Starting Dose of Prozac (Fluoxetine)

The recommended starting dose of fluoxetine for adults with depression 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, as controlled trials demonstrate this dose is sufficient to obtain a satisfactory response in major depressive disorder in most cases 1
Morning administration is preferred because fluoxetine is activating and may cause insomnia if taken later in the day 2
The full therapeutic effect may be delayed until 4 weeks of treatment or longer 1

Start with 10 mg every other morning in elderly patients, particularly those with dementia and depression, with a maximum of 20 mg every morning 3
A lower or less frequent dosage should be used in elderly patients due to altered pharmacokinetics 1

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

Consider starting at 10 mg daily (or even 10 mg every other morning) in patients with underlying anxiety disorders or panic disorder, as these patients are more likely to experience initial activation, agitation, and increased anxiety 2, 3
Research demonstrates that 28% of patients cannot tolerate the full 20 mg dose, with patients who have concurrent panic disorder being particularly intolerant 4
A "test dose" strategy starting at lower doses (5-10 mg) with gradual titration to 20 mg over 1 week can improve tolerability 4

For pediatric patients with depression, start with 10 mg/day, then increase to 20 mg/day after 1 week 1
In lower weight children, the starting and target dose may remain at 10 mg/day due to higher plasma levels 1

Fluoxetine has an exceptionally long half-life: 1-3 days for fluoxetine itself and 4-16 days for its active metabolite norfluoxetine 2, 5
Side effects may not manifest for several weeks due to this long half-life, so dose adjustments should not be made hastily 2
CYP2D6 poor metabolizers have 3.9 to 11.5-fold higher fluoxetine levels and are at significantly higher risk of toxicity even at standard doses 6
The FDA has issued safety warnings that fluoxetine should be used with caution in CYP2D6 poor metabolizers due to risk of QT prolongation and elevated drug levels 6

If 20 mg is insufficient after several weeks, doses may be increased, but should not exceed 80 mg/day 1
Doses above 20 mg/day may be administered once daily in the morning or divided (morning and noon) 1
When titrating in patients with anxiety concerns, increase at 3-4 week intervals (not 1-2 weeks) using the smallest available increments of 5-10 mg due to the long half-life 2

Do not start at 20 mg in patients with prominent anxiety symptoms or panic disorder—these patients frequently cannot tolerate this dose and may discontinue treatment 4
Do not make dose adjustments too quickly—the long half-life means steady state is not reached for weeks 2
Be aware that higher doses are associated with more adverse effects without clear evidence of superior efficacy 2
Monitor for activation symptoms (insomnia, anxiety, agitation) particularly in the first few weeks, as these are the most common reasons for discontinuation 4, 7