Formulary
Fluoxetine: Uses, Dosing, Side Effects and Indian Brand Names
Fluoxetine is the prototype SSRI antidepressant with the longest half-life in its class. It is first-line for depression and the drug of choice for bulimia nervosa, with a wide safety margin in overdose compared to tricyclic antidepressants.
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Clinically reviewed by Awaiting clinical review
Fluoxetine: Uses, Dosing, Side Effects and Indian Brand Names
Fluoxetine is the prototype SSRI antidepressant with the longest half-life in its class. It is first-line for depression and the drug of choice for bulimia nervosa, with a wide safety margin in overdose compared to tricyclic antidepressants.
NEET PG High-Yield: Fluoxetine is the drug of choice for bulimia nervosa -- a classic MCQ. It has the LONGEST half-life among SSRIs (5-week washout before MAOI). SSRIs cause sexual dysfunction (major compliance issue) and SIADH in elderly. Serotonin syndrome triad: mental status changes + autonomic instability + neuromuscular hyperactivity -- treated with cyproheptadine.
Clinical overview
Fluoxetine was the first SSRI to be introduced (1987) and revolutionised the treatment of depression by offering efficacy comparable to tricyclic antidepressants with a markedly improved side-effect profile and much greater safety in overdose. It is the most commonly prescribed antidepressant globally and is widely used in Indian psychiatric practice. Fluoxetine has the longest half-life among SSRIs (1-3 days for the parent compound, 4-16 days for the active metabolite norfluoxetine), which confers a clinical advantage: missed doses are less likely to cause discontinuation symptoms, and a washout period of 5 weeks is required before switching to an MAOI. Beyond depression, fluoxetine is the drug of choice for bulimia nervosa (the only FDA-approved pharmacotherapy for this condition) and is effective in obsessive-compulsive disorder, panic disorder, premenstrual dysphoric disorder, and body dysmorphic disorder. A critical safety concern is the initial activation of suicidal ideation in adolescents and young adults during the first few weeks of therapy (FDA black box warning), attributed to early energisation before mood improvement. Serotonin syndrome is the most dangerous adverse effect, occurring when fluoxetine is combined with other serotonergic drugs (MAOIs, tramadol, linezolid, St John's wort). The very long half-life also makes it a significant inhibitor of CYP2D6, with numerous drug interactions.
Pharmacological class
Fluoxetine belongs to the Selective serotonin reuptake inhibitor (SSRI) class. Fluoxetine selectively inhibits the reuptake of serotonin (5-HT) by blocking the serotonin transporter (SERT) at presynaptic nerve terminals. This increases serotonin concentration in the synaptic cleft, enhancing serotonergic neurotransmission. Unlike tricyclic antidepressants, fluoxetine has minimal effects on noradrenaline and dopamine reuptake, and negligible anticholinergic, antihistaminic, and alpha-adrenergic blocking activity.
Indian brand names and formulations
Available as: Fludac (Cadila/Zydus), Flunil (Intas Pharmaceuticals), Fluox (Sun Pharma), Prodep (Sun Pharma).
Capsules (10 mg, 20 mg, 40 mg, 60 mg), Oral solution (20 mg/5 mL), Dispersible tablets (20 mg)
Regulatory status
Fluoxetine is classified under Schedule H in India under the Drugs and Cosmetics Act, 1940. Schedule H prescription drug. Wide therapeutic index and low lethality in overdose compared to TCAs. Included in the NLEM 2022.
Indications
- Major depressive disorder (first-line)
- Obsessive-compulsive disorder (OCD)
- Bulimia nervosa (drug of choice -- only FDA-approved drug for this indication)
- Panic disorder (with or without agoraphobia)
- Premenstrual dysphoric disorder (PMDD)
- Body dysmorphic disorder
- Post-traumatic stress disorder (PTSD)
Dosing
Depression: 20 mg once daily in the morning (can increase to 40-80 mg/day after several weeks). OCD: 20-80 mg/day (higher doses often needed). Bulimia nervosa: 60 mg/day (higher than depression dose). PMDD: 20 mg daily or luteal-phase dosing (day 14 to menses). Elderly: start at 10 mg/day. Therapeutic effect takes 2-4 weeks to manifest (steady state is delayed due to long half-life). No dose adjustment needed for mild-moderate renal impairment.
