[Erebos]

The best way to get vitamins is through eating healthy. If you lack cyanocobalamin, eat more liver. If you lack folic acid, eat more green leaves. If you lack citric acid and developing scurvy, chow down on oranges.

Any chemical in excess or in shortage isn't good for the body. You can die from drinking too much water, after all.

As for vitamin supplements, the vitamins themselves probably aren't bad for you if you take the right amount. The other stuff that goes into the pill may be bad, but that will depend on the manufacturer. And yes, you do risk overdosing. On the other hand, overdosing on oranges would be difficult.