Vitamin and Mineral Supplements

There is a common perception, especially in North America, that we all need to supplement our diets with vitamins, minerals, and other nutrients. For some people who have an inadequate diet that may be true and taking a daily multivitamin is usually not harmful. Where we can get into trouble is by swallowing "mega" doses of these substances (or any other supplement or even prescribed medications), basing our too high consumption on the idea that "if this much is good for me, more will be even better." What we’re really doing is asking for trouble. Vitamin and mineral supplements can be toxic if taken in doses that exceed the recommended levels. Except for certain conditions, pernicious anemia for instance, treatment with high doses of vitamins is totally unnecessary and can produce undesirable or even dangerous side effects.

Minerals can be toxic, too. For instance, magnesium causes diarrhea if you take 400 mg or more per day. Zinc may cause vomiting or even depress the immune system.

Vitamin A can produce liver damage, problems with vision, and an increased risk of hip fracture at higher doses. It is also harmful to the unborn child at very high doses (e.g. 10,000 to 15,000 IU per day).

Beta carotene, promoted in the media as a cancer preventative, has been found instead to increase the risk of lung cancer in smokers and people who have been exposed to asbestos, or people with the condition asbestosis, at high intake levels, i.e., more than 33,000 IU per day. It can also produce joint pain, diarrhea, or yellow the skin.
High doses of Vitamin C (more than 2000 mg per day) can cause diarrhea or stomach upset.
More than 2000 IU per day of Vitamin D can cause calcium deposits in muscle or skin and can increase blood levels of calcium leading to muscle cramps and other problems.

Vitamin E at doses of 8000 IU per day can cause stomach problems, diarrhea, blurred vision, headaches, tiredness, and possible antiplatelet (anticlotting) effects.

Vitamin B6, if taken at doses higher than 200 mg per day, can cause nerve damage, particularly in the hands and feet, and can adversely affect balance and gait.

Niacin, in doses greater than 3000 mg per day, can cause liver damage. In lesser, but still excessive doses, it can cause stomach and intestinal upset, flushing, high blood sugar, and it may interact with statins (cholesterol lowering drugs) and blood pressure medications.

Adapted from Health and Drug Alerts: Too much of a good thing? Toxic effects of vitamin and mineral supplements.
Eric Wooltorton
CMAJ (Canadian Medical Association Journal) July 8, 2003; 169(1).