Starting Dose of Prozac (Fluoxetine)

The recommended starting dose of fluoxetine for adults with major depressive disorder is 20 mg once daily, administered in the morning. 1

Standard Adult Dosing for Depression

The FDA-approved initial dose is 20 mg/day taken in the morning, which is sufficient to obtain a satisfactory response in most cases of major depressive disorder 1
Clinical trials demonstrated that 20 mg/day is adequate for most patients, though doses ranging from 20-80 mg/day were studied 1
The maximum dose should not exceed 80 mg/day 1

For patients with anxiety concerns or heightened sensitivity, starting with 10 mg daily (or even 10 mg every other morning) is advisable, as fluoxetine is activating and may initially worsen anxiety 2
The American Academy of Family Physicians recommends 10 mg every other morning as an alternative starting strategy, particularly given fluoxetine's very long half-life (1-3 days for the parent compound, 4-16 days for the active metabolite norfluoxetine) 2
This lower starting approach allows assessment of tolerability before escalating to the standard 20 mg dose 2

For children and adolescents with depression, treatment should be initiated with 10 or 20 mg/day 1
After 1 week at 10 mg/day, the dose should be increased to 20 mg/day 1
In lower weight children, the starting and target dose may be 10 mg/day due to higher plasma levels in this population 1

Patients with hepatic impairment should receive a lower or less frequent starting dose 1
Elderly patients should be considered for lower starting doses 1
CYP2D6 poor metabolizers are at significantly higher risk of toxicity with 3.9 to 11.5-fold higher fluoxetine levels even at standard doses, and should start at 10 mg daily with cautious titration 3, 2
The FDA has issued safety warnings about QT prolongation risk in CYP2D6 poor metabolizers 3

Fluoxetine should be administered in the morning because it is activating and may cause insomnia if taken later in the day 2
Due to the extremely long half-life, side effects may not manifest for several weeks after initiation, and steady-state is not reached until approximately 5-7 weeks 2
This long half-life means dose adjustments should be made slowly, with intervals of 3-4 weeks between increases 2

If insufficient clinical improvement is observed after several weeks at 20 mg/day, a dose increase may be considered 1
Doses above 20 mg/day may be administered once daily in the morning or on a twice-daily schedule (morning and noon) 1
The full therapeutic effect may be delayed until 4 weeks of treatment or longer 1

Do not increase the dose too rapidly - fluoxetine's long half-life means drug accumulation continues for weeks, and premature dose escalation can lead to toxicity 2
Do not dismiss early anxiety or agitation - these are recognized initial adverse effects that may indicate the need for dose reduction back to a previously tolerated level 2
Do not overlook genetic factors - unexpected adverse effects at standard doses should prompt consideration of CYP2D6 testing 3, 2