Fluoxetine Dosing Frequency
Fluoxetine (Prozac) is administered once daily in the morning for most patients, with the standard dose being 20 mg/day. 1
Standard Dosing Schedule
The FDA-approved dosing for fluoxetine is once daily (morning administration) at an initial dose of 20 mg/day for adults with major depressive disorder. 1
Doses above 20 mg/day may be given either once daily in the morning OR twice daily (morning and noon), but this is only for higher doses, not the standard starting regimen. 1
The long elimination half-life of fluoxetine (4-6 days) and its active metabolite norfluoxetine (4-16 days) permits single daily dosing for most indications. 2, 3
When Twice-Daily Dosing May Be Considered
For OCD specifically, the FDA label states that doses above 20 mg/day can be split into a BID schedule (morning and noon) if needed. 1
In pediatric populations, atomoxetine (not fluoxetine) may require twice-daily dosing to reduce adverse effects, but this does not apply to fluoxetine. 2
The guideline evidence confirms that most SSRIs, particularly fluoxetine, have sufficiently long half-lives to permit single daily dosing, though at low doses of sertraline or any dose of fluvoxamine, youths may require twice-daily dosing—this is NOT the case for fluoxetine. 2
Special Populations
For elderly patients with dementia and depression, American Family Physician guidelines recommend starting at 10 mg every other morning, with a maximum of 20 mg every morning—still once daily dosing. 4
For pediatric depression, start with 10-20 mg once daily in the morning, increasing to 20 mg/day after one week if starting at 10 mg. 2, 1
Clinical Pearls
Blood for therapeutic drug monitoring should be collected 12-16 hours after the last dose for most antidepressants, or 24 hours if given once daily, which is the standard for fluoxetine. 2
The activating properties of fluoxetine make morning dosing preferable to avoid insomnia. 4
Once-weekly formulations (90 mg) exist for maintenance therapy after initial daily dosing, but this is a specialized formulation, not standard practice. 1, 5