Serotonin is a neurotransmitter, a chemical used to carry messages between neurons. Although only about 1% of the body's serotonin is localized in the brain, serotonin has profound effects on brain function. (The remaining 99% helps carry messages elsewhere in the body, such as the spinal cord and muscles.)

Serotonin's effects are complex and incompletely understood. Too little serotonin may be a cause of depression, and drugs that increase brain levels of serotonin (the serotonin-specific reuptake inhibitors, or SSRIs) can alleviate depression -- but they may also cause side effects such as insomnia, anxiety and loss of libido.

Several diet drugs, including flenfluramine (withdrawn from the US market in 1977) and sibutramine (brand name Meridia) increase brain levels of serotonin, increasing feelings of satiety and decreasing the desire to eat.

Various psychedelic drugs, including LSD, resemble serotonin chemically, and may work by mimicking some of the effects of serotonin in the brain.