Prozac (Fluoxetine) Available Dosage Forms and Strengths
Prozac is available in capsules of 10 mg, 20 mg, and 40 mg, an oral solution containing 20 mg/5 mL, and a delayed-release weekly formulation (Prozac Weekly) containing 90 mg per capsule. 1
Standard Immediate-Release Formulations
Capsules (Pulvules): Available in three strengths—10 mg, 20 mg, and 40 mg—each containing fluoxetine hydrochloride with inactive ingredients including starch, gelatin, silicone, titanium dioxide, and iron oxide 1
Oral Solution: Contains fluoxetine hydrochloride equivalent to 20 mg/5 mL (4 mg/mL concentration), formulated with 0.23% alcohol, benzoic acid, flavoring agent, glycerin, purified water, and sucrose 1
Extended-Release Formulation
- Prozac Weekly (Delayed-Release Capsules): Contains 90 mg of fluoxetine hydrochloride in enteric-coated pellets designed for once-weekly administration 1, 2
- This formulation was developed based on fluoxetine's long elimination half-life of 1-3 days for the parent compound and 4-16 days for its active metabolite norfluoxetine 3
- The once-weekly 90 mg formulation demonstrated similar efficacy to daily 20 mg dosing in preventing relapse over 25 weeks, with improved compliance rates (87.5% vs 79.4%) 2
Clinical Dosing Considerations
Standard dosing range: 20-40 mg once daily appears optimal for most indications, though doses up to 80 mg/day have been used 4
Low-dose initiation strategy: Starting at 5 mg daily and gradually increasing to 20 mg over one week may benefit patients who are sensitive to side effects or have comorbid panic disorder, as 28% of patients in one study could not tolerate the full 20 mg dose 5
Bioavailability: Fluoxetine demonstrates approximately 72% oral bioavailability, with peak plasma concentrations (Tmax) occurring between 4-8 hours after administration 4
Important Pharmacokinetic Properties
Fluoxetine exhibits nonlinear pharmacokinetics with a long elimination half-life averaging 4 days after chronic administration, while norfluoxetine averages 7 days 4, 3
The drug is approximately 94% protein-bound with a large volume of distribution 4, 3
Therapeutic drug monitoring suggests a potential therapeutic window, with combined fluoxetine plus norfluoxetine concentrations above 500 mcg/L associated with poorer clinical response 3