---

SNRIs, NDRIs & Serotonin Modulators — Complete Pharmacological Reference

---

PART I: SEROTONIN-NOREPINEPHRINE REUPTAKE INHIBITORS (SNRIs)

---

Overview & History

• Venlafaxine (Effexor, Effexor XR) — approved 1993
• Desvenlafaxine (Pristiq) — approved 2008; active metabolite of venlafaxine
• Duloxetine (Cymbalta) — approved 2004
• Levomilnacipran (Fetzima) — approved 2013; active enantiomer of milnacipran

---

Chemistry

• Venlafaxine/Desvenlafaxine: bicyclic compounds; differ by a methyl group (venlafaxine R=CH₃, desvenlafaxine R=H)
• Duloxetine: three-ring structure unrelated to TCAs
• Milnacipran/Levomilnacipran: cyclopropane ring; milnacipran is a racemate; levomilnacipran is the active (1S,2R) enantiomer

---

Mechanism of Action

---

1. VENLAFAXINE

History

Pharmacokinetics

• Absorption: Well absorbed orally; ~45% first-pass effect for immediate-release; XR formulation extends absorption across colon
• Bioavailability: ~45% (IR), higher with XR
• Tmax: 2–4 h (IR), 5.5 h (XR)
• Protein binding: ~27% (low — minimal displacement interactions)
• Metabolism: Primarily by CYP2D6 to active metabolite O-desmethylvenlafaxine (ODV = desvenlafaxine); also minor CYP3A4 and CYP2C19 contributions
• Half-life: Venlafaxine ~5 h; ODV ~11 h — clinically, the combination provides a longer effective duration
• Elimination: Primarily renal (87% of dose recovered in urine)
• Special populations: Dose reduction in renal impairment (GFR <30: reduce by 50%), hepatic impairment (reduce by 50%), elderly (no mandatory adjustment but monitor BP)

Pharmacodynamics

• Low dose (<150 mg/day): Predominantly SERT inhibition (SSRI-like profile)
• Medium dose (150–225 mg/day): SERT + NET inhibition
• High dose (>225 mg/day): SERT + NET + weak DAT inhibition
• No significant affinity for muscarinic, H1, D2, or adrenergic receptors

Indications (FDA-Approved)

1. Major Depressive Disorder (MDD) — all severities
2. Generalized Anxiety Disorder (GAD) — XR only; 6-month trials confirmed efficacy at 75–225 mg/day
3. Social Anxiety Disorder — XR only (12-week studies)
4. Panic Disorder — XR only

Off-Label Uses

• OCD, ADHD, chronic pain syndromes, neuropathic pain, fibromyalgia, PTSD, hot flashes (especially post-menopausal / breast cancer patients avoiding HRT), PMDD, migraine prophylaxis, agoraphobia, cocaine dependence with comorbid depression

Contraindications

• Concurrent or within 14 days of MAOI use (serotonin syndrome risk)
• Uncontrolled narrow-angle glaucoma
• Hypersensitivity
• Concurrent use of linezolid or IV methylene blue

Formulations & Doses

Administration Routes

Side Effects & Safety Profile

• Nausea — most common, peaks at initiation; preempt with food, gradual titration, or short-term ondansetron/mirtazapine
• Headache, dizziness, insomnia, somnolence, dry mouth
• Sweating (noradrenergic)
• Sexual dysfunction: decreased libido, delayed orgasm/ejaculation (30–40% with direct assessment)

• Constipation, asthenia, nervousness, tremor, blurred vision
• Hypertension — dose-dependent, sustained diastolic BP elevation at higher doses; monitor BP; unique among SNRIs in this prominence
• Tachycardia, palpitations

• Serotonin syndrome (especially with MAOIs, linezolid, triptans, tramadol, St. John's Wort)
• Hyponatremia / SIADH (especially elderly)
• Bleeding risk (antiplatelet effect from platelet 5-HT depletion)
• QTc prolongation (low risk, less than TCAs)
• Black box warning: Increased suicidal ideation in children, adolescents, young adults <25 (class effect)
• Activation of mania/hypomania in bipolar patients

