Complete Guide to MAO Inhibitors (MAOIs) and Tricyclic Antidepressants (TCAs)

PART I: MONOAMINE OXIDASE INHIBITORS (MAOIs)

1. History

• Isocarboxazid (Marplan) and phenelzine (Nardil) followed in the late 1950s
• Tranylcypromine (Parnate) — modeled on the amphetamine structure — was introduced in the early 1960s
• Selegiline (Deprenyl) was developed as a selective MAO-B inhibitor for Parkinson disease and later repurposed as an antidepressant

2. Biochemistry & Mechanism of Action

The MAO Enzyme

Pharmacodynamic Mechanism

1. Direct MAO inhibition → increased monoamine concentration in the synapse (hours)
2. Secondary receptor adaptations (weeks) — the actual therapeutic correlate:
   • Reduced density of α₂, β-adrenergic, 5-HT₁, and 5-HT₂ receptors
   • Correction of HPA axis hyperactivation
   • Increased BDNF-mediated signaling
   • Induction of hippocampal neurogenesis
   • Tranylcypromine also activates trace amine-associated receptors (TAAR1)

3. Classification of MAOIs

4. Pharmacokinetics

Phenelzine

• Absorption: Well absorbed orally
• T_max: ~2–4 hours
• Half-life: ~12 hours
• Elimination: Urinary metabolites
• Metabolized by liver; avoid in hepatic disease or severe renal impairment

Isocarboxazid

• Absorption: Readily absorbed from GI tract
• T_max: 3–5 hours
• Half-life: ~2.5 hours
• Metabolism/excretion: Hepatic, urinary metabolites
• Avoid in hepatic or renal impairment

Tranylcypromine

• Absorption: Well absorbed
• Note: Has amphetamine-like stimulant properties; less weight gain than hydrazines

Selegiline (Oral)

• Extensive first-pass metabolism → bioavailability only ~4%
• Metabolized by CYP2A6, 2B6, 2C9, 3A4, 3A5
• Active metabolites: methamphetamine and amphetamine (urinary excretion)

Transdermal Selegiline (Emsam)

• ~23–30% absorbed through skin over 24 hours
• Bioavailability: ~74% (vs. 4% oral) — bypasses first-pass metabolism
• Protein binding: 90%
• t½ under steady-state: 18–25 hours
• Avoids gut MAO inhibition at the 6 mg dose → no tyramine diet required at lowest dose

5. Indications

• Panic disorder
• Social anxiety disorder (phenelzine has strong evidence here)
• OCD (phenelzine)
• Atypical depression (phenelzine is particularly effective for leaden paralysis, hypersomnia, reactive mood, rejection sensitivity)
• Treatment-resistant depression — MAOIs remain among the most effective drugs for TRD; patients who fail multiple SSRIs/SNRIs frequently respond to classic MAOIs
• ADHD (selegiline)
• Alzheimer disease (selegiline, off-label)
• Melancholic depression (tranylcypromine)

6. Dosage and Administration

7. Side Effect Profile

Cardiovascular

• Orthostatic hypotension — most common significant side effect, particularly with phenelzine; dose-related and usually transient; manage with gradual dose titration, adequate hydration, support stockings
• Hypertension (tyramine reaction — see below)
• Tachycardia, palpitations

Neurological / CNS

• Insomnia — common with tranylcypromine (amphetamine-like stimulation); manage with gradual titration; may abate over time
• Agitation, anxiety, restlessness
• Headache
• Dizziness
• Hyperreflexia, myoclonus (serotonin-related)
• Sedation (more with phenelzine)

Anticholinergic

• Dry mouth, constipation, urinary hesitancy, blurred vision (less prominent than TCAs)

Metabolic

• Weight gain — significant with phenelzine and isocarboxazid (hydrazines); typically absent with tranylcypromine or transdermal selegiline
• Edema
• Sexual dysfunction (orgasm/ejaculation disorders)

Hepatic

• Hepatotoxicity risk — especially with phenelzine and isocarboxazid; monitor LFTs

8. Drug Interactions (Critical)

The Tyramine Reaction ("Cheese Effect")

