Prozac (Fluoxetine).
What It Does, Side Effects, and What to Expect.

Fluoxetine -- sold under the brand name Prozac -- was the first SSRI approved in the United States, and it remains widely used more than three decades later. It has a clinical profile that is genuinely distinct from other SSRIs, primarily because of its unusually long half-life. Understanding that difference helps explain both its advantages and its limitations.

What Is Prozac?

Prozac (fluoxetine) is a selective serotonin reuptake inhibitor (SSRI). Like other SSRIs, it works by blocking the reabsorption of serotonin in the brain, increasing the amount available between nerve cells. Its FDA-approved indications include:

• Major depressive disorder (MDD) -- including in children 8 and older
• Obsessive-compulsive disorder (OCD)
• Panic disorder
• Bulimia nervosa
• Bipolar depression (in combination with olanzapine, brand name Symbyax)
• Treatment-resistant depression (in combination with olanzapine)

It is also widely used off-label for generalized anxiety disorder, social anxiety, and PMDD.

The Long Half-Life: What It Means for You

The defining characteristic of fluoxetine is its half-life -- the time it takes for half the medication to leave the body. Most SSRIs have a half-life of 20 to 35 hours. Fluoxetine's half-life is 1 to 4 days, and its active metabolite (norfluoxetine) has a half-life of 4 to 16 days. This means fluoxetine can stay meaningfully present in your system for 4 to 6 weeks after you stop taking it.

This has practical implications in both directions:

Advantages of the long half-life:

• Minimal discontinuation syndrome. Because fluoxetine tapers itself slowly, stopping it abruptly causes far less of the "brain zaps," flu-like symptoms, and rebound anxiety that people experience when stopping shorter-acting antidepressants. This makes it the easiest SSRI to stop.
• Forgiving of missed doses. Missing a day or two has less impact because the medication is still active from previous doses.
• Used as a bridge medication. Some providers switch patients to a brief course of fluoxetine before stopping a harder-to-discontinue antidepressant (like venlafaxine), allowing the system to taper more gently.

Disadvantages of the long half-life:

• Slow to clear for medication switches. If fluoxetine is not working and you need to switch to another medication (especially an MAOI or certain other drugs), you need a washout period of 5 weeks -- much longer than with other SSRIs.
• Drug interactions. Fluoxetine inhibits certain liver enzymes (CYP2D6 and CYP2C19) that metabolize many other medications. This means it can raise blood levels of other drugs and requires more careful monitoring if you take other medications.

How Long Does It Take to Work?

The timeline for fluoxetine is similar to other SSRIs:

• Week 1 to 2: Possible early changes in sleep or energy. Some people feel more activating effects (increased energy, slight restlessness).
• Week 2 to 4: Gradual mood improvement begins. Anxiety may improve or, in some people, feel temporarily heightened as the activating effect kicks in.
• Week 4 to 8: Most people have a clear sense of whether fluoxetine is helping by this point.
• Week 8 to 12: Full effect.

Side Effects

Common side effects, especially in the first few weeks:

• Activation / agitation: Fluoxetine is more activating than most other SSRIs. For people with low energy and depression, this can be a benefit. For people with anxiety or insomnia, it can be disruptive early on. Starting at a lower dose (10 mg) reduces this.
• Insomnia: More common than with some other SSRIs, likely due to the activating effect. Taking it in the morning usually helps.
• Nausea: Common early on; taking with food helps.
• Headache
• Decreased appetite: Fluoxetine is more likely to suppress appetite than other SSRIs, particularly early in treatment. For most people this is mild; for some it is meaningful.

Sexual Side Effects

Like all SSRIs, fluoxetine can cause sexual side effects -- reduced libido, delayed orgasm, difficulty with arousal. Reported in 30 to 40 percent of users. Bring it up with your prescriber if it is a concern; options exist.

Weight

Fluoxetine is among the more weight-neutral SSRIs. Short-term studies often show slight weight loss or no change. Over longer periods, modest weight gain can occur, but less than with paroxetine or mirtazapine. See our full breakdown: antidepressants and weight gain.

What Dose Is Typical?

Fluoxetine is typically started at 10 to 20 mg daily. The therapeutic range is 20 to 80 mg. For OCD and bulimia, higher doses (60 to 80 mg) are often needed. For depression and anxiety, 20 to 40 mg is usually effective. Because of the long half-life, dose increases should be spaced further apart -- usually every 4 to 6 weeks -- to allow the medication to fully equilibrate.

Prozac vs. Other SSRIs

vs. Zoloft (sertraline): Zoloft has more FDA indications, especially for PTSD and PMDD. Zoloft is often better tolerated for anxiety without the activating effect. Prozac is easier to stop and more forgiving of missed doses.

vs. Lexapro (escitalopram): Lexapro is generally considered the cleanest, simplest SSRI in terms of drug interactions and side effect profile. Prozac's interaction potential (CYP2D6/2C19 inhibition) means more caution is needed with other medications. But Prozac is significantly easier to discontinue.

vs. Wellbutrin (bupropion): Wellbutrin has no sexual side effects, is weight-neutral or weight-negative, and is more activating. It is not effective for anxiety. Prozac and Wellbutrin are sometimes combined for depression when the individual medications alone are not enough.