Serotonin is an evolutionarily ancient neurotransmitter system and is found in animals as primitive as sea slugs. In humans, not surprisingly, it is involved with a very wide array of functions, ranging from the simplest to the most advanced. For example, serotonin is involved with hunger, sleep, migraine headaches, and sexual function. It is also involved with mood, anxiety, and anticipation of harm. People with low levels of serotonin demonstrate impulse control problems, while people with high levels of serotonin manifest excessive levels of caution and inhibition. The serotonin tracts originate in the raphe nuclei in the brainstem and project widely throughout the cerebral cortex. Certain serotonergic tracts also project downwards to the spinal cord.

Serotonin is targeted by the selective serotonin reuptake inhibitors (SSRIs), the widely used class of antidepressants. Examples of SSRIs include fluoxetine (Prozac), sertraline (Zoloft), and paroxetine (Paxil). SSRIs are also effective in the treatment of anxiety disorders and obsessive compulsive disorder.