Contraindications
- Concurrent use with MAO inhibitors (wait 2 weeks after stopping MAOI; wait 5 weeks after stopping fluoxetine before starting MAOI -- due to long half-life)
- Concurrent use with pimozide or thioridazine (QT prolongation risk)
- Known hypersensitivity to fluoxetine
- Unstable epilepsy (lowers seizure threshold)
Adverse effects
- GI disturbance (nausea, diarrhoea -- most common initial side effects)
- Sexual dysfunction (delayed ejaculation, anorgasmia -- major cause of non-compliance)
- Insomnia, anxiety, agitation (initial activation syndrome)
- Headache
- Weight loss initially (unlike many other antidepressants); weight neutral long-term
- Serotonin syndrome (with serotonergic drug combinations)
- Suicidal ideation in adolescents (FDA black box warning -- first 2-4 weeks)
- SIADH and hyponatraemia (especially in elderly)
- Bleeding risk (serotonin depletion in platelets)
Drug interactions
- MAO inhibitors -- risk of fatal serotonin syndrome (absolute contraindication; 5-week washout needed)
- Tramadol, linezolid, methylene blue -- serotonin syndrome risk
- Warfarin -- increased bleeding risk (platelet serotonin depletion + CYP2C9 inhibition)
- CYP2D6 substrates (TCAs, codeine, tamoxifen) -- fluoxetine is a potent CYP2D6 inhibitor; reduces tamoxifen activation
- NSAIDs -- additive GI bleeding risk
- St John's Wort -- serotonin syndrome risk
Pregnancy and lactation
Category C. SSRIs as a class are generally considered among the safer antidepressants in pregnancy. Fluoxetine has extensive human pregnancy data. First-trimester SSRI exposure may carry a small increased risk of cardiac septal defects (absolute risk low). Third-trimester exposure may cause neonatal adaptation syndrome (jitteriness, respiratory distress, feeding difficulty). The decision to continue SSRIs in pregnancy requires weighing relapse risk against neonatal effects.
Exam-style clinical scenario
A 25-year-old woman with recurrent binge-eating followed by self-induced vomiting, parotid gland enlargement, and dental erosion is diagnosed with bulimia nervosa. What is the pharmacotherapy of choice? Answer: Fluoxetine 60 mg/day is the drug of choice for bulimia nervosa. It is the only FDA-approved pharmacotherapy for this condition. The dose used for bulimia is higher than the standard antidepressant dose. It reduces binge-purge frequency by enhancing serotonergic transmission in the satiety centres. CBT combined with fluoxetine has the best outcomes.
Cost in India
INR 30-60 per strip of 10 capsules (20 mg)
Clinical governance
Author: MedNext Editorial Team. Clinical reviewer: Awaiting clinical review. Jurisdiction: India (Drugs and Cosmetics Act, 1940). Sources: CIMS India, Indian Pharmacopoeia. Publication state: Awaiting clinical review. Correction: Report errors at support@mednext.academy.
Frequently Asked Questions
Why does fluoxetine require a 5-week washout before switching to an MAOI?
Fluoxetine has a half-life of 1-3 days, but its active metabolite norfluoxetine has a half-life of 4-16 days. Complete elimination (5 half-lives of norfluoxetine) requires approximately 5 weeks. If an MAOI is started before fluoxetine is fully eliminated, the combination of increased synaptic serotonin (from residual SERT inhibition) plus blocked serotonin metabolism (from MAO inhibition) can precipitate fatal serotonin syndrome.
Why do SSRIs cause sexual dysfunction?
Serotonin (5-HT) inhibits sexual function through multiple pathways: 5-HT2A receptor activation in the spinal cord inhibits the ejaculatory reflex; 5-HT2C stimulation reduces dopaminergic tone in reward circuits (reducing libido); and increased serotonin in the spinal cord and brainstem interferes with orgasm. This pharmacological property is therapeutically exploited -- dapoxetine (a short-acting SSRI) is used specifically for premature ejaculation.
What is the FDA black box warning for SSRIs in adolescents?
SSRIs carry an FDA black box warning for increased suicidal thinking and behaviour in children, adolescents, and young adults (under 25 years) during the initial weeks of treatment. The proposed mechanism is that SSRIs may improve energy and motivation before improving mood, giving a previously retarded depressed patient the drive to act on pre-existing suicidal ideation. Close monitoring is recommended for the first 1-2 months, especially in young patients.
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