5-HTP can cause mild side effects which may improve over time. These include nausea, abdominal pain, and diarrhea, and less commonly fatigue, insomnia, and headaches. Intravenous administration of 5-HTP has been reported to cause confusion, anxiety, and memory impairment.[1][2]
At high doses (100–200 mg per kg of body weight), 5-HTP has been associated with serotonin syndrome in lab animals, a severe condition caused by high levels of serotonin in the body.[2] In rodents, the risk of serotonin syndrome was increased when 5-HTP was combined with SSRIs (commonly used antidepressant medications). There are no reports of toxicity at normal doses, and it should be noted that serotonin syndrome from 5-HTP supplementation has not been observed in humans.[2]
In 1989 and 1990, there were over 1,500 cases of eosinophilia myalgia syndrome (EMS) (a rare syndrome with symptoms of severe muscle pain and fatigue) leading to over 30 reported deaths in people taking tryptophan supplements, a direct precursor to 5-HTP. Additionally, there were a few cases of EMS reported with 5-HTP supplementation. It was determined that EMS likely occurred due to a toxic contaminant in the supplements, and there have been no confirmed cases in recent years. Regardless, this led to a stark decline in the use of and research on tryptophan and 5-HTP supplements. To avoid the risk of EMS, it is important that 5-HTP supplements are purified and tested for contaminants.[2][3]