Researchers have discovered that the unique microbiomes of healthy people cause their bodies to metabolize certain drugs in different ways. In fact, the bacteria living inside your body may very well control whether or not a drug works for you or not. Specifically, researchers in this recent study found that irinotecan, a chemotherapy drug, causes diarrhea in some patients because their bodies produce β-glucuronidases.
Normally, the body can modify irinotecan's chemical structure and those of other drugs with a chemical called glucuronidate, allowing the liver to take over and detoxify them. However, these bacterial enzymes remove glucuronidates, rendering the drug a toxic compound. Bacteria are mostly flexible in their diets and can eat whichever nutrients the body presents them with, including drugs. However, this flexibility, which is overall a good thing and helps the body, as well as the bacteria, to survive, can become harmful when it triggers the body to produce toxic compounds, as it is shown in this case.