Drug Interactions

• MAOIs: 14-day washout required each direction; 7 days washout from venlafaxine before starting MAOI
• CYP2D6 inhibitors (fluoxetine, paroxetine, bupropion, ritonavir): increase venlafaxine levels; may paradoxically reduce ODV
• CYP2D6 poor metabolizers (7–10% of Caucasians): higher venlafaxine, lower ODV; clinical effect generally preserved
• Triptans: serotonin syndrome risk
• Tramadol, fentanyl: serotonin syndrome
• Anticoagulants/NSAIDs/aspirin: increased bleeding risk
• Linezolid / IV methylene blue: avoid combination (serotonin syndrome)
• Alcohol: additive CNS depression; may worsen hepatic effects

Special Populations

• Pregnancy (Category C): Neonatal adaptation syndrome (jitteriness, irritability, respiratory distress) if used in third trimester; persistent pulmonary hypertension of newborn (PPHN) reported with SSRIs/SNRIs; weigh risk vs benefit
• Lactation: Low transfer to breast milk; consider infant monitoring
• Pediatrics: Not FDA-approved; use with caution; black box warning for suicidality
• Elderly: Lower clearance; hyponatremia risk; falls risk; monitor BP; no mandatory dose reduction but start low
• Renal impairment: Dose reduce 25–50% based on GFR
• Hepatic impairment: Reduce by ~50%; use IR (easier titration)

Discontinuation Syndrome

• "FINISH" mnemonic: Flu-like symptoms, Insomnia, Nausea, Imbalance/dizziness, Sensory disturbances (electric shock/zaps), Hyperarousal/agitation
• Paresthesias, vivid dreams, irritability, anxiety
• Management: Taper over ≥4 weeks (longer if on high doses or long duration); some patients require months-long tapers; bridging to fluoxetine (long t½) then stopping fluoxetine can help

---

2. DESVENLAFAXINE

History

Pharmacokinetics

• Bioavailability: ~80% (higher than venlafaxine because it bypasses CYP2D6-dependent first-pass conversion)
• Metabolism: Primarily by phase II glucuronidation (not CYP2D6 phase I) — more predictable across CYP2D6 phenotypes
• Half-life: ~11 h (consistent with ODV component of venlafaxine)
• Protein binding: ~30%
• Elimination: Renal (45% unchanged in urine)
• Advantage: No CYP2D6 drug interaction profile; no active metabolite complexity

Doses

• Standard: 50 mg once daily (fixed dose; no strong evidence that higher doses are more effective for MDD)
• Maximum studied: 400 mg/day (no added efficacy)
• Available as: 25, 50, 100 mg extended-release tablets (do not crush/chew)

Indications

• FDA: MDD only
• Off-label: Vasomotor symptoms (menopause) — strong evidence, especially for women with breast cancer contraindicated from estrogens; GAD; perimenopause depression

Side Effects

Discontinuation

---

3. DULOXETINE

History

Pharmacokinetics

• Bioavailability: ~50% (enteric coating prevents gastric degradation)
• Protein binding: >90% (high — clinically significant)
• Metabolism: CYP1A2 (major) and CYP2D6 (minor); hepatically cleared
• Half-life: ~12 h (range 8–17 h)
• Elimination: Primarily urinary metabolites; <1% unchanged drug
• Special note: Avoid in severe renal impairment (GFR <30) and contraindicated with substantial alcohol use — hepatotoxicity risk; avoid in hepatic impairment

Mechanism

Indications (FDA-Approved)

1. MDD
2. GAD
3. Diabetic Peripheral Neuropathic Pain (DPNP) — landmark approval, first antidepressant for this
4. Fibromyalgia
5. Chronic Musculoskeletal Pain (including chronic low back pain, osteoarthritis)
6. Social Anxiety Disorder (later approval)

Off-Label

• Stress urinary incontinence (approved in Europe as Yentreve, not USA)
• Chemotherapy-induced peripheral neuropathy
• Chronic pelvic pain, PTSD, ADHD

Formulations & Doses

Side Effects

• Nausea (most common, especially at initiation)
• Dry mouth, constipation, hyperhidrosis
• Somnolence or insomnia
• Sexual dysfunction
• Hepatotoxicity: Rare but serious; avoid with heavy alcohol use; check LFTs if jaundice develops
• Urinary retention: Due to NE effects on urinary sphincter — monitor in men with BPH
• BP elevation (less than venlafaxine)
• Orthostatic hypotension
• Hyponatremia

Contraindications

• MAOIs (within 14 days)
• Uncontrolled narrow-angle glaucoma
• Significant liver disease or heavy alcohol use
• Concurrent use of linezolid or methylene blue
• Severe renal impairment (GFR <30)

