An overdose of certain vitamins can cause health problems. Too much vitamin D can cause diarrhoea and weight loss. An overdose of vitamin A can also be harmful, particularly to pregnant woman because it can damage the foetus. The toxicity of vitamin A depends on several factors, so it is not possible to say precisely how much is too much.

It is possible to take large doses of the water soluble vitamins (Vitamin C and the B-complex) without any problems, except for vitamin B6. Long-term mega-doses of that vitamin may cause nerve damage in hands and feet. People at risk for kidney stones should be careful not to get too much of vitamin C.