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Selective serotonin reuptake inhibitor (SSRI)

Paroxetine

Paroxetine is a selective serotonin reuptake inhibitor (SSRI) antidepressant used primarily for major depressive disorder, generalized anxiety disorder, panic disorder, social anxiety disorder, obsessive-compulsive disorder, and post-traumatic stress disorder. The most testable fact is that paroxetine has the highest anticholinergic activity and shortest half-life among SSRIs, making it the most likely to cause discontinuation syndrome and antimuscarinic side effects.

Practice questionsHigh-yield facts
Drug classSelective serotonin reuptake inhibitor (SSRI)
MechanismInhibits presynaptic serotonin reuptake, increasing synaptic 5-HT
Main usesDepression, anxiety disorders, OCD, PTSD
Key adverse effectSevere discontinuation syndrome, sexual dysfunction, weight gain
6 min readFor medical studentsUpdated 2026-10-14

Mechanism of action

Paroxetine selectively inhibits the reuptake of serotonin (5-hydroxytryptamine, 5-HT) at the presynaptic neuronal membrane by blocking the serotonin transporter (SERT). This inhibition increases the concentration and duration of serotonin in the synaptic cleft, enhancing serotonergic neurotransmission in the central nervous system.

Unlike tricyclic antidepressants, paroxetine has minimal affinity for adrenergic, cholinergic (muscarinic), and histaminergic receptors, though it has the strongest anticholinergic activity among all SSRIs. It also has weak inhibition of nitric oxide synthase. The therapeutic effects in depression and anxiety disorders develop over 2-4 weeks as adaptive changes occur in postsynaptic receptor sensitivity and intracellular signaling pathways.

Paroxetine is a potent inhibitor of CYP2D6, which contributes to numerous drug interactions and affects its own metabolism.

High-yield for exams
  • Paroxetine has the shortest half-life (21-24h) and worst discontinuation syndrome of all SSRIs—never stop abruptly
  • Pregnancy category D: first-trimester use increases risk of cardiac septal defects (unlike other SSRIs which are category C)
  • Most anticholinergic SSRI: causes more dry mouth, constipation, and weight gain than other SSRIs
  • Potent CYP2D6 inhibitor: contraindicated with tamoxifen (reduces breast cancer treatment efficacy) and pimozide/thioridazine (QT prolongation)
  • Black box warning: increased suicidal thinking in patients under 25 during initial treatment
  • Highest rate of sexual dysfunction among SSRIs (25-40%): delayed ejaculation, anorgasmia, decreased libido
  • Can cause serotonin syndrome when combined with MAOIs, tramadol, triptans, or other serotonergic agents
  • Increases bleeding risk when combined with NSAIDs, aspirin, or anticoagulants due to impaired platelet function

Indications

Paroxetine is FDA-approved for:

  • Major depressive disorder (MDD): first-line pharmacotherapy
  • Generalized anxiety disorder (GAD)
  • Panic disorder: with or without agoraphobia
  • Social anxiety disorder (social phobia)
  • Obsessive-compulsive disorder (OCD)
  • Post-traumatic stress disorder (PTSD)
  • Premenstrual dysphoric disorder (PMDD): paroxetine CR formulation is FDA-approved for PMDD

Off-label uses

  • Premature ejaculation
  • Vasomotor symptoms of menopause (hot flashes)
  • Chronic pain syndromes

Paroxetine is often chosen for anxiety disorders due to its anxiolytic properties, though its side effect profile and discontinuation issues limit its use compared to other SSRIs like sertraline or escitalopram.

Pharmacokinetics

Absorption: Well absorbed orally with peak plasma concentrations in 5-8 hours. Bioavailability is approximately 50% due to first-pass metabolism. Food does not significantly affect absorption.

Distribution: Highly protein-bound (95%). Distributes widely into tissues including the CNS. Volume of distribution is 3-28 L/kg.

Metabolism: Extensively metabolized in the liver, primarily via CYP2D6 (which paroxetine itself potently inhibits, leading to nonlinear pharmacokinetics). The major metabolites are conjugated products of oxidation and methylation and are pharmacologically inactive.

