Fluoxetine vs Paroxetine

Both fluoxetine and paroxetine are effective for major depressive disorder. Large meta-analyses show comparable overall efficacy between SSRIs, though individual patient response varies.

Fluoxetine is FDA-approved for MDD, OCD, panic disorder, bulimia nervosa, and bipolar depression (in combination with olanzapine). It is also the only SSRI approved for MDD in children aged 8 and older. Paroxetine is approved for MDD, GAD, social anxiety disorder, panic disorder, OCD, and PTSD.

Fluoxetine has unique evidence for treatment-resistant depression when combined with olanzapine (Symbyax). Paroxetine has stronger evidence specifically for social anxiety disorder and PTSD. Both are effective for OCD, typically at higher doses than used for depression.

The side effect profiles of these SSRIs differ meaningfully:

Weight: Fluoxetine is considered weight-neutral or mildly weight-reducing, especially in the short term. Paroxetine is associated with significant weight gain — among the highest of all SSRIs.

Sedation vs. Activation: Fluoxetine is more activating and can cause insomnia, anxiety, and restlessness, especially early in treatment. Paroxetine is more sedating due to its anticholinergic and antihistaminic properties.

Discontinuation: Fluoxetine has a remarkably long half-life (parent: 1-3 days; active metabolite norfluoxetine: 4-16 days), which acts as a built-in taper and makes withdrawal symptoms rare. Paroxetine has the highest rate of discontinuation syndrome among SSRIs due to its short half-life and self-inhibiting metabolism.

Sexual dysfunction: Both cause sexual side effects, but paroxetine has among the highest rates. Fluoxetine falls in the middle range for SSRIs.

Pregnancy: Paroxetine is Category D (avoid in pregnancy). Fluoxetine is Category C and is one of the SSRIs with the most pregnancy safety data, though all SSRIs carry some risk.

Both are available as inexpensive generics. Fluoxetine has been generic since 2001, and paroxetine since 2003. A 30-day supply of either costs $4-$15 at most pharmacies.

Fluoxetine is also available in a weekly delayed-release capsule (Prozac Weekly, 90 mg) for patients who prefer once-weekly dosing, though this formulation may cost more.

Cost is not a meaningful differentiator between these medications.

Fluoxetine is typically taken once daily in the morning (due to its activating properties). Paroxetine can be taken morning or evening, with bedtime dosing preferred if sedation occurs. Both are available as tablets and oral solutions.

Fluoxetine's long half-life means a missed dose has less impact on blood levels, and the drug essentially self-tapers when discontinued. Paroxetine requires careful tapering and consistent daily dosing.

Fluoxetine's availability as a once-weekly formulation provides a unique convenience option not available with paroxetine.

Fluoxetine may be preferred for patients concerned about weight gain, those who may need to stop the medication (easier discontinuation), younger patients (FDA approval for pediatric depression), and patients with bipolar depression (combination with olanzapine).

Paroxetine may be preferred for specific anxiety disorders (social anxiety, PTSD) where it has the strongest evidence, for patients who benefit from a mildly sedating antidepressant, or for those who have responded well to it previously.

Paroxetine should be avoided in pregnancy and requires careful tapering when discontinuing. Both medications interact with CYP2D6 substrates. Consult your healthcare provider to determine which SSRI is right for you.