My doctor has recommended a large daily dose of vitamin D to counteract osteopenia (thinning bones). Is it possible to overdose on the vitamin? —J.S., Orinda, Calif.

Yes, but it takes huge doses: at least 10,000 international units (IU) a day for several years. Such amounts can damage the kidneys, bones, and nervous and cardiovascular systems. But reaching them would require a lot of pills or even injections. Most people should consume between 600 and 1,000 IU a day from food or supplements, depending on their age and level of sun exposure. Multivitamins usually supply about 400 IU, enough for most people to reach those levels. But you may need a larger supplement if you are older than 50 or so, heavier, have darker skin, or rarely get out in the sun. If the dose your doctor suggests is higher than the 1,000 IU a day typically recommended to strengthen bones and provide other benefits, ask why you need so much.

This article first appeared in the July 2008 issue of Consumer Reports on Health.

Read more on the surprising benefits of vitamin D (free) and on how vitamin D can help in the prevention of osteoporosis (subscribers only).