega-3s, but this might also cause GI distress for some… might be better off eating fish.


Getting exercise can be an effective way to quell appetite, increase energy expenditure, modify hormones, increase metabolic rate, and offset weight gain.

Exercise is also associated with improvements in gut microbiota composition. (R)


Administration of prebiotics during & after antibiotic treatment may be beneficial.

Prebiotics are basically food for healthy gut bacteria – they aren’t living organisms.

Prebiotics may be significantly less risky and more beneficial than probiotics in many cases for improving health & gut bacteria composition. (R)

This is a prebiotic I use and might be beneficial for some… (affiliate link).

Probiotics (?)

Some research suggests that use of probiotics after antibiotics results in slower recovery of the microbiome than doing nothing!

(A possibility that many are clueless about… mainstream advice: load up on probiotics after antibiotics – might be bad advice). (R)

That said, some have had luck with formulations like VSL #3 (affiliate link) – and these appear to help murine (animal) models. (R)

Fecal transplant (?)

An expensive, time-intensive, and risky procedure involving transplantation of fecal matter from a healthy individual into a person with antibiotic-induced dysbiosis may prove beneficial in some cases for restoration of healthy gut bacteria.

A small RCT by Taur et al. (2019) found that fecal microbiota transplantation (FMT) boosts microbial diversity and reestablishes intestinal microbiota composition that patients had before antibiotic treatment. (R)

Note: These ideas were mostly things I’d brainstormed & science papers seem to suggest similar things as possibly effective interventions. (The omega-3 fatty acids idea was something I stole from Wilkins & Reimer).

Do I think antibiotics cause weight gain?

Depends. If a person lost weight from an infection (e.g. tuberculosis) – then effective treatment of that infection with antibiotics should promote weight gain (reversion to normalcy).

I am skeptical of the idea that antibiotics definitively (100%) cause weight gain in humans – independent from regaining weight that was lost during infection.

Exposure to specific-spectrum antibiotics early in life and/or many courses – might: cause dysbiosis, alter fat storage & feeding patterns, alter growth trajectory, and ultimately induce overweightedness and/or obesity – in a subset of susceptible individuals.

It could be something like: antibiotics cause dysbiosis, the dysbiosis alters fat storage & feeding behavior, and this cycle is difficult to break post-antibiotics (e.g. body fat, food cravings, overeating, etc.) because it’s self-reinforcing.

However, for a majority of people, I don’t think that antibiotics typically cause weight gain that is both: (A) substantial (e.g. skinny to a hot air balloon) AND (B) permanent (impossible to lose).

I suspect that for most, gut microbiota will inevitably normalize (this may take some time) post-antibiotics – such that even if the antibiotics caused a bit of weight gain, the weight can be lost (either with a bit of effort or effortlessly).

Although there are associations between: (1) gut dysbiosis & obesity and (2) low-residue antibiotics in food supply & increasing obesity rate – obesity itself can cause dysbiosis & increasing obesity rate could be influenced by many variables.

These variables include: increased agricultural efficiency, increased availability of low cost food, more hyperpalatable options (foods formulated to be addictive), etc. – such that these alone could explain an obesity increase (independent of antibiotics).

Unless causality is proven in humans, I think it’s smart to remain skeptical that antibiotic regimens & antibiotic residues in the food supply significantly affect body fat & weight in the general population.

It would be interesting if studies controlled for: genetics & family (are immediate relatives overweight/obese?), other health conditions & medications (e.g. SSRIs), total calorie intake, food choices (high salt/sugar/fat vs. high protein/fiber/bland)… I doubt the association would persist.

Hopefully some higher-quality studies will emerge giving a better idea as to whether antibiotics (or specific antibiotics) likely cause weight gain – and strategies for mitigation (if necessary).

Have you gained weight after antibiotics?

Why do you think this occurred? (Gut dysbiosis?)

Could the weight gain have been from iatrogenic effects associated with antibiotics?

Could the weight gain have been from depression and/or the mental toll of battling an infection?

Were you able to eventually get your weight under control?

How does your body weight compare to your immediate family members?

How does your pre-infection body weight compare to post-infection weight?

Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.