Drug Interactions

• CYP1A2 inhibitors (fluvoxamine, ciprofloxacin): markedly increase duloxetine levels → increase side effects
• CYP1A2 inducers (rifampin, smoking): reduce duloxetine levels
• CYP2D6 inhibitors: modest additional interaction
• MAOIs: serotonin syndrome — 5-day washout from duloxetine before MAOI
• Thioridazine: avoid (duloxetine inhibits CYP2D6, increasing thioridazine levels and QTc risk)
• CNS depressants: additive sedation

Discontinuation

---

4. LEVOMILNACIPRAN

History

Mechanism — Special Feature

Pharmacokinetics

• Bioavailability: ~92% (high)
• Protein binding: ~22% (low)
• Metabolism: CYP3A4 (major) desethylation; minor CYP2C8, 2C19, 2D6
• Half-life: ~12 h
• Elimination: Renal (58% unchanged — highest renal excretion of any SNRI)
• Dose adjustment: Renal impairment — reduce dose significantly; avoid in severe renal failure

Indications

• FDA: MDD only

Formulations & Doses

• 20, 40, 80, 120 mg extended-release capsules
• Start: 20 mg/day × 2 days → 40 mg/day
• Target: 40–120 mg/day once daily
• Maximum: 120 mg/day

Side Effects

• Similar to other SNRIs; tachycardia and palpitations more prominent (NE-dominant)
• Urinary hesitancy/retention (most NE-prominent SNRI — most likely to cause this)
• Hypertension
• Hyperhidrosis, nausea, constipation
• Sexual dysfunction

Drug Interactions

• CYP3A4 inhibitors (ketoconazole, ritonavir): markedly increase levels — do not exceed 80 mg/day with strong CYP3A4 inhibitors
• CYP3A4 inducers: reduce efficacy

---

PART II: NOREPINEPHRINE-DOPAMINE REUPTAKE INHIBITOR (NDRI)

---

BUPROPION

History

Chemistry

Mechanism of Action

• Weakly blocks DAT (dopamine transporter) and NET (norepinephrine transporter)
• Does not inhibit SERT → no serotonergic side effects, no sexual dysfunction, no weight gain
• Also a nicotinic acetylcholine receptor antagonist → contributes to smoking cessation efficacy
• Its active metabolites (especially (+)-6-hydroxybupropion = radafaxine) are more potent NET/DAT inhibitors than the parent compound and are concentrated in the brain — bupropion is both an active drug and a prodrug
• PET studies show 10–30% striatal DAT occupancy at therapeutic doses — lower than classical stimulants but sufficient for antidepressant effect via accumulation of active metabolites

Pharmacokinetics

• Protein binding: 84%
• Metabolism: Primarily CYP2B6 (unique among antidepressants) → hydroxybupropion (active), threohydrobupropion, erythrohydrobupropion
• Half-life: Parent ~21 h; hydroxybupropion ~24 h
• Elimination: Urinary (87%); <1% unchanged
• CYP2D6 inhibitor: Bupropion and hydroxybupropion potently inhibit CYP2D6 — clinically important drug interaction

Indications (FDA-Approved)

1. Major Depressive Disorder (MDD) — all three formulations
2. Seasonal Affective Disorder (SAD) — XL only (Wellbutrin XL, Aplenzin)
3. Smoking Cessation — as Zyban (SR formulation, 150 mg); reduces craving by blocking nAChRs and boosting dopamine

Off-Label Uses

• ADHD (second-line after stimulants) — NE/DA mechanism provides benefit
• Bipolar depression — lower risk of inducing mania vs SSRIs/SNRIs; often used with mood stabilizer
• Sexual dysfunction induced by SSRIs — add bupropion to counteract
• Weight management — as Contrave (bupropion/naltrexone combination)
• PTSD, cocaine dependence, fatigue in cancer/MS patients
• Opioid use disorder (adjunctive)
• Dextromethorphan-bupropion (Auvelity) — FDA-approved 2022 for MDD (rapid onset; bupropion inhibits CYP2D6, raising dextromethorphan levels, which acts as NMDA antagonist)

