Serotonin levels can dictate if you feel depressed or not. Antidepressant medications like Paxil, Zoloft, St. John's Wort, and Prozac work by preventing serotonin destruction and loss. These antidepressants inhibit serotonin uptake ( or reuptake) by the neurons in the brains. Low serotonin levels cause depression.
Tryptophan is the dietary amino acid precursor for production of serotonin in the body. First it is converted into 5HTP, and then into serotonin. Both supplements are available, but tryptophan may require a doctor's prescription. It is recommended that these be taken under a doctor's supervision.