If you walk down the aisles of any health food or drug store, you'll find dozens, if not hundreds, of weight loss supplements - many claiming to have extraordinary benefits.

But before you're tempted to try one of these products, keep in mind that weight-loss aids and dietary supplements do not require FDA approval and aren't subject to the same rigorous standards as are prescription drugs. Therefore, they can be sold with limited proof of effectiveness or safety. Case in point: recent testing by ConsumerLab.com found a carcinogenic form of chromium, hexavalent chromium, present in some dietary supplements, including two popular weight loss products and a chromium supplement. They also found lower amounts of green tea than claimed in some weight loss supplements and noted concern for potential side effects from products containing high amounts of caffeine or other stimulants.

Chromium is a mineral that enables the hormone insulin to function properly. And although it's thought to be particularly helpful for people with insulin resistance or diabetes, chromium is also promoted as a weight-loss aid, sold as a single-ingredient supplement as well as in combination formulas. However, hexavalent chromium (which was at the centre of the movie Erin Brockovich) is toxic and should not normally be found in supplements. Long-term occupational exposure to hexavalent chromium can lead to skin problems, perforated nasal septum, and lung cancer.

Legal limits have not been established for the amount of hexavalent chromium in supplements, but the state of California is considering a public health goal limiting hexavalent chromium in drinking water to only 0.06 ppb-or about 0.12 mcg per day. ConsumerLab.com found that one widely sold weight loss pill delivered 26.4 mcg of hexavalent chromium per day, 220 times the maximum amount one could be exposed to from daily intake of water under California's proposed limit. A chromium supplement delivered 10.9 mcg in a daily serving and another nationally known weight loss supplement contained up to 3.2 mcg per daily serving. The contaminant was confirmed in three independent laboratories using blinded samples.

Other quality problems revealed by ConsumerLab.com included two supplements that provided less green tea than listed and a supplement with FDA labelling violations. In fact, due to the variety of quality problems discovered, all five of the weight loss supplements selected by ConsumerLab.com with listed amounts of green tea failed testing. ConsumerLab.com also noted the potential risks posed by weight loss supplements with high amounts of caffeine (three of which provided 600 mg per day-an amount equal to about 15 cans of cola), including insomnia, tremors, nausea, and headache. Potential cardiovascular risks were also raised with a product combining caffeine with synephrine, from the ingredient bitter orange, and with two products containing the ingredient yohimbine, an erectile dysfunction remedy.

ConsumerLab.com's reviews are available in two online reports, one focusing on Chromium Supplements and the other on Weight Loss Supplements (Green tea, 7-keto DHEA, and Stimulant Formulas). The new reports include reviews of Dexatrim, Green Tea Fat Burner, Hydroxycut, Mega-T Green Tea, Metabolife, MetaboSpeed, Orovo, Slimquick, Stacker3, and Xenadrine as well as supplements from Blaine Pharmaceuticals, FoodScience of Vermont, GNC, Natural Factors, Nature's Bounty, PhytoPharmica, Puritan's Pride, and Vitamin World.