Contraindications

• Seizure disorder (dose-dependent seizure threshold lowering)
• Eating disorders (anorexia nervosa, bulimia nervosa) — highest seizure risk (electrolyte imbalances)
• Current or recent MAOI use (within 14 days)
• Abrupt alcohol/benzodiazepine/antiepileptic withdrawal (seizure risk additive)
• Linezolid or IV methylene blue
• Hypersensitivity

Formulations & Doses

Administration

• Oral only
• Give with food to reduce nausea and peak plasma concentration
• XL tablets should not be crushed (SR and XL are different — do not substitute)
• Zyban (smoking cessation): Start 1–2 weeks before quit date; 150 mg/day × 3 days → 150 mg BID; treat for 7–12 weeks

Side Effects & Safety Profile

• Insomnia — most common; do not dose in evenings; morning dosing preferred
• Dry mouth
• Headache, dizziness
• Nausea (less than SSRIs/SNRIs)
• Agitation, restlessness, anxiety ("activating" profile)
• Tremor, sweating
• Tachycardia (mild)

• ✅ No sexual dysfunction (major advantage)
• ✅ Minimal weight gain (weight-neutral to weight-loss)
• ✅ No serotonin syndrome risk (no serotonergic mechanism)
• ✅ No anticholinergic effects

• Seizures (dose-dependent — see above)
• Hypertension (especially when combined with nicotine replacement therapy during smoking cessation)
• Neuropsychiatric events (hostility, agitation, depressed mood during smoking cessation — black box warning later removed for Zyban, re-evaluated)
• Black box warning: Suicidal ideation in children, adolescents, young adults <25 (class antidepressant effect); neuropsychiatric events in smoking cessation (reduced since 2016 re-evaluation)

Toxicity & Overdose

• Seizures are the hallmark toxicity — may occur even at therapeutic doses if maximum single-dose limits exceeded
• Symptoms: Agitation, tremor, seizures, tachycardia, dry mouth, nausea
• Management: Supportive; benzodiazepines for seizures; no specific antidote; activated charcoal if early ingestion; cardiac monitoring

Drug Interactions

• CYP2D6 substrates: Bupropion strongly inhibits CYP2D6 — increases levels of codeine (blocks activation — reduced analgesia), metoprolol (bradycardia risk), TCAs, antipsychotics, flecainide, propafenone
• MAOIs: Avoid — hypertensive crisis risk
• Ritonavir/lopinavir: Reduce bupropion efficacy
• Levodopa: Increased NE/DA may increase adverse effects
• Nicotine patches: Combined BP elevation during smoking cessation — monitor
• Alcohol: Lowers seizure threshold; avoid excessive use
• Carbamazepine, rifampin (CYP2B6 inducers): Reduce bupropion levels

Special Populations

• Pregnancy: Category C; no established teratogenicity; used when benefits outweigh risks; avoid in first trimester if possible; neonatal jitteriness reported
• Lactation: Excreted in breast milk; risk of infant seizures (case reports); weigh carefully
• Elderly: Lower seizure threshold with age; consider lower doses; minimal cardiac risk advantage over TCAs
• Pediatric: Not FDA-approved for depression; used off-label for ADHD; caution
• Renal impairment: Use caution — metabolite accumulation; reduce dose
• Hepatic impairment: Reduce dose significantly (SR: 100 mg/day; XL: 150 mg every other day)
• Eating disorders (AN/BN): Contraindicated — highest seizure risk

Discontinuation

• Bupropion has a low discontinuation syndrome risk compared to SSRIs/SNRIs (no serotonergic withdrawal)
• May experience increased fatigue, irritability, depression relapse
• Taper over 2 weeks if used long-term; generally well tolerated
• Nicotine craving may return after stopping Zyban

---

PART III: SEROTONIN MODULATORS

1. Mirtazapine — NaSSA (Noradrenergic & Specific Serotonergic Antidepressant)
2. Trazodone — SARI (Serotonin Antagonist and Reuptake Inhibitor)
3. Nefazodone — SARI (less used; hepatotoxicity)
4. Vilazodone — SPARI (Serotonin Partial Agonist and Reuptake Inhibitor)
5. Vortioxetine — SMS (Serotonin Multimodal/Modulator and Stimulator)

