Drugs that we take for common diseases might have more side effects than doctors realize. And not just for us – for the environment, too! Because the medicine we take is not completely used up in our bodies, it ultimately ends up in sewage systems. From there, it enters rivers and oceans, where it might impact fish and other living things. We set out to examine what happens when a certain type of fish (the Siamese fighting fish, also called the Betta fish) is exposed to a very common drug named metformin in its water. This drug is commonly given to people with diabetes and is also used for many other illnesses. And it did have an effect on our fish. We found that even low levels of the drug (as they are currently found in nature) change the behavior of the Betta fish: it makes males less aggressive, which can impact their chances of reproducing.