While in most cases you can get all the nutrients you need from a balanced diet, many people choose to take supplements, but taking some high-dose supplements over a long period of time could be harmful. An independent expert review of the scientific evidence on the safety of vitamins and minerals in the UK offers some warnings about supplements. This is not an all inclusive list, but covers only those supplements examined for safety at this time.

Most supplements are safe, however:

  • High levels of the supplement chromium picolinate may cause cancer, but 10 milligrams (mg) a day or less of chromium in forms other than chromium picolinate is safe.
  • More than 1,000 mg per day of vitamin C, 1,500 mg per day of calcium, and 17 mg of iron can cause abdominal pain and diarrhea in some people.
  • In another study from the US, consumption of high levels of iron appears to be associated with an increased risk of throat and stomach cancer.
  • More than 10 mg of vitamin B6 daily may also be harmful, unless under medical care.
  • Beta-carotene, an antioxidant found in orange and yellow fruits and vegetables, could cause irreversible harmful effects in smokers if taken long-term and in high doses.
  • Manganese could be dangerous for older people over an extended period of time, while nicotinic acid, zinc, and phosphorous could also be harmful in high doses over the long-term.

The good news is that, in most cases, the adverse effects seem to disappear when ingestion of the supplements stops. Always make sure you do not exceed the maximum dose unless supervised by your doctor. If you are taking more than one supplement, read the labels carefully to be sure you’re not doubling up with multiple products.

First published in the Inside Tract® newsletter issue 153 – January/February 2006