[pyotr]

Your question is too broad. Not all vitamins (and trace minerals) are the same and there are numerous vitamins and minerals we need. You don't need over-dosing of vitamins by supplements if you eat a healthy diet with plenty of vegetables, fruits, a variety of whole-grains, dairy or meat/fish/poultry. Many vitamins are absolutely required for you to live and function. Many of these vitamins are found in your typical food items.

For instance You get plenty of cyanocobalamin (B12) if you eat meat/dairy but get almost none if you are a total vegetarian - you will develop neurological problems and eventually die from it unless you take injection of B12. Another example Vitamin C is found plentifully in many foods we eat but many small poorly done studies suggested it could help fight infections, improve asthma, etc. Larger trials have shown that vitamin C in higher doesn't make a difference. Linus Pauling was getting senile when he started taking megadoses and he died anyway soon afterwards from prostate cancer.

You don't need to spend money to buy a big jug of multivitamins because you're basically spending money to produce expensive urine (something like >99% of these vitamins are urinated out un-incorporated). Many OTC vitamins are deadly if taken in large doses. Especially avoid oversupplementing yourself with the fat-soluble ones (A,D,E,K). VitA overdoses will kill you, especially kids. vitD ODs will give you calcinosis and a chain of syndromes which could kill you. VitE ODs are known to cause strokes (bleeding), VitK ODs will cause strokes (ischemic kind) and blood clots. Vit C ODs will give you kidney stones. Too much calcium will lock up your bowels and can cause all manner of other problems. Many B vitamins can wreak havoc with your nervous system at the neuronal level. All supplemental vitamins have the potential to kill off your liver from idiosyncratic toxic reactions at the level of the hepatic cells.

Short lesson: don't waste your money on supplemental vitamins unless specifically recommended by your doctor (example, folate for pregnant women so your fetus will be less likely to develop neural cord developmental problems, B1 taken while on therapy from meds that can injure periperal nerves, Vit C taken with iron if you're anemic to help in the absorption of the iron, etc.)