For 5-HTP and the compounds in marijuana to work in the body, they are metabolized in the same pathways. This can thus amplify the effects of each to an point that it can no longer be controlled, resulting in dangerous and unwanted effects. Serotonin syndrome may also occur when you mix both 5-HTP and marijuana. Serotonin syndrome is characterized by the production of too much serotonin in the body, resulting in aggression, seizures, increased heart rate, difficulty breathing, and psychosis.