Paroxetine

Paroxetine is a selective serotonin reuptake inhibitor (SSRI) that blocks the serotonin transporter (SERT) with the highest potency of any SSRI — approximately 5 times more potent than fluoxetine at SERT binding ^{[1, 3]}.

However, paroxetine is unique among SSRIs in that it also has significant off-target activity ^{[1, 3]}:

1. Muscarinic receptor antagonism: Paroxetine is the most anticholinergic SSRI, causing more dry mouth, constipation, and sedation than other SSRIs ^{[1, 3]}.

2. Norepinephrine transporter (NET) inhibition: Weak NET binding at higher doses may contribute to additional antidepressant and anxiolytic efficacy ^{[1, 3]}.

3. Potent CYP2D6 inhibition: Paroxetine is the strongest CYP2D6 inhibitor among SSRIs, causing numerous drug interactions. It also inhibits its own metabolism (non-linear pharmacokinetics), meaning dose increases cause disproportionate increases in blood levels ^{[1, 3]}.

4. Nitric oxide synthase inhibition: This may contribute to its prominent sexual side effects, which are the highest among SSRIs ^{[3, 6]}.

Paroxetine has the broadest FDA-approved indication list of any SSRI: MDD, GAD, panic disorder, social anxiety disorder, OCD, PTSD, and PMDD (as Paxil CR). Despite this versatility, its use has declined due to concerns about weight gain, discontinuation syndrome, and pregnancy safety compared to sertraline and escitalopram ^{[1, 3, 4]}.

Weeks 1-2: Start at 20 mg/day. Nausea is the most common early side effect. Anxiety may temporarily worsen (SSRI activation). Sedation is more common than with other SSRIs due to anticholinergic effects. Dry mouth and constipation may occur ^[1].

Weeks 2-4: Nausea improves. Anxiolytic effects begin to emerge. Sexual side effects (decreased libido, anorgasmia) become apparent — paroxetine has the highest rate of sexual dysfunction among SSRIs ^{[1, 6]}.

Weeks 4-8: Full antidepressant and anxiolytic effects typically achieved. Weight gain may begin — paroxetine is associated with more weight gain than other SSRIs. Dose may be increased to 40 mg if needed, but be aware of non-linear kinetics ^{[1, 4]}.

Long-term: Weight gain averages 3-7 kg and is the most common reason for discontinuation. Sexual dysfunction typically persists. If you need to stop paroxetine, plan for a very gradual taper — at least 4-8 weeks — due to the severe discontinuation syndrome ^{[1, 7, 8]}.

Paroxetine can be taken with or without food. Food slightly increases absorption but this is not clinically significant ^[1].

Alcohol: Alcohol should be avoided. Both are CNS depressants. The FDA labeling specifically recommends avoiding alcohol ^[1].

Grapefruit: No major interaction. Paroxetine is metabolized primarily by CYP2D6, not CYP3A4 ^[1].

Caffeine: No significant pharmacokinetic interaction. Caffeine may worsen anxiety in some patients ^[1].

NSAIDs and aspirin: SSRIs impair platelet function. Combining with NSAIDs or aspirin increases GI bleeding risk. Acetaminophen is a safer analgesic ^{[1, 10]}.

Depression (MDD): Start 20 mg/day. Increase by 10 mg/day at intervals of at least 1 week. Maximum: 50 mg/day. Usual range: 20-40 mg/day ^[1].

GAD: Start 20 mg/day. Range: 20-50 mg/day ^[1].

Panic disorder: Start 10 mg/day (low start to avoid SSRI activation). Target: 40 mg/day. Range: 10-60 mg/day ^[1].

Social anxiety disorder: Start 20 mg/day. Range: 20-60 mg/day ^[1].

OCD: Start 20 mg/day. Target: 40 mg/day. Range: 20-60 mg/day ^[1].

PTSD: Start 20 mg/day. Range: 20-50 mg/day ^[1].

PMDD (Paxil CR): 12.5-25 mg/day. Can be continuous or luteal phase only ^[1].

Elderly: Start 10 mg/day. Maximum: 40 mg/day ^[1].

Paxil CR (controlled-release): Start 25 mg/day. Maximum 62.5 mg/day for some indications. CR may have slightly fewer GI side effects initially ^[1].

Discontinuation: Taper very gradually — reduce by 10 mg every 1-2 weeks minimum. Some patients require liquid formulation for very slow tapers (2.5 mg decrements). Paroxetine has the worst discontinuation syndrome of any SSRI ^{[1, 7, 8]}.

Generic paroxetine is very affordable, costing approximately $4-12/month for IR tablets ^[1].

Pharmacy discount programs: Available on most $4 generic lists. GoodRx prices are $3-10/month ^[1].

Paxil CR (controlled-release): Generic CR is available but slightly more expensive ($15-30/month). Brand Paxil CR: $200-400/month ^[1].

Brisdelle: Paroxetine 7.5 mg capsule approved specifically for vasomotor symptoms (hot flashes). Brand-only and expensive ($150-200/month). Generic paroxetine at the same dose is much cheaper ^[1].

Insurance: Generic paroxetine IR is Tier 1 on all formularies ^[1].

Pregnancy: Paroxetine is the only SSRI classified as FDA Pregnancy Category D (positive evidence of human fetal risk). Studies have shown an approximately 1.5-2 fold increased risk of cardiac malformations (particularly ventricular septal defects) with first-trimester exposure ^{[1, 9, 11]}. The absolute risk increase is small (from ~1% to ~1.5-2%), but paroxetine is generally avoided during pregnancy when other SSRIs (particularly sertraline) are available. If a patient becomes pregnant while on paroxetine, do NOT stop abruptly — taper gradually and discuss alternatives with the prescriber ^{[1, 9, 11]}.

Breastfeeding: Paroxetine is excreted in breast milk at very low levels. The relative infant dose is approximately 0.5-2.8% — one of the lowest of any SSRI ^{[10, 12]}. This makes it paradoxically one of the preferred SSRIs during breastfeeding, despite being avoided in pregnancy. Most guidelines consider paroxetine compatible with breastfeeding ^{[10, 12]}.

Bioequivalence: Generic paroxetine IR tablets are FDA AB-rated as bioequivalent to brand Paxil ^[1].

Clinical equivalence: Decades of use confirm equivalent efficacy between generic and brand ^[1].

Formulation options: Generic available as IR tablets (10, 20, 30, 40 mg), CR tablets, and oral suspension. Paxil CR generics are also available ^[1].

Cost: Brand Paxil IR: $200-400/month. Generic paroxetine IR: $4-12/month ^[1].