When you should skip it: You have any type of illness, but especially a stomach bug.
Why it makes you feel worse: The caffeine in coffee is a diuretic, so it can make you pee a lot and leave you dehydrated.

Related: 11 Reasons to Have a Cup Of Coffee Right Now

That’s bad news when you’re battling a virus or other infection, since getting enough hydration helps your immune system fight more effectively, says Kristine Arthur, M.D., an internist at Orange Coast Memorial Medical Center in Fountain Valley, California.

Caffeine is even worse when you’re dealing with vomiting or diarrhea, since both can cause you to lose fluid. Taking in extra caffeine on top of that will only make that dehydration worse. (Find out how drinking too much caffeine affects your body.)

Plus, caffeine can actually stimulate the muscles in your digestive tract and make your diarrhea more intense, Dr. Arthur says.

Related: 5 Strange Causes Of Diarrhea