Prozac (Fluoxetine) Dosing for an 18-Year-Old
For an 18-year-old with major depressive disorder, start fluoxetine at 10-20 mg once daily in the morning, with a target dose of 20 mg/day, which is sufficient for most patients. 1
Initial Dosing Strategy
- Starting dose: 10-20 mg once daily in the morning 1
- The FDA label specifies that 20 mg/day is recommended as the initial dose for adults, administered in the morning 1
- For adolescents transitioning to adult dosing (which applies to an 18-year-old), treatment can be initiated with 10 or 20 mg/day 1
- After 1 week at 10 mg/day, increase to 20 mg/day if starting at the lower dose 1
Target and Maximum Dosing
- Target dose: 20 mg/day is sufficient to obtain satisfactory response in most cases 1
- Studies comparing 20,40, and 60 mg/day to placebo demonstrate that 20 mg/day achieves adequate response in major depressive disorder for the majority of patients 1
- Maximum dose: 80 mg/day 1
- Doses above 20 mg/day may be administered once daily (morning) or twice daily (morning and noon) 1
Dose Escalation Guidelines
- Consider dose increase after several weeks if insufficient clinical improvement is observed 1
- The full therapeutic effect may be delayed until 4 weeks of treatment or longer 1
- If relapse occurs on 20 mg/day during maintenance treatment, increasing to 40 mg/day is effective in approximately 67% of patients 2
Special Considerations for Young Adults
- Start with subtherapeutic "test" dose if anxiety or agitation is a concern, as these can be initial adverse effects of SSRIs 3
- Increase dose slowly in the smallest available increments at approximately 3-4 week intervals due to fluoxetine's long half-life 3
- The elimination half-life averages 4 days after long-term administration, with the active metabolite norfluoxetine having a 7-day half-life 4
Monitoring Requirements
- Monitor for adverse events including suicidal ideation, behavioral activation, or switch to mania, particularly in the first 24-48 hours after dosage changes 3
- Common side effects include nausea, anxiety, insomnia, anorexia, nervousness, and headache 4
- Fluoxetine is activating with a very long half-life; side effects may not manifest for a few weeks 3
Duration of Treatment
- Continue treatment for 4-12 months after first episode of major depressive disorder 3
- Systematic evaluation shows efficacy is maintained for up to 38 weeks following acute treatment at 20 mg/day 1
- Discontinue slowly by tapering to avoid withdrawal symptoms, though fluoxetine's long half-life makes discontinuation syndrome less likely than with shorter-acting SSRIs 3
Critical Safety Warnings
- Contraindicated with MAOIs: Allow at least 14 days after stopping an MAOI before starting fluoxetine, and at least 5 weeks after stopping fluoxetine before starting an MAOI 1
- Exercise caution when combining with other serotonergic drugs due to risk of serotonin syndrome 3
- Fluoxetine may interact with drugs metabolized by CYP2D6 3