Starting Dose of Prozac (Fluoxetine)
The recommended starting dose of fluoxetine is 20 mg once daily in the morning for adults with depression, though 10 mg daily (or even 10 mg every other morning) is preferred when anxiety is a concern or in lower-weight patients. 1, 2
Standard Adult Dosing
For most adults with major depressive disorder, start with 20 mg once daily in the morning, as FDA-approved labeling indicates this dose is sufficient to obtain a satisfactory response in most cases 1
The American Academy of Family Physicians recommends a more conservative approach of 10 mg every other morning or 10 mg daily for some patients, particularly given fluoxetine's very long half-life (1-3 days for fluoxetine, 4-16 days for norfluoxetine) 2
Morning dosing is strongly preferred because fluoxetine is activating and may cause insomnia if taken later in the day 2
Pediatric Dosing (Children and Adolescents)
Start with 10 mg daily for one week, then increase to 20 mg daily 1
For lower-weight children, the starting and target dose may remain at 10 mg daily due to higher plasma levels in this population 1
Critical Dosing Considerations for Anxiety-Prone Patients
When anxiety or panic disorder is present, use a subtherapeutic "test dose" strategy starting at 5-10 mg daily, as these patients are particularly vulnerable to initial activation and agitation from SSRIs 2, 3
Research demonstrates that 28% of depressed patients cannot tolerate the full 20 mg dose, with half of these patients discontinuing entirely—particularly those with comorbid panic disorder 3
Patients with anxiety should have dose increases at 3-4 week intervals using 5-10 mg increments, not the typical 1-2 week intervals, due to fluoxetine's exceptionally long half-life 2
Special Populations Requiring Lower Starting Doses
CYP2D6 poor metabolizers should start at 10 mg daily with cautious titration, as they have 3.9 to 11.5-fold higher fluoxetine levels and significantly increased toxicity risk even at standard doses 2
Elderly patients, those with hepatic impairment, and patients on multiple medications should receive lower or less frequent dosing 1
Renal impairment does not routinely require dosage adjustments 1
Common Pitfalls to Avoid
Do not expect immediate side effect manifestation—due to the very long half-life, adverse effects may not appear for several weeks after starting or changing doses, and steady-state is only reached after 5-7 weeks 2
Do not escalate doses too quickly in anxious patients—increased anxiety and agitation are recognized initial adverse effects that worsen with rapid dose escalation 2
The most common early adverse events (nausea, insomnia, nervousness, somnolence) occur in >5% of patients but significantly decrease in frequency over the first 4 weeks of treatment 4
Dose Escalation Strategy
If insufficient clinical improvement after several weeks at 20 mg, doses may be increased up to a maximum of 80 mg/day, administered once daily in the morning or split between morning and noon 1
Full antidepressant effect may be delayed until 4 weeks of treatment or longer 1
Higher doses above 20 mg are associated with more adverse effects without clear evidence of superior efficacy 2