Treatment with Belviq may cause serious side effects, including serotonin syndrome, particularly when taken with certain medicines that increase serotonin levels or activate serotonin receptors.

FORBES: FDA Approves A New Weight Loss Drug, Breaking a 13 Year Drought

"It isn't a huge surprise when you consider many positive effects exercise can have with regards to the brain chemistry: dopamine, serotonin, endorphin, epinephrine -- these are all associated with mood altering effects, " said Dr. Cedric Bryant, the chief science officer for the American Council on Exercise.

CNN: Former drug addicts find new fixation on triathlons

New drugs that work more directly to nourish neurons could be more effective than existing serotonin-based remedies, have fewer side effects or kick in slightly faster.

FORBES: Think Happy

These suggest that although serotonin may be central to Ecstasy's long-term effects, other neurotransmitters could be involved as well.

ECONOMIST: Recreational drugs