WASHINGTON (CNN) -- For the second time in a decade, researchers say they have found possibly harmful contamination in a popular supplement sold in health food stores to promote sleep, prevent headaches and aid in weight loss.

L-tryptophan was pulled from the market in 1990 after a contaminated form of the product, made in Japan, was associated with nearly 1,500 cases of eosinophilia myalgia (EMS), a rare and occasionally deadly blood disease. More than 30 deaths occurred.

L-tryptophan -- a version of a naturally occurring amino acid -- was reformulated, but Stephen Naylor and Dr. Gerald Gleich at the Mayo Clinic in Minnesota said Monday that tests on six different over-the-counter brands of the supplement showed evidence of the same chemical contaminant, known as "peak-X." Their findings were published in the September 1 edition of the journal Nature Medicine.

Gleich told a briefing Monday that they are not aware of any current cases of illness associated with products currently on the market. But, he added: "We believe the potential is there."

L-tryptophan had been used by an estimated 15 million Americans as a remedy for depression, premenstrual syndrome and insomnia when it was pulled from the market in 1990.

At that time, researchers could not determine whether the EMS resulted from peak X, excess L-tryptophan itself or a combination of the two.

Since then, the supplement was replaced by a version called 5-hydroxy-L-tryptophan (5-OH-Trp), which is sold over the counter.

Now, low levels of a similar contaminant have been found in both synthetic and natural versions of the new product, the research team says.

"This latter compound is freely available over the counter and is being recommended to overcome 'serotonin deficiency syndrome' as well as obesity, headaches and insomnia," they wrote in the findings. "Yet whether it works or even is safe has not been evaluated.

"Indeed, the onset of EMS-like symptoms has also been associated with the ingestion of 5-OH-trp as far back as 1980," they wrote.

They said no current cases of EMS have been traced to the new supplement, but said the Food and Drug Administration had been notified of their findings.

The FDA said is it trying to confirm the report.

Some health books recommend large doses of this supplement, Gleich noted, urging caution on anyone taking it. "We do not know what safe doses are," Naylor added.

"Many alternative medicine strategies seem to offer substantial promise to the consumer," they wrote in their findings. "However, this study emphasizes the need for tighter quality control for the production of both synthetic and 'naturally' produced nutritional supplements sold as medications."

In 1994, Congress passed a law prohibiting the FDA from regulating dietary supplements unless they are marketed as drugs -- by claiming to treat or prevent diseases -- or prove to be an unreasonable health risk.

Illnesses that the researchers called EMS-like were suffered by a family taking the newer version of the product in 1991, they said.

The Mayo researchers purchased six different samples of the new product in the Rochester, Minnesota, and New York City areas. They declined to name the manufacturers of the samples they tested.

"In all six commercial preparations of 5-OH-Trp analyzed, peak X was detected at levels between about 3 percent and 15 percent of that observed in" the product implicated in the 1991 case.

The lower levels of contaminant may be why no new cases of the illness have been reported, they suggested.

Many products being sold are capsules ranging from 25 milligrams to 100 milligrams.

In the 1991 case, a 28-year-old woman became ill after taking 50 milligrams to 70 milligrams daily, though that product had higher levels of the contaminant.

But the researchers said at least one book recommends taking 300 milligrams to 900 milligrams daily. That amount, they said, could raise intake to levels that might cause illness.

The Associated Press and Reuters contributed to this report.