What are the risks of taking vitamin and mineral supplements?

Vitamin overdose

There are certain vitamins, mainly water soluble ones, that do not cause any know toxicity when taken too much, like vitamin C and vitamin B12 (cobalamine), but in general, overdosing with certain vitamins can cause serious damage to the body which is the main risk in taking vitamin supplements that contain very high doses (several times the daily allowance) of vitamins. E.g. an overdose of preformed vitamin A can cause eye damage and in higher doses even fatalities due to damage to the cells in the entire body.

Retinol, which is one form of vitamin A, has been shown to cause birth defects at moderate doses, which is the reason why prenatal vitamin supplements usually do not contain retinol, but beta-carotene, another form of vitamin A, that does not cause birth defects. Women who are pregnant or want to become pregnant should therefore not take any supplements that contain retinol. Large doses of Thiamin (vitamin B1) can cause drowsiness and muscle relaxation.

An overdose with Niacin (vitamin B3) can cause rashes, irregular heart beat, and fatal liver damage. It can also cause birth defects. Vitamin B5 can cause diarrhea, and B6 can cause nerve damage. An overdose of vitamin D leads to too much calcium being in the blood stream which can damage muscles, bones, heart and kidneys. At slightly lower levels, too much vitamin D can cause kidney stones.

Multivitamin preparations usually contain vitamin levels below the dangerous limits, so it is unlikely to develop any toxicities, if they are taken at the recommended dose. However, some single vitamin supplements often contain large doses, and might be toxic when combined with certain foods that are rich in these vitamins (e.g. vitamin A and D are present in liver in large quantities). Some minerals like iron and copper are very toxic, when taken in too large quantities.

Caution with pre-existing conditions

Increased intake of certain vitamins by using supplements has also been shown to have negative effects in certain pre-existing health conditions e.g. vitamin E supplements worsened congestive heart failure in one large study. Vitamin K can decrease the effectiveness of certain blood thinning drugs (warfarin) that patients use to prevent stroke and heart attacks. Carotenoids (certain forms of vitamin A) have been shown to have negative effects in smokers.

Mineral poisonings

Iron poisoning due to the intake of brightly colored multivitamins by little children who mistook them for candy used to be one of the most common reasons for fatal poisonings in children until packaging requirements for child safety were changed for supplement containing iron.