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Too many supplements contain hidden and illegal drugs. Weight loss pills are the worst offenders.

Dietary supplements are barely regulated in the United States. The makers of these pills — which are commonly sold at pharmacies, natural health stores, and gyms — don't need to prove their products are safe or even effective before putting them on store shelves.

It was that fact that prompted Soo Oh and me to comb through government databases, court documents, and scientific studies to find out what dubious ingredients have been found to lurk in popular supplements.

We uncovered more than 850 products that have contained illegal and/or hidden ingredients — including banned drugs, pharmaceuticals like antidepressants, and other synthetic chemicals that have never been tested on humans.

Weight loss supplements are the most likely to have problems

One striking revelation: More than half of the offending supplements that have been found to contain illegal or hidden ingredients were marketed for weight loss (431 in all). The second most popular category was sexual enhancement supplements, followed by muscle builders.

Some weight loss supplements contained appetite suppressants like sibutramine. The government removed sibutramine from the market in 2010 for safety reasons: It can increase blood pressure and pulse rate, causing coronary artery disease, heart failure, arrhythmias, stroke, and even death.

Other weight loss pills contained antidepressants like Prozac, or hidden laxatives, including the drug phenolphthalein. Phenolphthalein was banned by the FDA after it was found to increase the risk of irregular heartbeat and cause cancer with long-term use.

Some products, such as Slim Xtreme, actually contained both phenolphthalein and sibutramine. Note: This supplement was marketed as "100% natural."

You can search our database for your favorite supplement and learn more about supplement regulation here. But just remember: Right now you have no way of knowing for sure what's in your supplement bottle. And you should be especially cautious if you're taking weight loss supplements.