Toxicity, allergies, interactions, and contraindications
January 1999

Toxicity refers to the degree to which a substance interferes with normal physiological functions. Almost any substance can be toxic if consumed in sufficient quantity. Most dietary supplements enjoy very low toxicity. Only about a dozen deaths in the United States were ever confirmed to be attributable to toxic dose of a dietary supplement throughout their history -- only about 1 per year. Most of these were associated with very high dosage levels. Compare this with over 100,000 deaths per year caused by the toxic effects of prescription drugs.

Most dietary supplements can have negative side effects when taken in excessively large quantities. Supplements that need to be taken with caution include the fat-soluble vitamins A and D, niacin, all minerals, and some of the stronger herbal products. Recommended dosage levels on products labels provide guidance as to what levels may cause unwanted side effects.

Some people are allergic to dietary supplements just as they are to food or drugs. Some substances such as comfrey are contraindicated for conditions such as pregnancy. Some can interact with other drugs or recently consumed supplements, thereby causing negative side effects. To avoid these problems, read all warnings on the product label. Ask a physician to assess whether any other substance you are taking or any medical condition you are experiencing precludes safe use of a dietary supplement.