---

1. MIRTAZAPINE

History

Mechanism of Action (NaSSA) — Unique

• α₂ autoreceptor blockade → disinhibits NE release from noradrenergic neurons (increased NE firing)
• α₂ heteroreceptor blockade on serotonin neurons → disinhibits 5-HT release (increased serotonin)
• Postsynaptic 5-HT₂A antagonism → antidepressant, antianxiety, improved sleep architecture (enhanced slow-wave sleep), reduced sexual dysfunction vs SSRIs
• Postsynaptic 5-HT₂C antagonism → increased appetite, weight gain (disinhibits dopamine release in reward areas)
• Postsynaptic 5-HT₃ antagonism → antiemetic effect (mechanism of ondansetron); reduced nausea; this makes mirtazapine useful alongside chemotherapy
• H₁ antagonism (potent) → sedation, weight gain
• Moderately potent α₁ and muscarinic antagonism

Pharmacokinetics

• Oral bioavailability: ~50% (first-pass effect)
• Absorption: Rapid and complete; Tmax ~2 h
• Protein binding: ~85%
• Metabolism: CYP1A2, CYP2D6, CYP3A4 (demethylation, hydroxylation, glucuronidation)
• Half-life: ~20–40 h (mean ~30 h) → once-daily dosing
• Elimination: Urinary (75%), fecal (15%)
• Special populations: Clearance reduced 30% in hepatic impairment, up to 50% in renal impairment; 40% slower in elderly men, 10% slower in elderly women

Indications (FDA-Approved)

• MDD (all subtypes)

Key Off-Label Uses

• Insomnia (especially in depressed patients)
• Nausea/vomiting (chemotherapy-associated; cancer patients)
• Appetite stimulation (cancer cachexia, HIV/AIDS)
• Augmentation of SSRIs/SNRIs (California rocket fuel = mirtazapine + venlafaxine — covers NE, 5-HT, blocks side effects of each)
• PTSD (reduces nightmares via 5-HT₂A blockade)
• GAD, OCD

Formulations & Doses

• Start: 15 mg at bedtime
• Titrate after 1–2 weeks to 30 mg, then 45 mg if needed
• Maximum: 45 mg/day
• Always give at bedtime due to sedation

Administration

• Oral only
• SolTab: place on tongue, allow to dissolve, can be swallowed without water — useful for patients unable to swallow tablets

Side Effects & Safety Profile

• Somnolence/sedation — >50% of patients; most pronounced at low doses; main reason to give at night; often considered a therapeutic side effect
• Weight gain — in ~30% of patients; due to H₁ + 5-HT₂C blockade; often significant (>7% body weight); prescribe cautiously in obese patients
• Increased appetite — ravenous hunger in some
• Dry mouth

• Dizziness (7%)
• Constipation
• Asthenia

• ✅ No sexual dysfunction (5-HT₂A blockade actually improves sexual function)
• ✅ No nausea (5-HT₃ antagonist)
• ✅ Antiemetic effect (beneficial in oncology)
• ✅ No drug-drug interactions at CYP level (minimal pharmacokinetic interactions)

• Agranulocytosis — rare (1–2/1000) but documented; monitor if fever, sore throat, infection signs
• Mania/hypomania induction in bipolar (similar to other antidepressants)
• Seizures (no increased risk demonstrated)
• Serotonin syndrome (rare; can occur with other serotonergic agents)
• Hyponatremia (SIADH)

Drug Interactions

• MAOIs: Contraindicated (serotonin syndrome risk despite different mechanism)
• CNS depressants (benzodiazepines, alcohol, opioids): Potentiates sedation — warn patients
• CYP inducers/inhibitors: CYP1A2 inducers (smoking, carbamazepine) reduce mirtazapine levels; CYP3A4 inhibitors (azole antifungals, ritonavir) increase levels
• Relatively minimal pharmacokinetic drug interactions — major advantage

Toxicity & Overdose

• Mirtazapine is much safer in overdose than TCAs
• Symptoms: Excessive sedation, confusion, tachycardia; rarely fatal alone
• Management: Supportive care; monitor cardiac rhythm; no antidote

Special Populations

• Elderly: Excellent choice (no anticholinergic effects, antiemetic, appetite stimulant, sedating = treats multiple geriatric symptoms); but sedation increases fall risk — counsel
• Pregnancy: Limited data; generally considered safer; neonatal adaptation syndrome possible
• Lactation: Low milk transfer; generally considered compatible
• Anorexia/cachexia patients: Beneficial weight gain; often used in cancer/HIV
• Hepatic/renal impairment: Reduce dose; monitor

