What is the typical starting dose of Prozac (fluoxetine) for a patient?

Starting Dose of Prozac (Fluoxetine)

Start fluoxetine at 20 mg once daily in the morning for most adult patients with depression. 1

Standard Adult Dosing for Depression

• The FDA-approved starting dose is 20 mg/day administered in the morning, which is sufficient to obtain a satisfactory response in major depressive disorder in most cases. 1
• This 20 mg/day dose has been shown to maintain efficacy for up to 38 weeks in controlled trials. 1
• The maximum dose should not exceed 80 mg/day. 1

Alternative Lower Starting Dose Strategy

Consider starting at 10 mg daily (or even 10 mg every other morning) in patients prone to anxiety or activation side effects. 2

• The American Academy of Family Physicians recommends 10 mg every other morning or 10 mg daily as an alternative starting approach, particularly when tolerability is a concern. 2
• This lower-dose strategy is especially useful because 28% of patients cannot tolerate the full 20 mg dose, with half of these patients benefiting from lower doses. 3
• Patients with concurrent panic disorder are particularly intolerant of the standard 20 mg dose and may require starting at 5-10 mg daily. 3

Pediatric Dosing (Children and Adolescents)

• Start with 10 mg/day in adolescents and higher-weight children, then increase to 20 mg/day after 1-2 weeks. 1
• For lower-weight children, initiate at 10 mg/day and consider this as both the starting and potentially the target dose due to higher plasma levels in this population. 1

Special Population Adjustments

Reduce the starting dose by approximately 50% in elderly patients. 2

• The American Academy of Family Physicians recommends lower or less frequent dosing in elderly patients due to significantly greater risk of adverse drug reactions. 2
• Patients with hepatic impairment require lower or less frequent dosing. 1

CYP2D6 poor metabolizers should start at 10 mg daily with cautious titration due to 3.9 to 11.5-fold higher fluoxetine levels and significantly increased toxicity risk, including FDA warnings about QT prolongation. 2

Condition-Specific Dosing Variations

Obsessive-Compulsive Disorder (OCD)

• Start at 20 mg/day in adults, though doses of 60 mg/day are typically needed for full therapeutic effect. 1
• Higher doses are generally necessary for OCD compared to depression. 2

Bulimia Nervosa

• The target dose is 60 mg/day, though it may be advisable to titrate up to this dose over several days rather than starting at this level. 1

Critical Timing and Pharmacokinetic Considerations

• Fluoxetine should be taken in the morning because it is activating and may cause insomnia if taken later in the day. 2
• The full therapeutic effect may be delayed until 4-5 weeks of treatment or longer due to fluoxetine's exceptionally long half-life (1-3 days for parent compound, 4-16 days for active metabolite norfluoxetine). 2, 1
• Side effects may not manifest for several weeks after starting treatment, as steady-state conditions are reached only after approximately 5-7 weeks. 2

Common Pitfalls to Avoid

• Do not increase the dose at 1-2 week intervals; wait 3-4 weeks between dose adjustments due to the long half-life. 2
• Common early adverse events (nausea, insomnia, nervousness, somnolence) resolve in the majority of patients and become significantly less frequent with continued treatment. 4
• If unexpected adverse effects develop at standard doses, consider CYP2D6 testing before escalating doses further. 2