Eosinophilia-myalgia syndrome is an incurable and sometimes fatal flu-like neurological condition that was caused by contaminated l-tryptophan supplements.

In 1989 an outbreak of eosinophilia-myalgia syndrome was traced to an improperly prepared batch of tryptophan. The bacterial culture used to synthesise tryptophan had recently been genetically engineered to increase tryptophan production: unfortunately, with the higher tryptophan concentration in the culture medium, the purification process had also been modified to reduce costs, and a purification step that used charcoal adsorption to remove impurities had been omitted. This allowed another bacterial metabolite through the purification, and this contaminant of the end-product had been responsible for the toxic effects. Regardless of the origin of the toxicity, tryptophan was banned from sale in the US, and other countries followed suit. Tryptophan remains banned from sale to the public.

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