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Researchers are also looking at yohimbine, which is derived from the bark of a tree typically found in parts of Africa, and hydrocortisone, better known as an anti-itch cream, but used in pill form.

Yohimbine, the alkaloid derived from the bark of the African yohimbe tree and the active ingredient in yohimbe supplements, is sold despite little or no evidence for its effectiveness as a weight-loss aid and for treating erectile dysfunction, depression, low libido in women and other ailments.

A study in Drug Testing and Analysis evaluated 49 brands of supplements presumably containing yohimbine and found that only 11 listed the quantity contained.

He’s also been studying yohimbine, a prescription drug that can be extracted from the bark of a species of West African evergreen tree and sometimes appears in bodybuilding capsules.

Like ephedrine, yohimbine “comes from a plant but is pharmaceutically active,” he says, blurring the line between drug and supplement.