Elimination: Metabolites are excreted in urine (64%) and feces (36%). The elimination half-life is approximately 21-24 hours, which is the shortest among commonly used SSRIs. This short half-life contributes to more severe discontinuation symptoms.

Special populations: Clearance is reduced in elderly patients, those with severe renal impairment, and those with hepatic dysfunction, requiring dose adjustment.

Adverse effects

Common adverse effects

  • Sexual dysfunction (25-40%): delayed ejaculation, anorgasmia, decreased libido—highest rate among SSRIs
  • Gastrointestinal: nausea, diarrhea, constipation, dry mouth
  • Central nervous system: somnolence, dizziness, insomnia, tremor, headache
  • Weight gain: more common with paroxetine than other SSRIs
  • Sweating and anticholinergic effects: due to mild antimuscarinic activity

Serious adverse effects

Discontinuation syndrome: Paroxetine causes the most severe and frequent discontinuation syndrome among SSRIs due to its short half-life and lack of active metabolites. Symptoms include dizziness, paresthesias ("brain zaps"), nausea, anxiety, irritability, confusion, and flu-like symptoms. Taper slowly over several weeks.

Serotonin syndrome: Risk increases with concomitant use of other serotonergic agents (MAOIs, tramadol, triptans, SNRIs). Presents with confusion, agitation, hyperthermia, hyperreflexia, myoclonus, and autonomic instability.

Suicidality: Black box warning for increased risk of suicidal thinking and behavior in children, adolescents, and young adults (under age 25) with major depressive disorder and other psychiatric disorders, especially during initial treatment.

Hyponatremia/SIADH: Particularly in elderly patients, can present with confusion, seizures, or falls.

Bleeding risk: SSRIs inhibit platelet serotonin uptake; risk increases with concurrent NSAIDs, aspirin, or anticoagulants.

QT prolongation: Rare, but paroxetine can prolong QTc interval.

Teratogenicity: First-trimester exposure is associated with increased risk of cardiac malformations (especially ventricular and atrial septal defects). Paroxetine is pregnancy category D. Also causes neonatal adaptation syndrome (jitteriness, respiratory distress) and persistent pulmonary hypertension in newborns.

Contraindications and warnings

Absolute contraindications

  • Concurrent use of MAOIs or within 14 days of MAOI discontinuation (risk of serotonin syndrome)
  • Use of thioridazine or pimozide (paroxetine's CYP2D6 inhibition increases their levels, causing QT prolongation)
  • Hypersensitivity to paroxetine

Relative contraindications and warnings

  • Pregnancy: Category D; avoid in first trimester due to cardiac teratogenicity
  • Mania/bipolar disorder: Can precipitate manic episodes; use mood stabilizer first
  • Seizure disorders: Lowers seizure threshold (though less than tricyclics)
  • Angle-closure glaucoma: Pupillary dilation can precipitate acute crisis
  • Bleeding disorders: Impairs platelet aggregation
  • Hepatic or severe renal impairment: Requires dose reduction
  • Elderly patients: Increased risk of hyponatremia, falls, and cognitive impairment

Withdrawal considerations

Paroxetine should never be stopped abruptly. Gradual tapering over 4-8 weeks (or longer) is essential. Some clinicians switch to fluoxetine (long half-life) before discontinuation to minimize withdrawal symptoms.

Drug interactions

Paroxetine is a potent CYP2D6 inhibitor, leading to numerous clinically significant interactions.

Major interactions

MAOIs: Contraindicated due to serotonin syndrome risk. Allow 14-day washout.

Serotonergic agents: Increased serotonin syndrome risk with:

  • Other antidepressants (SNRIs, TCAs, other SSRIs)
  • Tramadol, meperidine, fentanyl
  • Triptans (sumatriptan, etc.)
  • Linezolid (weak MAOI)
  • St. John's wort
  • Dextromethorphan

Thioridazine and pimozide: Paroxetine increases their levels via CYP2D6 inhibition, causing dangerous QT prolongation. Contraindicated.

