Taking vitamins on an empty stomach can irritate your stomach lining. This can cause nausea, cramping, or diarrhea in some people.
The reaction depends on the vitamin. Iron, vitamin C, and zinc are common causes because they directly irritate stomach tissue when nothing else is in your digestive system.
Fat soluble vitamins behave differently. Instead of irritation, the main problem is poor absorption, since your body needs fat present to move them out of your gut and into your bloodstream.