David L. Katz, MD
Photo: Mackenzie Stroh
Q: I'm in good health and work out regularly. I started taking 5,000 micrograms of biotin daily to help my hair grow thicker and longer—is there any danger in that?
— Rakia Green, Norton, Massachusetts

A: Biotin, a water-soluble B vitamin, helps convert calories into energy and is involved in the synthesis of fatty acids, and we typically get it from yeast, liver, egg yolks, cheese, avocados, and raspberries.

A deficiency can cause hair loss, which may help explain why people believe taking it will give them thick hair. But biotin deficiencies are rare in the United States. (Unless you're in the habit of eating raw eggs, that is. The whites contain a compound that interferes with absorption of the vitamin; it's deactivated by cooking.)

You're taking about 160 times more than experts recommend Americans get on a daily basis, but you'll be relieved to hear that there doesn't seem to be any danger in the amount. Although the Institute of Medicine (the national advisory group tasked with setting daily values for nutrients) recommends just 30 micrograms a day, it hasn't issued a safe upper limit for biotin; people have taken very high doses without any adverse effect.

One reason biotin is so safe is that, with watersoluble vitamins, you absorb what you need and your body flushes the rest out through your kidneys. That's not true in the case of fat-soluble vitamins such as A, D, E, and K, which can build to toxic levels in your body.

Here's the bad news: There is no conclusive evidence that high-dose biotin will give you a thick, lustrous mane. While you may not be putting your health at risk, your wallet is definitely suffering.