When parts of our bodies are sick, doctors can take them out to put new parts in. After they do, they give “feel-good-drops” to make you feel better. These make sure that you don’t get sicker by attacking and killing other bad things living in your body. At the same time, though, the good things that live in our stomach, and help us eat and go to the bathroom, can be killed. This is bad for us and can make it harder to get better. In our study, we included people who had new parts put into their bodies. We asked them to go to the bathroom and give us what comes out every week and then every month for up to a year. We figured out what was living in each person’s body, and how that changed over the year. We saw that the same feel-good-drops doctors were using to make people better were actually killing a lot of the good, helping things in their stomachs. Even 14 days after taking the feel-good drops, the people still had fewer good things in their bodies. It was also important that the type of drops the doctors used changed how many and which of the good things were killed. What we found is important for how doctors deal with sick people, especially after they put new body parts into them. Now, doctors know that the feel-good-drops they use to make people less sick can also hurt them later, and that it matters exactly which drops they use. Going forward, they can use this to decide how many and which feel-good drops to use. We hope that this study will help really sick people get better faster and stay better for a long time after going to the hospital.