Antibiotics and Weight Gain

Antibiotics and Weight Gain

In observational studies, antibiotics have been linked to an increased risk of childhood obesity. For this reason, a recent study examined this association.

This is the first large-scale observational study to examine this link across a large range of healthy children and adolescents.

Below is a detailed summary of the results.

Antibiotics May Promote Weight Gain In Children

Antibiotics and Weight Gain


Antibiotics are sometimes added to animal feed to increase growth in livestock.

For this reason, scientists have speculated that antibiotics may have similar effects in humans.

Many observational studies have examined this association in humans. Here are some of their findings from over the years:

  • 2013: Antibiotic use in the first 6 months of life was associated with increased body weight at 10–38 months. Later exposure was not consistently linked.
  • 2014: Repeated use of broad-spectrum antibiotics in the first 2 years of life was linked with a greater risk of obesity at 24–59 months of age.
  • 2014: Using antibiotics in the first year of life was linked with an increased risk of being overweight or obese at ages 9 and 12.
  • 2014: Receiving antibiotics in the first year of life was associated with a slight increase in body mass index in boys aged 5–8 years.
  • 2014: Antibiotics improved weight gain in children with malnutrition.
  • 2015: Antibiotic use within the first 6 months of life was linked with higher body mass in childhood. This was strongest for macrolide antibiotics.
  • 2015: When women received antibiotics in the second or third trimester of pregnancy, their children were at a higher risk of becoming obese.
  • 2016: Receiving antibiotics within the first 6 months of life was not associated with weight gain up to age 7.

As a possible explanation, studies indicate that antibiotics may imbalance the gut microbiota, affecting metabolism and increasing calorie absorption.

Following short-term antibiotic use, the composition of the microbiota may return to normal after a while, but in some cases it takes several years.

Article Reviewed

This study examined the association between childhood antibiotic exposure and the risk of weight gain and obesity.

Antibiotic use and childhood body mass index trajectory.

Study Design

This longitudinal observational study examined the association between antibiotics and weight, using the electronic health record data of 163,820 children and adolescents, aged 2–18 years.

It tested three main hypotheses:

  • Hypothesis 1: Antibiotics have a reversible effect — affecting people’s risk only temporarily, or until the gut microbiota recovers.
  • Hypothesis 2: Antibiotics have a persistent effect — affecting people’s risk of weight gain or obesity for a long time afterward.
  • Hypothesis 3: Antibiotics have a progressive effect — influencing children’s weight gain in a way that strengthens over time.

The researchers used mixed-effects linear regression models to calculate the association, while taking other obesity-related factors into account.

Bottom Line: This was an observational study examining the association between antibiotic use and childhood weight gain and obesity.

Finding: Antibiotics Are Linked to Weight Gain

The study showed that antibiotic use in childhood was linked to weight gain.

Specifically, taking antibiotics was associated with an estimated 1.6–3.3 lbs (0.73–1.50 kg) greater body weight at 15 years of age.

Additionally, children who were prescribed antibiotics 12 or more times were 3.5 % heavier when they were 8 years old, compared to the average for that age.

This weight gain was equivalent to 2.4 lbs (1.1 kg).

The study also confirmed all 3 hypotheses — using antibiotics was reversibly, progressively and persistently associated with body mass index (BMI).

  • Reversible association: Antibiotics’ effects on body weight may weaken with time. Other studies also show that the gut microbiota may recover with time.
  • Persistent association: The effects of antibiotics may persist for a long time, as supported by previous studies.
  • Progressive association: Associations differed by age. The study suggests that as children get older, antibiotics’ effects on weight gain may become stronger.

When different classes of antibiotics were analyzed separately, macrolides turned out to be the most strongly associated with weight gain at age 15.

Penicillin and cephalosporin were also separately linked with weight gain.

However, this is the first study to indicate that antibiotic use may affect weight gain in children at any age.

Bottom Line: The study suggests that using antibiotics may increase the risk of weight gain throughout childhood.


This study was the largest to examine the association of antibiotic use with obesity, and tried to address many limitations commonly seen in previous studies.

As an observational study, it couldn’t prove that antibiotics caused weight gain, only that taking them was associated with it.

However, given the evidence from animal studies, it seems likely that antibiotics may have a similar effect in humans.

Additionally, the researchers did not have the full antibiotic history of the participants, and only examined a few years of each person’s life (usually 3–5 years).

Data on antibiotic use in the first two years of life — when they may have the strongest effects on weight — was only available for some of the participants.

Finally, the researchers relied on prescriptions for the antibiotic dose and duration. Therefore, they couldn’t be certain all the prescribed antibiotics were actually used.

Bottom Line: This study observed associations, but couldn’t prove a causal relationship. Additionally, the patient history data was incomplete.


Summary and Real-Life Application

In short, the study suggests that taking antibiotics may increase the risk of weight gain in children and adolescents.

However, this was an observational study. It doesn’t prove that antibiotics are to blame, only that taking them is linked with weight gain.

Nevertheless, antibiotics are used to increase growth in farm animals, and a similar effect in humans seems plausible.


Written by Aline Pilani

Hey, I’m Aline Pilani. I am a certified personal trainer and nutritionist. I have spent the last 10 years of my life helping people losing weight, increase their health and confidence, and I truly want to do the same for you.