Some people always say that too much of everything is not always good, this saying also apply to some supplements. Vitamins, Minerals and other food supplements can also be harmful when taken in large amounts, because they build up to dangerous levels in the body tissue. A toxic effect is rare with the water soluble vitamin B and C because they cannot be stored in the body in significant amounts, any excess is simply excreted in the urine. Very high doses of Vitamin C have been known to cause diarrhea and increase the production of kidney stones in people who are already prone to it.

Vitamin B6 when taken in very large quantities for month or years (as some women have done to relieve them from premenstrual syndrome) can cause some severe nervous system problems. The problems cease without leaving any lasting effects once dosage stops.

Fat soluble vitamins (A, D, E, and K) are potencially more of a risk since they are not excreted but stored in the liver and in fatty tissue. The most serious reported dangers arise with Vitamin A. Excessive use of very high dosage Vitamin A can result in damage to the liver and bones, hair loss, double vision, vomiting and headaches. Birth defects is also seen in babies of a woman who overdose themselves with Vitamin A during pregnancy. It is advice that pregnant women avoid all vitamin A supplements, and also liver and liver pate' as animal feed is often highly supplemented with Vitamin A. For most people, the best advice is to simply to avoid Vitamin A supplements. Nearly everyone gets more than enough from food such as oily fish, liver butter, margarine and milk.

Vitamin D can also have ill effect in very large quantities. Most people obtain enough from exposure to sunlight which cause it to be manufactured in the body, and from such foods like oily fish and eggyolk. Pregnant and nursing women, who need extra, and people who are confined at home are generally advised to increase their intake by diet.

Although also fat-soluble, vitamin E and K appear remarkably free of side effects.