Prozac is the brand name for fluoxetine, the first SSRI approved by the FDA (1987). It's FDA-approved for major depressive disorder, OCD, bulimia nervosa, and premenstrual dysphoric disorder, and is used off-label for panic disorder and generalized anxiety. Its very long half-life (4 to 6 days) is its defining feature: dose changes take longer to reach steady state, and discontinuation symptoms are milder.
What Prozac is
Prozac is the brand name for fluoxetine, a medication in the SSRI class. It was the first SSRI approved by the FDA and remains widely prescribed worldwide. Generic fluoxetine is inexpensive and clinically equivalent. The MedlinePlus drug information page for fluoxetine covers the consumer essentials.
Fluoxetine is FDA-approved for major depressive disorder, OCD, bulimia nervosa, and premenstrual dysphoric disorder. Off-label use for panic disorder and generalized anxiety is well-established. It's approved for adolescent depression and pediatric OCD, which gives it a larger evidence base in younger patients than most other SSRIs.
What it does
Fluoxetine blocks the serotonin transporter, raising serotonin in the synapse. Over weeks the brain's anxiety circuitry adapts. The mechanism is the standard SSRI mechanism described in the SSRI and CBT reference. PsychiatryRx has an animated way to see how serotonin reuptake inhibitors work.
What's different about fluoxetine is its pharmacokinetics. The parent drug has a half-life of about four to six days, and its active metabolite norfluoxetine has a half-life of one to two weeks. Steady-state concentrations take roughly four to five weeks to reach after a dose change. Conversely, when fluoxetine is stopped, drug levels fall slowly over weeks rather than days.
What it feels like
Patients describe the same general arc as other SSRIs: a gradual reduction in worry, rumination, and physical anxiety symptoms over weeks. Fluoxetine is somewhat more activating than escitalopram or sertraline. Some patients experience jitteriness, mild restlessness, or insomnia in the first weeks. These usually settle. For patients with prominent fatigue, the activating quality can be welcome; for patients with prominent insomnia or panic, the activation can be a poor fit.
Early side effects include nausea, headache, mild sleep disruption, and (in some patients) an initial uptick in anxiety. Sexual side effects are common across the SSRI class and may persist.
When it's chosen
Prozac is often chosen when:
- The patient is an adolescent. Fluoxetine has the largest evidence base in adolescents for both depression and OCD.
- OCD is the primary diagnosis. Effective OCD doses are higher than for depression, typically 60 to 80 mg daily.
- The patient is unreliable about taking medication every day. The long half-life forgives missed doses.
- The patient has had discontinuation symptoms with other SSRIs. The slow decay of fluoxetine makes stopping easier.
It's less often chosen as first-line for panic disorder or generalized anxiety because the activating quality can transiently worsen symptoms in the first weeks. For these conditions, sertraline or escitalopram is typically preferred.
Typical dosing and timeline
Starting dose for depression or anxiety is 10 to 20 mg daily, taken in the morning to reduce the chance of insomnia. The typical effective dose for depression and anxiety is 20 to 40 mg. The OCD dose is often 40 to 80 mg.
Because of the long half-life, dose changes are usually spaced four to six weeks apart so the previous dose has reached steady state before the next adjustment.
Side effects and cautions
Activation (jitteriness, restlessness, insomnia) is more common than with sertraline or escitalopram, particularly early in treatment. Sexual side effects are common. QT-interval effects exist but are typically clinically insignificant at standard doses.
The FDA boxed warning on increased suicidal thoughts in patients under 25 applies; the NIMH mental health medications page discusses the framing. Close monitoring early in treatment is the standard response.
Fluoxetine inhibits the CYP2D6 enzyme, which can raise levels of certain other medications including some beta-blockers, certain antipsychotics, and some opioids. In patients on multiple medications, particularly older adults, this is worth checking before starting.
What Prozac doesn't do
Prozac doesn't produce an immediate calming effect. Its long half-life makes the first few weeks especially flat-feeling for patients who expect rapid change. As with all SSRIs, the therapeutic effect comes from weeks of receptor adaptation, not from individual doses.
Prozac isn't a controlled substance and doesn't produce addiction. The slow drug-elimination curve means abrupt discontinuation rarely produces dramatic symptoms, but a supervised taper is still recommended.
Comparison with related medications
Compared with Lexapro, fluoxetine is more activating, has a much longer half-life, and is the preferred SSRI in adolescents. See the Lexapro vs Prozac comparison. Compared with sertraline, fluoxetine has a stronger pediatric evidence base but a less convenient day-to-day dosing profile. Compared with SNRIs, fluoxetine has no norepinephrine effect, which is sometimes the deciding factor when fatigue is prominent.