Tamoxifen: Paroxetine reduces conversion of tamoxifen to its active metabolite (endoxifen), potentially reducing breast cancer treatment efficacy. Avoid combination; use alternative SSRI like escitalopram or venlafaxine.

CYP2D6 substrates: Paroxetine increases levels of:

  • Tricyclic antidepressants (desipramine, nortriptyline)
  • Beta-blockers (metoprolol, propranolol)
  • Antiarrhythmics (flecainide, propafenone)
  • Atomoxetine
  • Codeine (reduces conversion to morphine, decreasing analgesic effect)

NSAIDs, aspirin, anticoagulants: Increased bleeding risk.

Alcohol: Avoid; may increase sedation and cognitive impairment.

Moderate interactions

Drugs affecting CYP2D6: Strong CYP2D6 inhibitors (quinidine, fluoxetine) may increase paroxetine levels. CYP2D6 inducers may reduce efficacy.

Anticholinergic drugs: Additive antimuscarinic effects (dry mouth, constipation, urinary retention, confusion).

CNS depressants: Additive sedation with benzodiazepines, opioids, antihistamines.

Clinical pearls

  • Worst discontinuation syndrome: Paroxetine has the shortest half-life and most severe withdrawal among SSRIs. Always taper slowly; consider switching to fluoxetine before discontinuation.
  • Most anticholinergic SSRI: Expect more dry mouth, constipation, and blurred vision compared to other SSRIs. Avoid in elderly with cognitive impairment.
  • Pregnancy category D: Unlike other SSRIs (mostly category C), paroxetine has clear evidence of cardiac teratogenicity. Use sertraline or citalopram as safer alternatives in pregnancy.
  • Tamoxifen interaction: Critical to avoid in breast cancer patients. Paroxetine significantly impairs tamoxifen efficacy by inhibiting CYP2D6-mediated conversion to active metabolite.
  • Potent CYP2D6 inhibitor: Consider all medications metabolized by CYP2D6 when prescribing paroxetine. Adjust doses of tricyclics, beta-blockers, and antiarrhythmics.
  • Weight gain: More common with paroxetine than fluoxetine, sertraline, or escitalopram. Consider alternative SSRI if weight is a concern.
  • Sexual dysfunction: Highest rates among SSRIs (up to 40%). Can manage with dose reduction, drug holidays, or switching to bupropion or mirtazapine.
  • Delayed onset: Like all antidepressants, therapeutic effects require 2-4 weeks. Counsel patients on adherence.
  • Once-daily dosing: Usually given in morning or evening depending on whether it causes sedation or insomnia in individual patient.
  • Controlled-release formulation: Paroxetine CR may reduce nausea and allow once-daily dosing with more stable levels.
Mnemonic

PAROXETINE discontinuation = PAINFUL: Paresthesias, Anxiety, Rebound depression, Orthostatic dizziness, Xtra sweating, Electric shock sensations (brain zaps), Tremor, Irritability, Nausea, Equilibrium problems (dizziness)

Practice questions

1. A 34-year-old woman with major depressive disorder has been taking paroxetine for 8 months with good symptom control. She decides to stop the medication abruptly because she feels better. Five days later, she presents to the emergency department with dizziness, electric shock-like sensations in her head, nausea, sweating, and severe anxiety. Her vital signs are normal. Physical examination shows hyperreflexia but no focal neurological deficits. What is the most likely diagnosis?

Answer: B. This patient has classic SSRI discontinuation syndrome, which occurs most severely and frequently with paroxetine due to its short half-life (21-24 hours) and lack of active metabolites. The electric shock sensations ("brain zaps"), dizziness, paresthesias, nausea, and anxiety within days of abrupt cessation are pathognomonic. Serotonin syndrome would require addition of serotonergic agents and presents with hyperthermia, confusion, and autonomic instability. Depression recurrence would take weeks to develop, not days, and would not cause brain zaps or dizziness.

2. A 52-year-old woman with breast cancer is receiving tamoxifen therapy. She develops depressive symptoms and her oncologist refers her to psychiatry. Her medications include tamoxifen, anastrozole, and calcium supplements. Which antidepressant should be avoided due to a significant drug interaction that could compromise her cancer treatment?

