5-HTP: Uses, Side Effects, Interactions and Warnings - WebMDDonít use 5-HTP until more is known. 5-HTP might be UNSAFE. Some people who have taken it have come down with eosinophilia-myalgia syndrome (EMS), a serious condition involving extreme muscle tenderness (myalgia) and blood abnormalities (eosinophilia). Some people think the EMS might be caused by an accidental ingredient (contaminant) in some 5-HTP products. But 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, avoid taking 5-HTP.
I found this on the first site I clicked on which is WebMD.