Serotonin is a neurotransmitter that plays a major role in the regulation of mood. In higher levels it is associated with feelings of well-being and contentment. Serotonin has other functions, including modulating synaptic strength, appetite, sleep, sexual desire, and digestion. Antidepression drugs such as selective serotonin reuptake inhibitors (which tend to increase serotonin levels available to receptors) tend to have far-reaching effects, not all of them desirable (such as suppressing libido).
