SSRI (Selective Serotonin Reuptake Inhibitor) Pregnancy: Pregnancy Category C (US) - use during pregnancy only if the potential benefit justifies the potential risk to the fetus; neonates exposed late in the third trimester have developed complications.

Fluoxetine

Brand names: Prozac

Used in: Depression & Anxiety

Fluoxetine is a selective serotonin reuptake inhibitor (SSRI) used for depression, obsessive-compulsive disorder and bulimia. It has a long half-life.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: 20 mg/day

Route: Oral

Frequency: Once daily in the morning

Max: 80 mg/day

US labelling (no UK SPC posology in bundle — verify against UK SPC). Major Depressive Disorder: initiate 20 mg/day orally in the morning; consider a dose increase after several weeks if insufficient improvement; doses above 20 mg/day given once daily (morning) or twice daily (morning and noon); maximum 80 mg/day. OCD: initiate 20 mg/day; recommended range 20-60 mg/day (up to 80 mg/day tolerated); maximum 80 mg/day. Bulimia nervosa: 60 mg/day in the morning. Panic disorder: initiate 10 mg/day, increase to 20 mg/day after one week. Depressive episodes in Bipolar I disorder: only in combination with olanzapine (5 mg olanzapine + 20 mg fluoxetine once daily initial); fluoxetine monotherapy is not indicated for this. Use a lower or less frequent dosage in hepatic impairment, the elderly, and patients with concurrent disease or on multiple concomitant medications. Full effect in MDD may be delayed 4 weeks or longer. Paediatric (children/adolescents): MDD initiate 10 or 20 mg/day (10 mg/day may be the target in lower-weight children); OCD initiate 10 mg/day (range 20-60 mg/day in adolescents/higher-weight children, 20-30 mg/day in lower-weight children); no per-kg regimen stated — verify against a children's formulary.

Dose auto-extracted from US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

US labelling (FDA)

Reference — US labelling, may differ from UK

Indication Adult Pediatric MDD ( 2.1 ) 20 mg/day in am (initial dose) 10 to 20 mg/day (initial dose) OCD ( 2.2 ) 20 mg/day in am (initial dose) 10 mg/day (initial dose) Bulimia Nervosa ( 2.3 ) 60 mg/day in am Panic Disorder ( 2.4 ) 10 mg/day (initial dose) Depressive Episodes Associated with Bipolar I Disorder ( 2.5 ) Oral in combination with olanzapine: 5 mg of oral olanzapine and 20 mg of fluoxetine once daily (initial dose) Oral in combination with olanzapine: 2.5 mg of oral olanzapine and 20 mg of fluoxetine once daily (initial dose) A lower or less frequent dosage should be used in patients with hepatic impairment, the elderly, and for patients with concurrent disease or on multiple …

Source: US FDA prescribing information (openFDA / DailyMed), label dated 2019-02-01. Accessed 2026-06-12. US dosing and indications can differ from UK practice — use UK sources for prescribing decisions.

Contraindications

Do not use MAOIs intended to treat psychiatric disorders with fluoxetine or within 5 weeks of stopping fluoxetine; do not use fluoxetine within 14 days of stopping such an MAOI (serotonin syndrome risk)
Do not start fluoxetine in a patient being treated with linezolid or intravenous methylene blue
Pimozide: do not use (QT prolongation and drug interaction)
Thioridazine: do not use (QT prolongation and elevated thioridazine levels); do not use within 5 weeks of discontinuing fluoxetine

Side effects

Abnormal dreams, abnormal ejaculation
Anorexia, altered appetite and weight
Anxiety and insomnia
Nausea and other gastrointestinal effects
Serotonin syndrome; abnormal bleeding; hyponatraemia; QT prolongation (see warnings)

Interactions

Monoamine oxidase inhibitors (MAOIs) - contraindicated (serotonin syndrome)
Drugs metabolised by CYP2D6 - fluoxetine is a potent CYP2D6 inhibitor
Tricyclic antidepressants - monitor TCA levels during coadministration or after recent discontinuation
Other serotonergic drugs (triptans, tramadol, lithium, tryptophan, buspirone, St John's Wort) - serotonin syndrome risk
Drugs that interfere with haemostasis (NSAIDs, aspirin, warfarin) - may potentiate bleeding risk
Drugs that prolong the QT interval - do not use (e.g. pimozide, thioridazine)

Clinical monograph

How it works

It selectively inhibits serotonin reuptake, increasing synaptic serotonin over several weeks.

Prescribing in practice

The long half-life means slow onset and offset, fewer discontinuation symptoms, but prolonged potential for interactions.
Initiation can transiently worsen anxiety; be alert to serotonin syndrome with other serotonergic drugs and to increased bleeding risk with NSAIDs/antithrombotics.
It is the SSRI with the best evidence in children and young people, where indicated.

Monitoring

Review mood, anxiety and suicidal ideation early after starting and at dose changes, particularly in younger patients.

Counselling the patient

It can take a few weeks to work, and anxiety may briefly increase at first.
Report worsening mood or thoughts of self-harm.
Discontinuation effects are usually mild because it is long-acting.

Evidence & guidelines

A first-line SSRI for depression (NICE NG222) and the preferred antidepressant in young people where drug treatment is indicated.

Reference: NICE CG90; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

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