In addition, because of concerns that some products are impure or weak, we do not routinely recommend these products to our patients," he added.
Some of the most commonly used products include horny goat weed, ginseng, DHEA, Ginkgo biloba, fenugreek and maca, the researchers found.
For many of the products, there's no scientific evidence to support claims they can improve libido, erectile dysfunction or sexual performance, the researchers said.
They also found that some of the supposedly "natural" products have traces of phosphodiesterase-5-inhibitors (PDE5Is), the medication found in prescription drugs -- such as Viagra -- used to treat impotence.
One study reviewed by the researchers found that 81 percent of tested samples of over-the-counter male sexual enhancement products bought in the United States and Asia contained PDE5Is.
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