What is the recommended starting dose of Prozac (fluoxetine) for adults and adolescents with depression or anxiety disorders?

Prozac (Fluoxetine) Starting Dose

For adults with depression or anxiety disorders, start fluoxetine at 20 mg once daily in the morning, as this is the FDA-approved initial dose that is sufficient to obtain a satisfactory response in most cases. 1

Standard Adult Dosing

• The FDA label recommends 20 mg/day administered in the morning as the initial dose for major depressive disorder in adults. 1
• Studies comparing 20,40, and 60 mg/day to placebo demonstrate that 20 mg/day is sufficient for most patients with major depression. 1
• For obsessive-compulsive disorder, the same 20 mg/day starting dose is recommended, though doses up to 60 mg/day may ultimately be needed. 1

Lower Starting Doses for Anxiety-Prone Patients

However, for patients with panic disorder or significant anxiety symptoms, starting at 10 mg/day (or even 5 mg/day) is strongly advisable to minimize initial agitation and improve tolerability. 2, 3

• Fluoxetine carries a greater risk of agitation and overstimulation compared to other SSRIs, particularly concerning in anxiety-prone patients. 2
• In a study of 133 depressed outpatients, 28% could not tolerate the full 20 mg dose, with patients having comorbid panic disorder being most likely to discontinue due to intolerance. 3
• Starting at 5-10 mg/day and gradually increasing to 20 mg over 1-2 weeks allows identification of patients who benefit from lower doses while minimizing dropout. 3

Pediatric and Adolescent Dosing

For children and adolescents (ages 7-17), start with 10 mg/day, increasing to 20 mg/day after 1-2 weeks. 1, 2

• Lower-weight children should remain at 10 mg/day initially, with a target range of 20-30 mg/day. 2
• Higher-weight adolescents can follow the adult titration schedule of 10 mg/day for 2 weeks, then 20 mg/day. 2, 1
• Close monitoring for suicidal ideation is essential, especially in the first months and after dose changes. 2

Special Populations Requiring Lower Doses

Elderly patients, those with hepatic impairment, or those on multiple medications should start at 10 mg daily or 10 mg every other day with slower titration. 2, 1

• Older adults are at significantly greater risk of adverse drug reactions and require approximately 50% of the standard adult starting dose. 4
• Dose reduction is required in hepatic impairment but not in renal disease. 1
• CYP2D6 poor metabolizers have 3.9 to 11.5-fold higher fluoxetine levels and should start at 10 mg daily with cautious titration. 4

Critical Timing Considerations

• Do not expect full therapeutic effect before 4-6 weeks of treatment, with maximal benefit requiring 12 weeks or longer. 1, 2
• Fluoxetine's exceptionally long half-life (1-3 days for parent compound, 4-16 days for active metabolite norfluoxetine) means steady-state is not reached for 5-7 weeks. 4
• Wait at least 4-6 weeks at each dose before escalating, as side effects may not manifest for several weeks after dose changes. 2, 4

Common Pitfalls to Avoid

• Do not start at 20 mg in patients with panic disorder or prominent anxiety symptoms—this increases risk of early dropout due to agitation. 3, 2
• Do not increase doses too rapidly—fluoxetine's long half-life means plasma levels continue rising for weeks after each dose change. 4
• Do not assume lack of response before 6-12 weeks at therapeutic dose—premature dose escalation increases side effects without allowing adequate time for response. 2
• Consider that other SSRIs (sertraline, escitalopram, citalopram) may be preferred first-line agents for anxiety disorders due to more favorable adverse effect profiles and less risk of initial agitation. 2