The eosinophilia-myalgia syndrome was the devastating neurological and muscular condition linked to tryptophan supplements that led to them being restricted — an epidemic of the late 1980s and early 1990s. Users of L-tryptophan, sold as a sleep and mood aid, developed soaring eosinophil counts with severe muscle pain, skin thickening, nerve damage, and in some cases death; investigation traced the outbreak to a contaminated batch from a single manufacturer whose altered production process introduced a toxic impurity. Thousands were affected before the link was made. The episode reshaped supplement regulation — it remains the textbook case for why “natural” does not mean safe and why manufacturing quality matters. Tryptophan itself was exonerated; the contaminant was the killer. The syndrome’s legacy is caution: supplements need the same quality discipline as drugs.