L-tryptophan from Showa Denko killed 37 and disabled 1,500 — a contaminated supplement

In 1989, L-tryptophan was a widely sold dietary supplement used for insomnia, depression, and premenstrual syndrome. It was produced by several manufacturers, including the Japanese chemical company Showa Denko, which used a genetically engineered strain of Bacillus amyloliquefaciens to produce the amino acid in open fermentation vats. In an effort to cut costs, Showa Denko had reduced the amount of activated charcoal used in purification and, in some batches, bypassed the reverse-osmosis filtration step entirely.

These manufacturing changes introduced trace contaminants into the L-tryptophan. By November 1989, the US Centers for Disease Control had received the first reports of a mysterious new syndrome — eosinophilia-myalgia syndrome (EMS) — characterised by severe muscle pain, skin hardening, nerve damage, and respiratory failure. More than 1,500 cases were reported to the CDC, and at least 37 people died. The contaminated batches were traced to a single manufacturer: Showa Denko.

The FDA recalled L-tryptophan supplements in 1989 and banned most public sales in 1990. The ban remained in place for over a decade, with restrictions loosening only in 2001 and fully lifting in 2005. The case was one of the first major public health disasters linked to a genetically engineered product and a stark demonstration of how cost-cutting in manufacturing can produce deadly contaminants. Showa Denko faced multiple lawsuits, and the disaster permanently changed the regulation of dietary supplements.