Fluoxetine (Prozac) is the original SSRI antidepressant, approved for major depression (adults and children 8+), OCD, bulimia nervosa, panic disorder, and bipolar I depression (combined with olanzapine). It inhibits the serotonin transporter (SERT) and is distinguished by its exceptionally long half-life — the active metabolite norfluoxetine persists 4–16 days in the body. Common side effects include insomnia, nausea, and sexual dysfunction. A 5-week washout after stopping fluoxetine is required before starting MAO inhibitors — longer than any other SSRI — due to this extended half-life.
Fluoxetine
Fluoxetine was the first selective serotonin reuptake inhibitor (SSRI) approved in the United States and remains one of the most widely prescribed antidepressants worldwide. Introduced in 1987, it transformed the treatment of depression by offering a safer alternative to tricyclic antidepressants and MAO inhibitors. It is distinguished by its exceptionally long half-life, which allows for once-weekly dosing formulations and a more forgiving response to missed doses.
Uses & FDA Indications
Fluoxetine has one of the broadest FDA-approved indications of any antidepressant. It is approved for major depressive disorder (MDD) in adults and children aged 8 and older — making it one of the few antidepressants approved for pediatric depression. Additional FDA approvals include obsessive-compulsive disorder (OCD) in adults and children aged 7+, bulimia nervosa (as Prozac), panic disorder, bipolar I depression (as Symbyax, combined with olanzapine), and treatment-resistant depression (also as Symbyax).
Off-label uses include premenstrual dysphoric disorder (PMDD — marketed as Sarafem), social anxiety disorder, PTSD, borderline personality disorder, and binge-eating disorder.
Fluoxetine's long half-life makes it uniquely forgiving — missing an occasional dose is less clinically significant than with shorter-acting SSRIs, and discontinuation syndrome is rare compared to other antidepressants.
How It Works
Like all SSRIs, fluoxetine inhibits the serotonin transporter (SERT), preventing the reuptake of serotonin from the synaptic cleft back into the presynaptic neuron. This increases serotonin availability to activate postsynaptic receptors, gradually modulating mood, anxiety, appetite, and sleep regulation circuits over weeks of treatment.
Fluoxetine is also a moderate inhibitor of CYP2D6 and CYP2C19 liver enzymes, which is clinically important because it can significantly raise the blood levels of many co-administered drugs that are metabolized by these pathways. Its active metabolite, norfluoxetine, has a half-life of up to 16 days and contributes meaningfully to both its therapeutic effects and interaction profile.
Fluoxetine also has mild antagonist effects at 5-HT2C receptors, which may contribute to its activating, sometimes stimulating quality — a characteristic that distinguishes it from more sedating SSRIs like paroxetine.
Side Effects
Common
- Insomnia or vivid dreams (more activating than most SSRIs)
- Nausea and decreased appetite
- Headache
- Nervousness, restlessness, or anxiety (especially early in treatment)
- Sexual dysfunction (reduced libido, delayed orgasm)
- Dry mouth
- Sweating
- Tremor
Serious
- Serotonin syndrome — risk elevated when combined with other serotonergic drugs or MAOIs
- Suicidal ideation — black box warning, especially in patients under 25
- Hyponatremia — low sodium, particularly in elderly patients (SIADH mechanism)
- Seizures — rare; caution in patients with seizure history
- Prolonged QTc — rare but clinically relevant, particularly with cardiac comorbidities
- Abnormal bleeding — SSRIs reduce platelet aggregation; risk increases with anticoagulants or NSAIDs
- Mania activation — fluoxetine can precipitate manic episodes in patients with undiagnosed bipolar disorder
Drug Interactions
| Drug / Class | Interaction | Clinical Significance |
|---|---|---|
| MAO Inhibitors (phenelzine, tranylcypromine) | Severe, potentially fatal serotonin syndrome | Contraindicated — 5-week washout required after stopping fluoxetine |
| Tricyclic antidepressants (amitriptyline, nortriptyline) | Fluoxetine inhibits CYP2D6, dramatically raising TCA levels; toxicity risk | High — dose reduction and monitoring required |
| Thioridazine / Pimozide | QTc prolongation and risk of fatal arrhythmia | Contraindicated |
| Tamoxifen | CYP2D6 inhibition reduces conversion of tamoxifen to active metabolite endoxifen, reducing efficacy | High — alternative antidepressants preferred in breast cancer patients |
| Opioids (codeine, tramadol) | CYP2D6 inhibition reduces codeine conversion to morphine; tramadol serotonin syndrome risk | Moderate–High |
| Warfarin | Possible increased anticoagulant effect; monitor INR closely | Moderate |
| Lithium | Enhanced serotonergic effects; monitor for toxicity | Moderate |
Warnings & Contraindications
BLACK BOX WARNING: Antidepressants increase the risk of suicidal thinking and behavior in pediatric and young adult patients (under 25). Monitor for clinical worsening, agitation, and unusual behavioral changes, especially during the first few months of therapy or after dose changes.
Contraindications: Concurrent or recent use of MAO inhibitors (5-week washout needed after discontinuing fluoxetine due to its long half-life). Use with thioridazine or pimozide. Known hypersensitivity to fluoxetine.
Bipolar disorder precaution: Fluoxetine monotherapy can trigger manic episodes in patients with bipolar disorder. Always screen for bipolar history before initiating. When treating bipolar depression, fluoxetine is used in combination with a mood stabilizer or olanzapine.
Long washout period: Because of norfluoxetine's extended half-life, drug interactions and discontinuation considerations apply for weeks after stopping fluoxetine — longer than any other SSRI.
Weight changes: Long-term use may cause weight gain, though short-term use is associated with appetite suppression and possible weight loss.
Frequently Asked Questions
Why does fluoxetine have such a long half-life?
Fluoxetine is metabolized in the liver to an active compound called norfluoxetine, which is eliminated very slowly — with a half-life of 4 to 16 days. This means the drug and its active effects persist in the body for weeks after the last dose, which is why such a long washout period is required before starting an MAOI.
Is fluoxetine safe for children?
Fluoxetine is one of the few antidepressants with FDA approval for major depression in children (age 8 and older) and OCD in children (age 7 and older). It is generally considered among the better-studied antidepressants for pediatric use, though all patients require careful monitoring for behavioral changes and suicidal ideation.
Does fluoxetine cause weight gain?
Early in treatment, fluoxetine often suppresses appetite and may cause slight weight loss. With long-term use, weight gain becomes more common, though it tends to be less pronounced than with some other antidepressants (e.g., paroxetine or mirtazapine).
Can I stop fluoxetine suddenly?
Fluoxetine's long half-life means abrupt discontinuation rarely causes the withdrawal symptoms (brain zaps, flu-like symptoms) seen with shorter-acting antidepressants. However, gradual tapering under medical supervision is still recommended to allow the prescriber to monitor for relapse of the underlying condition.
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