Courtesy of Wiki:
In February 2001 the FDA loosened the restrictions on marketing (though not on importation), but still expressed the following concern:
"Based on the scientific evidence that is available at the present time, we cannot determine with certainty that the occurrence of EMS in susceptible persons consuming L-tryptophan supplements derives from the content of L-tryptophan, an impurity contained in the L-tryptophan, or a combination of the two in association with other, as yet unknown, external factors."
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