Discontinuation

• Lower risk than SSRIs/SNRIs; taper over 2 weeks
• Rebound insomnia possible; rebound anxiety; vivid dreams
• No "electric shock" sensations (serotonergic withdrawal)

---

2. TRAZODONE

History

Mechanism of Action (SARI)

• 5-HT₂A antagonism → improves sleep architecture, reduces anxiety; underpins hypnotic use
• α₁-adrenergic antagonism → orthostatic hypotension; sedation
• H₁ antagonism → sedation

• SERT inhibition → antidepressant effect
• 5-HT₂C antagonism → antidepressant, activating
• 5-HT₁A partial agonism → anxiolytic
• α₂ antagonism → increased NE/5-HT release

Pharmacokinetics

• Bioavailability: ~75% (increases with food — take with food)
• Protein binding: 89–95%
• Metabolism: CYP3A4 primarily → m-CPP
• Half-life: ~5–9 h (IR); controlled-release (OAD formulation) achieves smoother, blunted peak concentrations
• Elimination: Urinary

Indications

• FDA-approved: MDD (though rarely used as primary antidepressant due to sedation at antidepressant doses in IR formulation)
• Off-label (most common clinical use): Insomnia — extremely widely used; the most-prescribed non-benzodiazepine sleep aid
• Other off-label: Anxiety, PTSD (nightmare reduction), alcohol dependence, fibromyalgia, eating disorders

Formulations & Doses

Side Effects & Safety

• Sedation — profound at all doses; this is the primary hypnotic mechanism but limits daytime use at antidepressant doses
• Orthostatic hypotension (α₁ blockade) — significant; risk of falls, syncope
• Dizziness, headache, dry mouth
• Nausea (especially m-CPP effects)
• Blurred vision

• ⚠️ Priapism — rare (1/6,000–8,000 men) but urological emergency; may require surgical intervention; counsel male patients; discontinue immediately if prolonged erection occurs
• Cardiac arrhythmias: Less arrhythmogenic than TCAs but may prolong QTc; avoid in recent MI or unstable angina
• Hyponatremia/SIADH

• ✅ No significant anticholinergic effects (vs TCAs)
• ✅ No sexual dysfunction (5-HT₂A blockade)
• ✅ No significant weight gain
• ✅ Safer in overdose than TCAs

Drug Interactions

• MAOIs: Avoid (serotonin syndrome)
• CYP3A4 inhibitors (azole antifungals, ritonavir): Increase trazodone levels → sedation, hypotension
• CYP3A4 inducers (rifampin, carbamazepine): Reduce trazodone levels
• Other serotonergic drugs: Serotonin syndrome risk
• CNS depressants: Additive sedation
• Antihypertensives: Additive hypotension
• Digoxin/phenytoin: Trazodone may increase their serum levels

Special Populations

• Elderly: Preferred for insomnia over benzodiazepines (no dependence) BUT orthostatic hypotension and falls risk is significant — start at 12.5–25 mg
• Pregnancy: Limited data; considered relatively safe for insomnia; category C
• Hepatic impairment: Use with caution; reduce dose

Discontinuation

• Mild; taper over 1–2 weeks if used at higher doses
• Rebound insomnia possible
• No significant serotonergic discontinuation syndrome

---

3. NEFAZODONE

History

Mechanism

• 5-HT₂A antagonism (primary)
• SERT inhibition (stronger than trazodone at therapeutic doses)
• α₁ antagonism, weak H₁ antagonism
• Metabolized to active metabolite hydroxynefazodone and m-CPP (same as trazodone)
• Potent CYP3A4 inhibitor — significant drug interaction burden

Key Differences from Trazodone

• More potent SERT inhibition → better antidepressant efficacy
• Less α₁ blockade → less orthostatic hypotension
• No priapism
• But: CYP3A4 inhibition makes it dangerous with many co-medications; and black box warning for hepatic failure (rare but potentially fatal)

Doses

• IR tablets: 100–600 mg/day in divided doses; start 100 mg BID

Contraindications

• Active hepatic disease; prior nefazodone-induced hepatic injury
• Concurrent terfenadine, astemizole, cisapride, pimozide (CYP3A4 interactions → QTc)
• MAOIs

Clinical Status

---

4. VILAZODONE

History

Mechanism (SPARI — Serotonin Partial Agonist and Reuptake Inhibitor)

