Starting Dose of Prozac (Fluoxetine)
The starting dose of Prozac (fluoxetine) is 20 mg once daily, administered in the morning, for adults with major depressive disorder. 1
Adult Dosing
Initial dose: 20 mg/day in the morning is recommended as the starting dose for major depressive disorder, as controlled trials demonstrate this dose is sufficient to obtain a satisfactory response in most cases 1
The FDA-approved dosage range is 20-80 mg/day, though 20 mg/day is adequate for most patients 1
A dose increase may be considered after several weeks if insufficient clinical improvement is observed, but the full therapeutic effect may be delayed until 4 weeks of treatment or longer 1
Doses above 20 mg/day can be administered once daily (morning) or twice daily (morning and noon), with a maximum dose of 80 mg/day 1
Pediatric Dosing (Children and Adolescents)
Initial dose: 10 mg/day for lower weight children 1
Initial dose: 10-20 mg/day for adolescents and higher weight children 1
After 1 week at 10 mg/day, the dose should be increased to 20 mg/day 1
Due to higher plasma levels in lower weight children, the starting and target dose may remain at 10 mg/day, with consideration for increasing to 20 mg/day after several weeks if needed 1
Special Populations
Elderly patients: Use a lower or less frequent dosage due to altered pharmacokinetics and increased sensitivity to adverse effects 1
Hepatic impairment: Use a lower or less frequent dosage as fluoxetine metabolism is reduced 1
Renal impairment: Dosage adjustments are not routinely necessary 1
Obsessive-Compulsive Disorder
Adults: 20 mg/day in the morning as the initial dose 1
Adolescents and higher weight children: 10 mg/day initially, increased to 20 mg/day after 2 weeks 1
Lower weight children: 10 mg/day initially, with a recommended dose range of 20-30 mg/day 1
The full therapeutic effect may be delayed until 5 weeks of treatment or longer 1
Clinical Context from Guidelines
The 2015 American Family Physician guidelines confirm that fluoxetine dosing ranges from 20-80 mg per day for depression, though they note fluoxetine has a greater risk of agitation and overstimulation and should not be used in older adults 2
The 2002 guidelines for managing Alzheimer's disease recommend an initial dose of 10 mg every other morning for elderly patients with dementia and depression, with a maximum of 20 mg every morning, noting that fluoxetine is activating with a very long half-life 2
Important Considerations
Activating properties: Fluoxetine tends to be activating and may cause initial anxiety or agitation, which is why starting with the standard 20 mg dose (or lower in vulnerable populations) is appropriate 2, 1
Long half-life: Fluoxetine has an elimination half-life of 1-3 days after single dose and 4 days after long-term administration, with its active metabolite norfluoxetine having a half-life of 7 days 3
Common adverse effects: Nausea, insomnia, nervousness, and somnolence are common early in treatment but resolve in the majority of patients and become significantly less frequent with continued treatment 4
Drug interactions: Fluoxetine inhibits CYP2D6 and other cytochrome P450 enzymes, increasing potential for drug interactions 5