Some vitamins can be overdone. These are the ones that build up in your system, like E, and A. Vitamin A can be very toxic at high levels. B and C vitamins flush through your system. There is some debate about the value of supplementing with them, but they are not thought to be harmful, even at fairly high levels. Eating a balanced, pre-industrial (i.e. low processed) diet should give you all you need. Some typical modern American diets may actually require supplements.

I try to eat a balanced diet. This is not easy since the wife prefers the highly processed crap. I try to limit saturated fats. I eat peanut butter on wheat toast for breakfast almost every morning. Almost no red meat. I get most of my fish from canned tuna, the only form of fish the wife will eat. Oh well.