Here's a quick one from wikipedia - you will find more when you search:
''Because 5-HTP has not been thoroughly studied in a clinical setting, possible side effects and interactions with other drugs are not well known.
Administered serotonin has been shown to increase the risk of heart valve disease in animals.[19][20] 5-HTP has not been subjected to this test. Oral 5-HTP results in an increase in urinary 5-HIAA, a serotonin metabolite, indicating that 5-HTP is peripherally metabolized to serotonin, which is then metabolized. This might cause a false positive test in tests looking for carcinoid syndrome.[21]
5-HTP can cause hypertension by increasing plasma renin activity, when not co-administered with a peripheral aromatic L-amino acid decarboxylase inhibitor, such as carbidopa or benserazide.[22]
Direct and indirect evidence for possible yet unproven risks and side effects associated with 5-HTP when overdosed:
* Heart valve damage or disease (cardiac fibrosis).[19][20]
* When combined with MAOIs or SSRIs, 5-HTP can cause acute serotonin syndrome.[23][24]
* When combined with carbidopa (as a treatment for symptoms of Parkinson's disease), 5-HTP causes nausea and vomiting; however this can be alleviated via administration of granisetron.[25] As mentioned above under pharmacology, cases of scleroderma-like illness have been reported in patients using carbidopa and 5-HTP.[26]
5-HTP may cause eosinophilia-myalgia syndrome, a serious condition which results in extreme muscle tenderness, myalgia, and blood abnormalities. Some think this could be caused by a contaminant in most 5-HTP products.[27]''
Interestingly enough the reason why you know more about St Johnswort and its side effect is because it has been properly tested in a clinical setting (it is prescribed by doctors in Germany and other countries, including to children, although not for severe cases)