Mechanism of action
Fluoxetine selectively inhibits the reuptake of serotonin (5-HT) at the presynaptic neuronal membrane by blocking the serotonin transporter (SERT). This inhibition increases synaptic serotonin concentrations in the central nervous system, enhancing serotonergic neurotransmission.
Unlike tricyclic antidepressants, fluoxetine has minimal affinity for muscarinic, histaminergic, and alpha-adrenergic receptors, which accounts for its more favorable side effect profile. The therapeutic effects typically take 2-4 weeks to manifest as neuroadaptive changes occur, including downregulation of postsynaptic 5-HT2 receptors and modulation of brain-derived neurotrophic factor (BDNF).
Fluoxetine is metabolized to norfluoxetine, an active metabolite with similar potency and an even longer half-life. Both parent drug and metabolite are potent inhibitors of CYP2D6 and moderate inhibitors of CYP3A4 and CYP2C19, leading to clinically significant drug interactions.
- Fluoxetine has the longest half-life of all SSRIs (4-6 days for norfluoxetine), minimizing withdrawal symptoms
- Strong CYP2D6 inhibitor: increases levels of TCAs, beta-blockers, and antipsychotics metabolized by this pathway
- First-line SSRI approved for bulimia nervosa and pediatric depression (age ≥8 years)
- Serotonin syndrome risk when combined with MAOIs; requires 5-week washout before starting MAOI
- Can cause QT prolongation, especially in overdose or when combined with other QT-prolonging drugs
- Activating SSRI: often causes initial insomnia and anxiety; best dosed in the morning
- Safe in overdose compared to TCAs; toxicity mainly involves serotonin syndrome and seizures
- Sexual dysfunction (anorgasmia, decreased libido) occurs in 30-50% of patients on chronic therapy
Indications
Fluoxetine is FDA-approved for:
- Major depressive disorder (adults and pediatric patients aged 8 years and older)
- Obsessive-compulsive disorder (adults and pediatric patients aged 7 years and older)
- Bulimia nervosa (moderate to severe)
- Panic disorder (with or without agoraphobia)
- Premenstrual dysphoric disorder (as fluoxetine or sarafem formulation)
- Treatment-resistant depression (in combination with olanzapine)
- Bipolar I depression (in combination with olanzapine)
Off-label uses include generalized anxiety disorder, social anxiety disorder, post-traumatic stress disorder, and premature ejaculation. Fluoxetine is often preferred in pediatric depression due to the most favorable benefit-risk profile among SSRIs in this population.
Pharmacokinetics
Absorption and distribution
Fluoxetine is well absorbed after oral administration with bioavailability of approximately 70-80%. Peak plasma concentrations occur 6-8 hours after dosing. It is highly protein-bound (approximately 95%) and has a large volume of distribution.
Metabolism
Fluoxetine undergoes extensive hepatic metabolism primarily via CYP2D6 to form norfluoxetine, an active metabolite with equivalent pharmacological activity. Both fluoxetine and norfluoxetine are potent CYP2D6 inhibitors, which creates potential for significant drug interactions.
Elimination
Fluoxetine has a half-life of 1-3 days, while norfluoxetine has a half-life of 4-16 days (average 7-9 days). This exceptionally long half-life means:
- Steady state is reached in 4-5 weeks
- Drug discontinuation does not require tapering
- Complete elimination takes 5-6 weeks after stopping
- Drug interactions may persist for weeks after discontinuation
Renal excretion accounts for approximately 80% of elimination. Dose adjustment may be needed in severe hepatic impairment but is generally not required in renal impairment.
Adverse effects
Common adverse effects
- Gastrointestinal: Nausea, diarrhea, dry mouth, decreased appetite (most common early in treatment)
- Central nervous system: Insomnia, anxiety, nervousness, headache, dizziness, somnolence
- Sexual dysfunction: Delayed ejaculation, anorgasmia, decreased libido (20-30% of patients)
- Other: Sweating, tremor, weight changes, fatigue
Serious adverse effects
- Serotonin syndrome: Life-threatening condition characterized by mental status changes, autonomic instability, and neuromuscular abnormalities; risk increased when combined with other serotonergic agents (MAOIs, SNRIs, triptans, tramadol, linezolid)
- Suicidality: Black box warning for increased suicidal thinking and behavior in children, adolescents, and young adults (under 25 years) during initial treatment; close monitoring required especially in first few months
- Bleeding: Increased risk due to serotonin's role in platelet aggregation; use caution with NSAIDs, aspirin, or anticoagulants
- Hyponatremia/SIADH: Especially in elderly patients; monitor for confusion, weakness, or seizures
- QT prolongation: Rare but documented; caution in patients with cardiac disease or on QT-prolonging drugs
- Mania/hypomania activation: Particularly in patients with undiagnosed bipolar disorder
- Seizures: Occurs in 0.1-0.2% of patients
Discontinuation syndrome
Fluoxetine is least likely among SSRIs to cause discontinuation syndrome due to its long half-life. When symptoms occur, they include dizziness, sensory disturbances, anxiety, and flu-like symptoms.
Contraindications and warnings
Absolute contraindications
- MAO inhibitor use: Contraindicated within 14 days of MAOI discontinuation or 5 weeks after fluoxetine discontinuation due to serotonin syndrome risk
- Pimozide or thioridazine co-administration: Risk of QT prolongation and ventricular arrhythmias due to CYP2D6 inhibition
- Known hypersensitivity to fluoxetine or formulation components
Warnings and precautions
- Pregnancy category C (now replaced by PPRM labeling): Use during third trimester associated with neonatal complications including persistent pulmonary hypertension of the newborn (PPHN), though absolute risk is low. Weigh risks versus benefits of untreated maternal depression.