• SERT inhibition — potent (similar to SSRIs)
• 5-HT₁A partial agonism — anxiolytic, putative faster antidepressant onset
• Net: Increases synaptic 5-HT via reuptake inhibition AND stimulates 5-HT₁A receptors → additive serotonergic enhancement

Pharmacokinetics

• Bioavailability: ~72% — food-dependent: bioavailability increases ~147% with a high-fat meal and ~65% with a light meal; must take with food
• Protein binding: ~96–99%
• Metabolism: Primarily CYP3A4; minor CYP2C19, CYP2D6; non-CYP pathways
• Half-life: ~25 h → once-daily dosing
• Active metabolites (minor)

Indications

• FDA-approved: MDD only
• Off-label: GAD, anxiety symptoms in depression

Formulations & Doses

• Tablets: 10, 20, 40 mg
• Start: 10 mg/day × 7 days → 20 mg/day × 7 days → 40 mg/day
• Must titrate slowly to reduce GI side effects
• Maximum: 40 mg/day
• Must be taken with food (otherwise inadequate absorption)

Side Effects

• Diarrhea (most common — more than other antidepressants; due to 5-HT₁A GI effects)
• Nausea, vomiting
• Dizziness, insomnia
• Sexual dysfunction (similar to SSRIs — SERT mechanism)

• Similar efficacy and tolerability
• Some trials suggest slightly faster onset of anxiolytic effect (5-HT₁A agonism)
• No weight gain advantage over SSRIs
• More GI side effects (diarrhea)

Drug Interactions

• CYP3A4 inhibitors: Increase vilazodone levels → limit dose to 20 mg/day with strong inhibitors
• CYP3A4 inducers: Reduce efficacy; may need dose increase to 80 mg/day (max) with strong inducers
• MAOIs: Contraindicated
• Serotonergic agents: Serotonin syndrome risk
• Anticoagulants: Increased bleeding risk

Special Populations

• Pregnancy: Limited data; similar precautions to SSRIs
• Elderly: Hyponatremia risk; start low; no mandatory dose adjustment
• Hepatic/Renal impairment: No dose adjustment needed for mild-moderate; data limited in severe impairment

Discontinuation

• Moderate risk (SERT inhibition component similar to SSRIs)
• Taper over 2+ weeks; watch for dizziness, nausea, paresthesias

---

5. VORTIOXETINE

History

Mechanism (SMS — Serotonin Modulator and Stimulator)

1. SERT inhibition (like SSRIs)
2. 5-HT₃ receptor antagonism → disinhibits glutamate release in prefrontal cortex → enhances downstream DA, NE, ACh, histamine; pro-cognitive
3. 5-HT₇ receptor antagonism → modulates circadian rhythms, cognitive flexibility
4. 5-HT₁D receptor antagonism → enhances 5-HT release (presynaptic)
5. 5-HT₁B receptor partial agonism → fine-tuning of 5-HT release
6. 5-HT₁A receptor agonism → anxiolytic; enhances 5-HT activity via autoreceptor stimulation

Pharmacokinetics

• Bioavailability: ~75%; not food-dependent
• Protein binding: ~98%
• Metabolism: Primarily CYP2D6 (major), with CYP3A4/5, CYP2A6, CYP2C8/9/19 contributing; glucuronide conjugation
• Half-life: ~66 h — very long; once-daily dosing; slower drug levels build-up and washout
• Elimination: Urinary (>59%), fecal (26%)
• CYP2D6 poor metabolizers: Levels ~2× higher — reduce maximum dose to 10 mg/day

Indications

• FDA-approved: MDD only
• Off-label: Cognitive impairment in depression, GAD

Formulations & Doses

• Tablets: 5, 10, 20 mg
• Start: 10 mg once daily
• Target: 10–20 mg/day
• Poor CYP2D6 metabolizers: Maximum 10 mg/day
• CYP2D6 inhibitors co-prescribed: Maximum 10 mg/day
• CYP2D6 inducers co-prescribed: Consider up to 3× the original dose (max 40 mg)

Side Effects

• Nausea (most common, especially at initiation; dose-dependent; peaks week 1–2)
• Diarrhea, constipation, dry mouth
• Dizziness
• Sexual dysfunction (present but may be less than SSRIs — 5-HT₁A agonism partially ameliorates)

