Prozac Starting Dose for Adults
The recommended starting dose of Prozac (fluoxetine) for adults with major depressive disorder is 20 mg once daily, administered in the morning. 1
Standard Adult Dosing
Initial dose: 20 mg/day taken in the morning is sufficient to obtain a satisfactory response in most cases of major depressive disorder. 1
Doses of 20,40, and 60 mg/day have been compared to placebo in controlled trials, confirming that 20 mg/day is adequate for initial treatment. 1
A dose increase may be considered after several weeks if insufficient clinical improvement is observed, but the initial starting point should remain 20 mg/day. 1
The maximum dose should not exceed 80 mg/day, and doses above 20 mg/day can be given once daily (morning) or twice daily (morning and noon). 1
Important Timing Considerations
Full therapeutic effect may be delayed until 4 weeks of treatment or longer, so patience is required before considering dose adjustments. 1
The optimal dosage range for most patients appears to be 20-40 mg once daily, with higher doses used only when clinically necessary. 2
Special Population Adjustments
Lower or less frequent dosing should be used in patients with hepatic impairment, as fluoxetine undergoes extensive hepatic metabolism. 1
Elderly patients should receive a lower or less frequent dosage due to altered pharmacokinetics and increased sensitivity to side effects. 1
Patients with concurrent diseases or on multiple concomitant medications should also be considered for lower starting doses. 1
Dosage adjustments for renal impairment are not routinely necessary. 1
Pharmacokinetic Rationale
Fluoxetine has a very long elimination half-life of 1-3 days after a single dose, averaging 4 days after long-term administration. 2
The active metabolite norfluoxetine has an even longer elimination half-life, averaging 7 days after long-term administration. 2
This extended half-life allows for once-daily dosing and essentially precludes withdrawal phenomena. 3
Common Pitfalls to Avoid
Do not start at doses higher than 20 mg/day in typical adult patients, as this increases the frequency of dose-related side effects without improving initial efficacy. 2, 3
Do not increase the dose before 4 weeks unless there are compelling clinical reasons, as the full antidepressant effect takes time to manifest. 1
Be aware that fluoxetine is a potent inhibitor of cytochrome P450 2D6, which increases potential for drug interactions, though most are not clinically significant. 3
The most common adverse effects at the recommended 20 mg/day dose include nausea, anxiety, insomnia, anorexia, diarrhea, nervousness, and headache—these are generally mild and dose-related. 2, 3