Some "herbal" weight-loss products may in fact contain powerful drugs that could put consumers at risk of serious side effects, according to researchers.

Writing in the journal Deutsches Arzteblatt International, German researchers describe the cases of 17 patients who were poisoned after taking "Chinese slimming capsules" sold over the Internet.

Though the products were marketed as "herbal," tests showed that they contained high doses of sibutramine -- the active ingredient in an anti-obesity drug sold as Meridia in the U.S. and Reductil in Europe.

The patients, who were reported to poison-control centers in two German cities, suffered symptoms such as elevated heart rate, chest pain, nausea, vomiting and breathlessness.

In two cases, patients developed temporary psychosis when they used the weight-loss product in combination wit prescribed psychiatric drugs.

Sibutramine is chemically similar to amphetamine, and it enhances the activity of the hormone norepinephrine and the chemical serotonin, which transmits messages to nerve cells and plays a role in appetite, metabolism and mood.

Among the people who should not take sibutramine are those with heart disease, psychiatric illnesses, uncontrolled high blood pressure or kidney disease.

Of the 17 poisoning cases, four patients had leftover pills that the researchers tested. They found that the purportedly herbal products contained doses of sibutramine that were nearly twice the maximum dose allowed in prescriptions sold in Germany.

The bottom line for consumers is that they should be aware of the potential risks and not assume that "herbal" means "safe," according to lead researcher Dr. Dieter Muller, a pharmacologist at Giftinformationszentrum Nord in Gottingen, Germany.

Muller told Reuters Health that he would not warn consumers to avoid all herbal weight-loss products sold over the Internet, as "there are doubtless a lot of slimming products without any health risks."

But patients should talk to their doctors first, particularly if they are taking any prescription medications. Not all doctors are familiar with herbal products, Muller noted, but they do know their patients' medical history and the potential risks of combining over-the-counter products with prescription drugs.