Patients also need to be educated to watch for symptoms of eosinophiliamyalgia syndrome such as muscle pain or weakness, headache, and vomiting, as said at the seminar, which was sponsored by Harvard Medical School. They also need to exercise caution if they are already taking a selective serotonin reuptake inhibitor because inexplicable worsening of mood has been reported in normal control given 5-htp in conjunction with an SSRI. The Food and Drug Administration issued a "talk paper" on 5-HTP in 1998, confirming that peak X had been identified in some products. While the agency has not taken any action to remove the supplement from the market, it has stated that vigilance is warranted. Much of the peak X-contaminated L-tryptophan implicated in the 1980 outbreak ultimately was traced to a single Japanese manufacturer.
L-tryptophan was pulled from the US market in 1990, but remains available by prescription in Canada. The rationale for using L-tryptophan and 5-HTP for depression is that 5-htp is an immediate precursor of the neurotransmitter serotonin. L-tryptophan is metabolized to 5-HTP in the body.
Numerous studies have investigated the safety and efficacy of these products for depression, but most have been flawed. A systematic review suggested that L-tryptophan and 5-HTP may be more effective than placebo, but noted the evidence was of insufficient quality to be conclusive. The starting dose is 300mg twice daily; this can be increased to 300mg three times per day.