Answer: B. Paroxetine is a potent CYP2D6 inhibitor that blocks the conversion of tamoxifen to its active metabolite endoxifen, potentially reducing breast cancer treatment efficacy and worsening outcomes. This interaction is clinically significant and paroxetine should be avoided in patients on tamoxifen. Safe alternatives include venlafaxine, escitalopram, citalopram, or mirtazapine, which do not significantly inhibit CYP2D6.

3. A 29-year-old woman in her 10th week of pregnancy presents for prenatal care. She has been taking paroxetine for generalized anxiety disorder for the past 6 months. She asks whether she should continue the medication. Fetal ultrasound is scheduled for next week. What is the most appropriate counseling regarding paroxetine use?

Answer: C. Paroxetine is pregnancy category D with established evidence of increased cardiac malformations (especially septal defects) with first-trimester exposure. The patient should be counseled about this risk, and switching to a safer alternative such as sertraline (category C) should be discussed with consideration of her clinical stability. Abrupt discontinuation is inappropriate due to severe withdrawal risk and potential relapse of anxiety. Simply continuing paroxetine ignores established teratogenic risk. While cardiac organogenesis is largely complete by 10 weeks, switching to a safer agent is still appropriate for the remainder of pregnancy.

Frequently asked questions

How does paroxetine work?

Paroxetine works by selectively inhibiting the reuptake of serotonin at presynaptic nerve terminals, increasing serotonin availability in the synaptic cleft. This enhanced serotonergic neurotransmission improves mood and reduces anxiety over 2-4 weeks as adaptive changes occur in brain receptor systems. Unlike older antidepressants, paroxetine has minimal effects on other neurotransmitter systems, though it has mild anticholinergic activity.

What are the side effects of paroxetine?

Common side effects include sexual dysfunction (25-40%, highest among SSRIs), nausea, weight gain, dry mouth, drowsiness, and sweating. Serious adverse effects include severe discontinuation syndrome when stopped abruptly, serotonin syndrome with other serotonergic drugs, increased suicidal thinking in young adults, hyponatremia especially in elderly, increased bleeding risk, and cardiac birth defects if used in first trimester of pregnancy. Paroxetine causes more anticholinergic effects and weight gain than other SSRIs.

Is paroxetine a controlled substance?

No, paroxetine is not a controlled substance and has no abuse potential. It is not classified under the Controlled Substances Act. However, it should never be stopped abruptly due to severe discontinuation syndrome, and it does require a prescription. Physical dependence (withdrawal symptoms upon discontinuation) is not the same as addiction or controlled substance status.

Paroxetine vs sertraline—what is the difference?

Both are SSRIs, but paroxetine has a shorter half-life (21-24h vs 26h) causing worse discontinuation syndrome, more anticholinergic effects, more weight gain, and higher sexual dysfunction rates. Paroxetine is pregnancy category D (teratogenic) while sertraline is category C (safer in pregnancy). Paroxetine is a potent CYP2D6 inhibitor with more drug interactions, while sertraline has fewer interactions. Sertraline is generally preferred due to better tolerability, safety profile, and easier discontinuation.

Why is paroxetine contraindicated in pregnancy?

Paroxetine is pregnancy category D because first-trimester exposure increases the risk of congenital cardiac malformations, particularly ventricular and atrial septal defects, by approximately 1.5-2 fold. Other SSRIs are generally category C with less clear teratogenic risk. Paroxetine also causes neonatal adaptation syndrome (jitteriness, respiratory distress, irritability) and persistent pulmonary hypertension in newborns when used in the third trimester. Sertraline or citalopram are preferred SSRI choices during pregnancy.

Conditions it is used for

major depressive disordergeneralized anxiety disorderpanic disordersocial anxiety disorderobsessive-compulsive disorderpost-traumatic stress disorderpremenstrual dysphoric disorderpremature ejaculation
fluoxetinesertralineescitalopramcitalopramfluvoxaminevenlafaxineduloxetinebupropion

Sources and further reading

DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: N06AB05

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.