(AP) -- Federal regulators are ordering several companies to stop selling an unproven weight loss remedy that uses protein from the human placenta.

The Food and Drug Administration issued warning letters to seven companies that sell the protein as drops, pellets or sprays. Human chorionic gonadotropin is produced by the placenta and found in the urine of pregnant women. While it is approved for certain infertility treatments, the FDA says there is no evidence it helps reduce weight.

Many of the products cited in the warning letters claim to change "abnormal eating patterns," and help people lose 20 to 30 pounds in as little as a month when used with a low-calorie diet, usually around 500 calories per day.

Health experts say such restrictive diets can be dangerous.

"These products are marketed with incredible claims and people think that if they're losing weight, HCG must be working," said Elizabeth Miller, of FDA's division for non-prescription drugs and health fraud, in a statement. "But the data simply does not support this - any loss is from severe calorie restriction. Not from the HCG."

The products are advertised as homeopathic remedies, or highly diluted drugs made from natural ingredients and sold with medical claims. Many doctors view homeopathic remedies as ineffective but mostly harmless because the drugs in them are present in such tiny amounts.

But the FDA said the products targeted Tuesday could be dangerous when taken as directed.

Homeopathic treatments are based on principles unverified by mainstream science, and often reference mystical-sounding concepts like "vital force" and "healing energy."

In 1938, Congress passed a law granting homeopathic remedies the same legal status as regular pharmaceuticals. The law's principal author was Sen. Royal Copeland of New York, a trained homeopath.

While regulation of the products is minimal, the FDA maintains a list of approved ingredients that can be used for homeopathic drugs. HCG is not among the approved ingredients.

After decades of declining use, homeopathic remedies have revived in recent decades with the burst of interest in vitamins, herbs and other unconventional treatments.

The companies cited by regulators have 15 days to respond and detail how they intend to remove their products from the market. Failure to do so may result in legal action, including seizure and injunction, or criminal prosecution.

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