In a battle against an infection, antibiotics can bring victory over enemy germs. Yet that war-winning aid can come with significant collateral damage; microbial allies and innocents are killed off, too. Such casualties may be unavoidable in some cases, but a lot of people take antibiotics when they’re not necessary or appropriate. And the toll of antibiotics on a healthy microbiome can, in some places, be serious, a new study suggests.

In two randomized, placebo-controlled trials of healthy people, a single course of oral antibiotics altered the composition and diversity of the gut microbiome for months, and in some cases up to a year. Such shifts could clear the way for pathogens, including the deadly Clostridium difficile. Those community changes can also alter microbiome activities, including interacting with the immune system and helping with digestion. Overall, the data, published Tuesday in the journal mBio, suggests that antibiotics may have more side effects than previously thought—at least in the gut.