Is a high-dose vitamin D regimen safe?

Q. My doctor found that my blood levels of vitamin D were very low. She gave me a prescription for 50,000 international units of vitamin D to be taken every two weeks. But I've read that the recommended dose is more like 1,000 IU a day, and that more than 4,000 IU a day is unsafe. I have thin bones but not osteoporosis yet. I'm a healthy woman and don't want to do more harm than good. What's your advice?

A. Your doctor's prescription makes sense. But I can see why you're confused.

When people are found to have very low levels of vitamin D, it's often recommended that they "tank up" on large amounts for several weeks. Many doctors recommend a 50,000 IU vitamin D supplement once a week for six to eight weeks. However, that same dose once every two weeks for two months is also very reasonable. After that, people usually switch to a daily dose of 1,000 international units (IU).

Don't worry: Your body stores of the vitamin are very low. So weekly doses well over the usual 4,000 IU upper limit won't overload you and be toxic when taken for a limited period of time.

Vitamin D comes in two forms: vitamin D₂ (ergocalciferol) and vitamin D₃ (cholecalciferol). I recommend vitamin D₃. It is believed to be more effective than D₂ at boosting the biologically active form of the vitamin.

Once your vitamin D levels are in the normal range, you should continue to take a daily vitamin D supplement, but at a much lower dose. Since your bones are already somewhat thin (osteopenic), I recommend 1,000 IU daily.

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