Kiera Butler and Jaeah Lee review research on the question:
[A] growing body of evidence suggests that antibiotics might be linked to weight gain. A 2012 New York University study found that antibiotic use in the first six months of life was linked with obesity later on. Another 2012 NYU study found that mice given antibiotics gained more weight than their drug-free counterparts. As my colleague Tom Philpott has noted repeatedly, livestock operations routinely dose animals with low levels of antibiotics to promote growth.
No one knows exactly how antibiotics help animals (and possibly humans) pack on the pounds, but there are some theories. One is that antibiotics change the composition of the microbiome, the community of microorganisms in your body that scientists are just beginning to understand. (For a more in-depth look at the connection between bacteria and weight loss, read Moises Velasquez-Manoff’s piece on the topic.)