I want to know what's the side effects of multivitamin?

Nothing: They generally don't hurt or help when taken in recommended dosages. Save your money and eat whole foods and wild salmon.

Variable responses: Multivitamins vary widely in their contents and quality. Some can cause stomach upset, diarrhea, and other problems. Some vitamins contain inferior ingredients, such as cheaper forms of cobalamin (cyanocobalamin), synthetic vitamin a and the wrong tocopherols centrum silver is one of these. Some also contain iron unneeded iron. Much helpful info at: http://www.Multivitaminguide.Org/.