Title: Impact of Antibiotic Therapy on Weight in Adult Patients
Authors: Nghi Nguyen, Kate O’Connor, Joshua Eudy, Elizabeth Spitzer, Bintaben Patel, Emmie Wu, Daniel Anderson
Objective: To evaluate short‑ and long‑term weight changes and metabolic effects associated with antibiotic use in adults.
Background: Approximately 80–90% of antibiotic prescriptions occur in the outpatient setting, and an estimated 30% are considered inappropriate. This unnecessary overuse of antibiotics is often cited as a risk factor for increased rates of antimicrobial resistance and adverse drug events. Another often-overlooked consequence of antibiotic use is gut dysbiosis, a disruption of the microbial composition involved in host energy metabolism, such as modulation of Glucagon-like peptide-1 (GLP-1). As such, gut dysbiosis associated with antibiotic exposure may contribute to unintended weight gain. While pediatric studies have linked early and broad-spectrum antibiotic exposure to increased rates of obesity by age five, data on weight and metabolic changes following antibiotic exposure in adults are limited. Understanding these long‑term physiological effects is necessary to support personalized antimicrobial stewardship and improve shared decision-making with patients in the ambulatory care setting.
Method: This was a single-center retrospective cohort study evaluating adult patients at the Wellstar MCG Health Internal Medicine Clinic between December 1, 2024, and February 28, 2025. Cohorts included patients who received at least three days of antibiotics and those who received no antibiotics. Patients were excluded if they were prescribed a GLP-1 receptor agonist, if they were pregnant or postpartum during the inclusion window, had a recent major surgery, had prolonged hospitalization (intensive care unit stay > 72 hours and/or medical‑surgical unit admission > seven days), active cancer treatment, or uncontrolled HIV. The primary outcome was the change in weight from baseline at 12-months. Secondary outcomes included weight changes at three and six months and changes in A1C at six and twelve months. Statistical analysis was completed using an independent t-test for continuous data and a chi-squared test for categorical data.
Results: A total of 158 patients were included (66 antibiotic‑exposed and 92 controls). Baseline characteristics were balanced, although a higher proportion of patients in the antibiotic arm were receiving medications with potential for weight gain than in the control arm (54.5% and 27.2%, respectively). Antibiotic‑exposed patients also had a higher median number of clinic visits per year than controls: 8.5 and 6.5 visits. Patients received a mean of 2.8 antibiotic courses and a median of 15 days per course. Amoxicillin/clavulanate, azithromycin, and doxycycline were the most frequently prescribed antibiotics. There was no difference in weight change at 12 months between the antibiotic and no-antibiotic groups (0.84 kg vs -0.2 kg, p=0.16). No differences were observed in weight at three or six months, nor in A1C at six and twelve months.
Conclusion: There was no difference in 12‑month weight change between antibiotic-exposed and unexposed adults. However, this data indicates a need for further investigations that include a prolonged follow-up period, targeted evaluation of high-dysbiosis-risk antibiotics, and cumulative antibiotic consumption, while accounting for exposure to other weight-impacting factors such as medications and comorbid conditions.
