Fluoxetine (brand name Prozac) is a selective serotonin reuptake inhibitor (SSRI) commonly used to treat major depressive disorder, anxiety disorders, and obsessive-compulsive disorder. It works by increasing serotonin levels in the brain, which helps regulate mood and emotional balance. Fluoxetine has a long half-life, which means it stays in the body longer and has a lower risk of withdrawal symptoms compared to other SSRIs. It is often preferred for patients who may struggle with medication adherence or need a more gradual taper. Common side effects include insomnia, nervousness, nausea, and sexual dysfunction. Fluoxetine is generally effective and well-tolerated, and may be especially helpful in patients with both depression and fatigue or low energy.