Detective work by epidemiologsts in Britain and the United States led the health department in London to impose a ban on a food supplement linked to at least 22 deaths in America. The supplement is tryptophan, an essential amino acid which has been sold by health food stores and chemist shops in tablet form as a folk remedy for insomnia, premenstrual tension, stress and depression. It is also an ingredient of prescription-only anti-depressants, which have been withdrawn by their manufacturers on government advice.

Researchers in the United States have found that tryptophan as a supplement is closely connected to outbreaks of a rare blood disorder called eosinephilia myalgia syndrome, or EMS. More than 1,500 cases of the illness have been documented. It causes potentially fatal nerve and muscle damage, as well as skin rashes and flu-like symptoms.

Seven cases of the condition have been reported in Britain, including three Americans who had used the tablets. Investigations by epidemiologists at the Centers for Disease Control in Atlanta, Georgia, showed that virtually all the sufferers had been taking tryptophan products imported from Japan. Health authorities in America alerted their British counterparts who, in November, issued a warning that people should stop taking any dietary supplements containing tryptophan as the sole or major ingredient, unless prescribed by a doctor. The name “L-tryptophan” or “DL-tryptophan” usually features on the labels.

In January, France banned sales of the supplement, and in March, the US Food and
Drug Administration recalled products containing the supplement.

Louis Sullivan, the US government’s health and human services secretary, warned that it could be fatal. “We are confronted with a major public health problem,” he says. Tryptophan was causing deaths and long-term illness with some patients failing to respond to treatment. “I urge everyone to stop taking these supplements immediately.”