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SSRI (Selective Serotonin Reuptake Inhibitor) Pregnancy: Use if benefit outweighs risk. Long half-life means neonatal exposure after stopping.

Fluoxetine

Brand names: Prozac

Adult dose

Dose: 20 mg once daily; increase to 40–60 mg if needed
Route: Oral
Frequency: Once daily (morning — activating effect)
Max: 60 mg/day (80 mg/day for bulimia)
Depression: 20 mg OD, increase to 40–60 mg after 4 weeks if poor response. Bulimia nervosa: 60 mg/day. OCD: 20 mg initially, increase to 40–60 mg/day. Due to long half-life (1–4 days + active metabolite), can be given alternate days in elderly. Fewer discontinuation effects than other SSRIs.

Paediatric dose

Route: Oral
Frequency: Once daily
Max: 20 mg/day (child); 40 mg/day (adolescent)
Licensed for moderate-severe depression and OCD (8–17 years): 10 mg OD initially (8–17 years); after 1–2 weeks, increase to 20 mg OD. Adolescents with greater body weight: may need 20 mg/day initially. Max 20 mg/day (children); 40 mg/day (adolescents).

Dose adjustments

Renal

No dose adjustment required for mild-moderate impairment; consider dose reduction in severe renal failure.

Hepatic

Reduce dose or use alternate-day dosing in severe hepatic impairment.

Clinical pearls

  • Longest half-life SSRI — fewer discontinuation effects; once-weekly dosing possible in stable patients
  • Avoid in breast cancer patients on tamoxifen (CYP2D6 inhibition reduces active tamoxifen metabolite)
  • Preferred SSRI for eating disorders (particularly bulimia nervosa)
  • Licensed for depression and OCD in children in UK (only SSRI with UK paediatric licence)
  • 5-week washout before starting MAOI (long half-life)

Contraindications

  • MAO inhibitors within 14 days
  • Pimozide or thioridazine
  • Linezolid or IV methylene blue

Side effects

  • Nausea and GI upset (usually transient)
  • Insomnia (activating — take in morning)
  • Anxiety and agitation
  • Sexual dysfunction
  • Weight loss initially then possible weight gain
  • Hyponatraemia (SIADH)
  • Suicidal ideation (young patients)

Interactions

  • MAOIs — serotonin syndrome; 5-week washout required after fluoxetine before starting MAOI
  • Tamoxifen — significantly reduced efficacy (potent CYP2D6 inhibitor — use sertraline instead)
  • Tricyclic antidepressants — increased TCA levels
  • Warfarin — increased bleeding risk
  • Atomoxetine — increased levels (risk of toxicity)

Monitoring

  • Mood and suicidality
  • Weight
  • Serum sodium (elderly)
  • Drug interactions (CYP2D6 inhibitor)

Reference: BNFc; BNF; NICE CG90; BNFc. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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