Epidemiology of potential association between L-tryptophan ingestion and eosinophilia-myalgia syndrome

This article examines the methodology of the epidemiological studies of the association between L-tryptophan and eosinophilia-myalgia syndrome (EMS) and evaluates the validity of the conclusions from these studies. In the initial case-control studies of L-tryptophantryptophan and EMS there were a variety of methodological problems, including different sources and different exclusion criteria for cases and controls, which could have resulted in selection bias, as well as problems with information bias and confounding. The studies of manufacturer and brand also had similar potential for bias. The L-tryptophan-EMS association is based on two small studies that had important methodological inadequacies. Subsequent studies of brand of L-tryptophan also contained errors in design, which may have produced biased results and call the conclusions into question. The cause of eosinophilia-myalgia syndrome remains unknown.