• ✅ Pro-cognitive effects — superior to escitalopram in processing speed, executive function, verbal learning in multiple RCTs; likely the most cognitively enhancing antidepressant
• ✅ Minimal weight gain (compared to SSRIs)
• ✅ Minimal discontinuation syndrome (long half-life provides its own natural taper)
• ✅ No QTc prolongation

• Serotonin syndrome (with MAOIs, triptans, tramadol)
• Hyponatremia (SIADH)
• Increased bleeding risk

Drug Interactions

• CYP2D6 inhibitors (bupropion, fluoxetine, paroxetine): Double vortioxetine levels → reduce dose to 10 mg/day
• CYP2D6 inducers (rifampin, carbamazepine): May need dose increase (max 3× baseline)
• MAOIs: 14-day washout; 21-day washout from vortioxetine before MAOI (long t½)
• Other serotonergic agents: Additive serotonin syndrome risk
• Linezolid, methylene blue: Contraindicated

Special Populations

• Elderly: Well tolerated; pro-cognitive effects particularly beneficial in age-related cognitive slowing; minimal anticholinergic burden; start 5 mg/day; no mandatory reduction
• Cognitive impairment/MCI: Emerging evidence for benefit
• Pregnancy: Limited data; category C; similar precautions to SSRIs
• Hepatic impairment: No dose adjustment for mild-moderate; not studied in severe

Discontinuation

• Lowest discontinuation risk among serotonergic antidepressants (long t½ of 66 h provides intrinsic gradual washout)
• Still taper over 1–2 weeks for doses >10 mg or prolonged use
• Less "electric shock" / zap sensation than SSRIs

---

COMPARATIVE SUMMARY TABLE

---

SEROTONIN SYNDROME — CROSS-CLASS EMERGENCY

• MAOIs
• Other serotonergic antidepressants
• Triptans (sumatriptan, etc.)
• Opioids (tramadol, fentanyl, meperidine)
• Stimulants (MDMA, cocaine)
• Dextromethorphan
• Linezolid, IV methylene blue
• St. John's Wort
• Lithium

---

DISCONTINUATION SYNDROME — RISK HIERARCHY

---

RECENT ADVANCES (2023–2025)

1. Dextromethorphan-Bupropion (Auvelity): FDA-approved 2022 for MDD; bupropion's CYP2D6 inhibition elevates dextromethorphan → NMDA antagonism (rapid-onset antidepressant). Represents the first NDRI-NMDA combination product. 2024–2025 data support broader MDD use.
2. Vortioxetine for cognitive dysfunction: Multiple RCTs and meta-analyses confirm superiority over escitalopram in cognitive processing speed, executive function, and verbal memory in depressed patients — increasingly recognized in treatment guidelines for patients with prominent cognitive complaints.
3. Levomilnacipran for anhedonia and motivation: Growing literature supporting its NE-preferring profile for energy and motivational symptoms, including post-COVID fatigue/depression.
4. Hyponatremia risk across all antidepressants: 2024 systematic review confirmed hyponatremia risk across all marketed antidepressants, including newer agents (vortioxetine, desvenlafaxine, vilazodone) — monitor sodium in elderly patients starting any agent in this class.
5. Antidepressant discontinuation in pediatrics: 2025 systematic review (PMID 39469761) highlighted need for individualized, prolonged tapering strategies in children and adolescents — abrupt stopping of SNRIs especially problematic.
6. Neuroimmune modulation: 2025 Nature data showing SNRIs modulate cytokine networks (TNF-α, IL-6) — potentially explaining partial benefit in inflammatory conditions and chronic pain, opening new research directions.
7. Menopause guidelines (2024): Venlafaxine and desvenlafaxine confirmed as first-line alternatives to HRT for vasomotor symptoms of menopause in women with contraindications to estrogen (e.g., breast cancer) — now formally recommended in multiple society guidelines.

---

Sources: Kaplan & Sadock's Synopsis of Psychiatry (10th ed.), Stahl's Essential Psychopharmacology (5th ed.), Katzung's Basic and Clinical Pharmacology (16th ed.), Lippincott Illustrated Reviews: Pharmacology, Goodman & Gilman's Pharmacological Basis of Therapeutics, Maudsley Prescribing Guidelines (15th ed.), Maudsley Deprescribing Guidelines; PubMed 2023–2025 (PMIDs 39469761, 38186654, 38014714, 37794650).