Which is probably a clear case of the placebo effect for the premise for its use is faulty.
Despite the popular myth, anxiety disorders and depression are not caused by having low brain serotonin levels, nor do SSRIs increase serotonin, except in the first few weeks which is what produces most of the side-effects, including heightened anxiety. Levels soon drop back to baseline and in areas of the brain associated with anxiety and depression to well below baseline. See my:
Serotonin - The 'chemical imbalance' myth
The second problem is the brain is not the only serotonergic organ. The
enteric nervous system (ENS), the mini brain which controls the gut, makes and uses about 50 times as much as the brain and given orally taken 5-HTP enters the gut first the ENS will scavenge almost all of it for its own use. In research 5-HTP is usually injected subcutaneously (under the skin), or directly into the brains of research animals. Even then most of it is still converted to serotonin in the gut
as explained by the discoverer of the serotonin-ENS connection, Michael D. Gershon M.D.
Also be aware that 5-HTP may contain Peak-X contaminates. Peak-X is though to trigger the immune system disorder
Eosinophilia-myalgia syndrome (EMS). Peak-X contaminated L-Tryptophan caused the deaths of 37 people in the late 1980s and permanently damaged the health of another 1,500+. See:
Notes on the Tryptophan Disaster.