L-tryptophan isn't the only amino acid that has been sold in "health food" stores in apparent contravention of FDA rules. According to the National Nutritional Foods Association, a trade group for health food retailers, 13 kinds of amino acid supplement pills are available in stores around the United States.

None has been approved by the FDA, as required by federal regulations for the last 18 years. None comes with any kind of package insert describing possible side effects or potential risks. None of the manufacturers of these pills, in fact, has conducted any government-reviewed research documenting the safety and effectiveness of the pills.

And, according to some scientific experts, the safety of these products is unproven.

Consider, for example, the amino acid tyrosine.

"People are buying it because they have seen papers showing that it can improve performance in people who are exhausted," said Richard Wurtman, a professor of neuroscience at the Massachusetts Institute of Technology. "But it is not benign. It has side effects. It has a dosage range. It is contraindicated. I have a great concern that someone with high blood pressure will take tyrosine and will suddenly have a major fall in blood pressure. These are potent compounds."

Wurtman also said that carnitine, which is sold as an amino acid, can modify blood sugar levels. "Someone taking it could have a automobile accident because of hypoglycemia," he said.

FDA officials confirm that they are now considering taking action against the sale of these and other compounds sold under similar terms.

"The L-tryptophan situation has encouraged the agency to be more vigilant in the dietary supplement area, and we are now in the process of reexamining the way in which we regulate the dietary supplement industry," said FDA spokesman Jeff Nesbit.

This will not entirely resolve the issue of the safety of dietary supplements, however. Because of the protection offered the supplement industry by the Proxmire Amendment, the FDA is not allowed to limit the dosages in which vitamin and mineral supplements are sold. Yet many experts say that some nutrients -- especially Vitamin B-6 in high doses -- are toxic to the nervous system.