5-HTP is very reliable for increasing serotonin levels in presynaptic neurons in the brain and is actually used as a test drug to help determine the potency of other serotonin-increasing substances.[1]
Most of the research has looked at 5-HTP for its potential role in treating depression. This is based on the serotonin theory of depression — the debatable theory that depression is the result of low serotonin in the brain.[2] While some research has been promising, the body of evidence is weak overall. Most of this research was conducted in the 1970s and 1980s with poor methodological quality.[3][4]
5-HTP may reduce symptoms of depression compared to a placebo when taken alone or in addition to standard treatment.[3] Also, two short-term studies (6 to 8 weeks) reported that 5-HTP reduced symptoms of depression to a similar degree as the SSRI antidepressant medications fluoxetine and fluvoxamine.[5][6] While this is promising, higher quality research is needed to confirm these effects.
5-HTP may also play a role in appetite regulation. In people with overweight or obesity, researchers have observed weight loss due to reduced calorie intake and increased satiety, or the feeling of fullness, during supplementation with 5-HTP.[7][8][9]
5-HTP has also been studied for its effects on migraine or tension headache prevention,[10][11][12] fibromyalgia,[13][14] and Parkinson’s disease.[15][16] Currently, research is mixed and largely inconclusive.