What can be some side effects of multivitamin tablets?

Nausea, diarrhea: Most multivitamins are very well tolerated but some can cause stomach upset, especially if taken on an empty stomach. They could cause diarrhea in some, especially if they contain large amounts of poorly-absorbed forms of magnesium like magnesium oxide. Some people get stimulated by taking b vitamins, especially if taken late in the day. One may also be allergic to some of the ingredients.