- Breastfeeding: Fluoxetine and norfluoxetine are excreted in breast milk; consider alternative SSRIs with shorter half-lives (sertraline, paroxetine) as preferred agents
- Hepatic impairment: Reduced clearance; consider lower or less frequent dosing
- Seizure disorders: Use with caution; may lower seizure threshold
- Angle-closure glaucoma: Risk of mydriasis-induced acute angle closure
- Bleeding risk: Caution in patients with history of bleeding disorders or concurrent anticoagulant use
- Diabetes: May affect glycemic control; monitor blood glucose
- Electrolyte abnormalities: Correct before initiating therapy to reduce arrhythmia risk
Drug interactions
Fluoxetine is a potent CYP2D6 inhibitor and moderate CYP3A4 and CYP2C19 inhibitor, creating numerous clinically significant interactions.
Contraindicated combinations
- MAO inhibitors: Risk of serotonin syndrome; requires 14-day washout before starting MAOI, 5-week washout after stopping fluoxetine
- Pimozide, thioridazane: QT prolongation via CYP2D6 inhibition leading to elevated antipsychotic levels
Major interactions requiring monitoring or dose adjustment
- Serotonergic agents: Triptans, tramadol, other SSRIs/SNRIs, tryptophan, St. John's wort, linezolid—increased serotonin syndrome risk
- Tricyclic antidepressants: Fluoxetine inhibits metabolism, increasing TCA levels and toxicity risk
- Antipsychotics: Increased levels of CYP2D6-metabolized agents (haloperidol, risperidone, aripiprazole)
- Beta-blockers: Increased metoprolol, carvedilol levels; may cause bradycardia or hypotension
- Warfarin: Enhanced anticoagulant effect; monitor INR closely
- NSAIDs and aspirin: Increased bleeding risk
- Tamoxifen: Fluoxetine inhibits conversion to active metabolite endoxifen; consider alternative SSRI
- Phenytoin: Increased phenytoin levels; monitor for toxicity
- Benzodiazepines: Increased alprazolam and diazepam levels
- Antiplatelet agents: Additive bleeding risk
Alcohol
Although fluoxetine does not potentiate alcohol's cognitive impairment, concurrent use is generally discouraged in depressed patients.
Clinical pearls
- Longest SSRI half-life: The 4-16 day half-life of norfluoxetine means fluoxetine is self-tapering and has the lowest risk of discontinuation syndrome. However
FLUOXETINE: Forget withdrawal (Long half-life), Liver enzyme inhibitor (CYP2D6), Used in bulimia, OCD treatment effective, eXtremely long norfluoxetine (active metabolite), Energizing/activating, Teenage depression approved, Interacts with many drugs, Nausea common, Extended washout before MAOIs
Practice questions
1. A 28-year-old woman with major depressive disorder has been taking fluoxetine 40 mg daily for 6 weeks with good response. She now reports difficulty achieving orgasm and decreased libido, which is affecting her relationship. She asks to switch antidepressants. What is the most appropriate next step?
2. A 35-year-old man with obsessive-compulsive disorder has been successfully treated with fluoxetine for 2 years. He presents to the emergency department with agitation, confusion, tremor, hyperreflexia, diaphoresis, and a temperature of 39.2°C (102.6°F). His wife reports he started taking an herbal supplement 2 days ago. Which supplement most likely caused this presentation?
3. A 19-year-old woman with bulimia nervosa begins fluoxetine therapy. After 3 weeks, she reports feeling more anxious and having difficulty sleeping. She denies suicidal ideation but says she feels 'jittery' and restless. Her vital signs are normal. What is the most appropriate management?
Frequently asked questions
How does fluoxetine work?
Fluoxetine selectively inhibits the reuptake of serotonin (5-HT) at the presynaptic neuronal membrane, increasing serotonin availability in the synaptic cleft. This enhances serotonergic neurotransmission in the brain, which improves mood, reduces anxiety, and helps regulate obsessive thoughts and compulsive behaviors over 2-4 weeks of treatment.
What are the side effects of fluoxetine?
Common side effects include nausea, headache, insomnia, anxiety, sexual dysfunction (delayed orgasm, decreased libido), and weight changes. Fluoxetine can initially be activating, causing restlessness or jitteriness. Serious but rare effects include serotonin syndrome (when combined with other serotonergic drugs), increased suicidal ideation in young adults, hyponatremia (especially in elderly patients), and QT prolongation.
Is fluoxetine a controlled substance?
No, fluoxetine is not a controlled substance. It is a prescription-only antidepressant with no abuse potential or physical dependence. However, abrupt discontinuation can cause SSRI discontinuation syndrome, though this is less common with fluoxetine than other SSRIs due to its long half-life.
How long does fluoxetine stay in your system?
Fluoxetine has a half-life of 1-3 days, but its active metabolite norfluoxetine has a half-life of 4-16 days (average 4-6 days). This means fluoxetine can remain in the body for 4-6 weeks after discontinuation. This long duration minimizes withdrawal symptoms but requires a 5-week washout period before starting MAOIs to prevent serotonin syndrome.
Fluoxetine vs sertraline — what is the difference?
Both are SSRIs, but fluoxetine has a much longer half-life (4-6 days for norfluoxetine vs 26 hours for sertraline), making withdrawal less likely but drug interactions more prolonged. Fluoxetine is more activating and better for bulimia, while sertraline is better tolerated for anxiety and PTSD. Fluoxetine is a stronger CYP2D6 inhibitor, causing more drug interactions. Sertraline has more GI side effects due to mild dopamine reuptake inhibition.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: N06AB03
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.