5-HTP acts as a direct precursor for the neurotransmitter serotonin. 5-HTP readily crosses the blood-brain barrier, which can lead to increased serotonin levels in presynaptic neurons in the brain. This may play a role in regulating mood, sleep, appetite, memory, and various other processes.[1]
However, it’s also important to note that the relationship between serotonin levels and depression is not as clear as many people think. The current body of evidence does not consistently show that lower levels of serotonin are associated with or cause depression.[2]