Many dietary supplements, especially those that claim to enhance male sexual performance, are spiked with prescription drugs. That's one of the conclusions of our new report on 10 surprising dangers of vitamins and supplements.

Case in point: Mojo Nights, a sexual-enhancement product made by Evol Nutrition Associates, Inc. Earlier this month, the Food and Drug Administration said tests it conducted on samples of the supposedly "all-natural" product found that it actually contained sildenafil and tadalafil, the active ingredients in the prescription drugs Viagra and Cialis.

The agency also warned that the product's claims, such as "stop pre-mature ejaculations" and "increase the STRENGTH and POWER of your ERECTION," go beyond what's permissible for dietary supplements, which can't claim to directly affect the structure or function of a body part. (Only drugs, which have to go through an approval process in which they demonstrate efficacy, can make those claims.)

Just three weeks earlier, the FDA warned another company, ABCO Laboratories, that samples of its Sexual Virility Max supplement contained synthetic chemical compounds that are structurally similar to sildenafil.

Those are just the latest in a stream of so-called "natural" products that have turned out to be spiked with powerful prescription drugs. Since 2008 more than 400 such products have been recalled, most of them marketed for bodybuilding, weight loss, and sexual enhancement.

Those adulterated products can cause some of the same side effects and interactions that consumers may have been trying to avoid by choosing supplements over drugs.

Bottom line: Don't assume "natural" means safe, or in some cases even natural. For details, see our new report, Vitamins and Supplements: 10 Dangers that May Surprise You.