Starting Dose of Fluoxetine (Prozac) for Adults

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

Begin with 20 mg/day taken in the morning as this dose is sufficient to obtain a satisfactory response in most cases of major depressive disorder 1
The FDA-approved dose range extends from 20-80 mg/day, though 20 mg is the established initial dose 1
If insufficient clinical improvement occurs after several weeks, consider increasing the dose, with administration either once daily (morning) or twice daily (morning and noon) 1
Maximum dose should not exceed 80 mg/day 1

Full therapeutic effect may be delayed until 4 weeks of treatment or longer, so avoid premature dose escalation 1
The long half-life of fluoxetine and its active metabolite norfluoxetine means steady-state plasma levels are not reached for nearly 4 weeks 2

Use a lower or less frequent dosage in elderly patients 1
For elderly patients with dementia and depression, start with 10 mg every other morning, with a maximum of 20 mg every morning 3
Fluoxetine should generally not be used in older adults due to greater risk of agitation and overstimulation 3

A lower or less frequent dosage should be used in patients with hepatic impairment 1

Consider lower starting doses for patients with concurrent disease or on multiple concomitant medications 1

Consider starting at 5 mg/day and gradually increasing to 20 mg over 1 week in patients with concurrent panic disorder 4
Research shows 28% of patients cannot tolerate the full 20 mg dose, with those having panic disorder being particularly sensitive 4
Half of these patients benefit from lower maintenance doses (5-15 mg/day) 4

Fluoxetine is activating and may cause initial anxiety, agitation, or insomnia, which is why some patients cannot tolerate the standard 20 mg starting dose 3, 5
The most frequently occurring adverse event is insomnia 2
Common adverse events at the 20 mg dose are referable to the gastrointestinal and nervous systems 5
When higher doses are used, adverse events become more common 5

Fluoxetine is an inhibitor of CYP2D6 and other cytochrome P450 enzymes, increasing potential for drug interactions 5
The FDA has issued safety warnings about using fluoxetine with caution in patients with conditions predisposing to QT prolongation, including CYP2D6 poor metabolizers 6

In children and adolescents, treatment should be initiated with 10 or 20 mg/day 1
After 1 week at 10 mg/day, increase to 20 mg/day 1
Lower weight children may require a starting and target dose of only 10 mg/day due to higher plasma levels 1