What is the starting dose of Prozac (fluoxetine) for adults?

Starting Dose of Fluoxetine (Prozac) for Adults

The recommended starting dose of fluoxetine (Prozac) for adults is 20 mg once daily in the morning. 1

Dosing Guidelines

• The FDA-approved starting dose for adults with major depressive disorder is 20 mg once daily, typically administered in the morning 1
• Clinical trials supporting fluoxetine's efficacy used morning doses ranging from 20 to 80 mg/day, with 20 mg/day being sufficient to obtain a satisfactory response in most cases 1
• The therapeutic effect may not be fully apparent until 4 weeks of treatment or longer 1

Dose Adjustments

• After several weeks of treatment, if insufficient clinical improvement is observed, a dose increase may be considered 1
• Doses above 20 mg/day can be administered either once daily (morning) or twice daily (morning and noon) 1
• The maximum recommended dose should not exceed 80 mg/day 1

Special Populations

• Lower or less frequent dosing should be considered for:
  • Elderly patients 1, 2
  • Patients with hepatic impairment 1
  • Patients with concurrent disease or on multiple medications 1
• Dosage adjustments for renal impairment are not routinely necessary 1

Alternative Dosing Strategies

• Some clinicians have explored starting at lower doses (5-10 mg/day) and gradually increasing to 20 mg/day over a 1-week period, particularly for patients who might be sensitive to side effects 3
• This approach may be beneficial for patients with comorbid panic disorder or anxiety 3

Maintenance Treatment

• For maintenance treatment of major depressive disorder, the same dose that induced remission (typically 20 mg/day) is generally continued 1
• Efficacy has been demonstrated for periods up to 38 weeks following 12 weeks of acute treatment at a dose of 20 mg/day 1

Common Side Effects

• Most common side effects include nausea, anxiety, insomnia, anorexia, diarrhea, nervousness, and headache 4
• Side effects are typically dose-related and tend to decrease in frequency over time with continued treatment 5
• Common adverse events that occur early in treatment (first 4 weeks) significantly decrease in frequency with continued treatment over a 6-month period 5

Clinical Pearls

• If a patient relapses while on 20 mg/day during long-term treatment, increasing the dose to 40 mg/day has been shown to be an effective strategy, with about 61% of patients maintaining their response during follow-up 6
• Fluoxetine has a long half-life (1-3 days after single dose, averaging 4 days after long-term administration), which can be advantageous for patients who occasionally miss doses 4

Remember that while 20 mg is the standard starting dose for most adults, clinical response and tolerability should guide subsequent dosing decisions.