5-HTP is POSSIBLY SAFE when taking by mouth appropriately. 5-HTP has been used safely in doses up to 400 mg daily for up to one year. However, some people who have taken it have developed a condition called eosinophilia-myalgia syndrome (EMS), a serious condition involving extreme muscle tenderness (myalgia) and blood abnormalities (eosinophilia). Some people think EMS might be caused by an accidental ingredient or contaminant in some 5-HTP products. However, there is not enough scientific evidence to know if EMS is caused by 5-HTP, a contaminant, or some other factor. Until more is known, 5-HTP should be used cautiously. Other potential side effects of 5-HTP include heartburn, stomach pain, nausea, vomiting, diarrhea, drowsiness, sexual problems, and muscle problems.