Fluoxetine

Fluoxetine works by stopping nerve cells in the brain from reabsorbing serotonin, a chemical messenger involved in mood, anxiety, and appetite, so more of it stays active between brain cells. Unlike older antidepressants, it's fairly selective for serotonin and doesn't strongly affect other brain chemicals, which is part of why it tends to have a more tolerable side-effect profile. It usually takes several weeks of consistent use before people notice the full mood-lifting effect, because the brain needs time to adjust to the higher serotonin activity. The body also breaks fluoxetine down into an active byproduct that lingers for days to weeks, which is why effects fade out more slowly than with many other antidepressants.