• Mechanism: MAO-A in the gut normally inactivates dietary tyramine. When MAO-A is inhibited, tyramine is absorbed systemically, enters presynaptic vesicles, and causes massive norepinephrine release → acute hypertensive crisis (occipital headache, diaphoresis, flushing, hypertensive hemorrhage)
• Onset: Minutes to hours after ingesting tyramine-rich food
• Foods to avoid: Aged cheeses (e.g., cheddar, camembert, stilton), fermented/cured meats (salami, pepperoni), broad bean pods (fava beans), aged/fermented soy products (miso, soy sauce, tofu), tap/draft beer, aged red wine, pickled herring, overripe/spoiled fruits
• Note: Modern expert consensus confirms that with contemporary dietary guidance, serious hypertensive events are now very rare

Serotonin Syndrome

• Combining MAOIs with serotonergic drugs (SSRIs, SNRIs, TCAs, tramadol, meperidine, dextromethorphan, triptans, linezolid) risks life-threatening serotonin syndrome: hyperthermia, clonus, agitation, rigidity
• Washout: Fluoxetine requires a 5-week washout before starting MAOI (long t½); other SSRIs require 2-week washout; TCAs require 2-week washout

Sympathomimetic Drugs

• Indirect sympathomimetics (amphetamines, pseudoephedrine, dopamine, ephedrine) → hypertensive crisis / sympathomimetic crisis

Meperidine (Pethidine)

• Absolute contraindication — can cause hyperthermia, rigidity, convulsions, death
• Morphine and other opioids are safer alternatives (use cautiously)

Hypoglycemics

• MAOIs potentiate insulin and oral hypoglycemics → hypoglycemia risk

9. Contraindications

• Cerebrovascular disease / recent stroke
• Pheochromocytoma
• Concomitant use of: other MAOIs, SSRIs, SNRIs, meperidine, dextromethorphan, tramadol, sympathomimetics, direct/indirect vasopressors, buspirone, carbamazepine, cyclobenzaprine
• Severe hepatic impairment (phenelzine, isocarboxazid)
• Hypersensitivity to the drug
• Age < 16 years (insufficient data for classic MAOIs)
• Elective surgery requiring general anesthesia (hold 2 weeks prior)

10. Special Populations

11. Discontinuation

• Abrupt discontinuation can cause confusion, irritability, vivid dreams, and rebound anxiety/depression
• Taper gradually when possible
• After stopping an MAOI, wait at least 14 days before starting another antidepressant (2 weeks is standard; this is essential to prevent serotonin syndrome)
• After stopping fluoxetine, wait 5 weeks before starting an MAOI
• After stopping an MAOI, wait 14 days before starting a TCA

12. Toxicity / Overdose

• Supportive: Benzodiazepines for agitation/seizures
• Cyproheptadine (serotonin antagonist) for serotonin syndrome features
• Phentolamine or nitroprusside for hypertensive crisis (avoid propranolol — unopposed α blockade)
• Avoid sympathomimetics
• ICU monitoring for 24 hours minimum

PART II: TRICYCLIC ANTIDEPRESSANTS (TCAs)

1. History

• Amitriptyline (1961) — became the most prescribed TCA worldwide
• Nortriptyline, desipramine — secondary amine metabolites with better tolerability
• Clomipramine (1960s) — the most serotonin-selective TCA; became first proven OCD treatment
• Doxepin, trimipramine, protriptyline expanded the class through the 1960s–70s
• TCAs dominated antidepressant therapy for 30 years until SSRIs arrived in the late 1980s–90s

2. Chemistry

• Amitriptyline (Elavil), imipramine (Tofranil), clomipramine (Anafranil), doxepin (Sinequan), trimipramine (Surmontil)

• Nortriptyline (metabolite of amitriptyline), desipramine (metabolite of imipramine), protriptyline (Vivactil)

• Maprotiline (Ludiomil), amoxapine (Asendin)

3. Pharmacodynamics

• Week 1–2: Initial reuptake blockade → serotonin/NE rise → presynaptic autoreceptor activation → firing rate decreases (paradoxical)
• Week 2–4: Autoreceptor desensitization → normalization of firing → enhanced neurotransmission
• Chronic: β-adrenergic receptor downregulation, 5-HT₁A upregulation, 5-HT₂ downregulation

4. Pharmacokinetics

• Most TCAs: 2–8 hours
• Protriptyline: 6–12 hours
• Maprotiline: 8+ hours

• Highly lipophilic, basic amines → large volume of distribution (concentrated in tissues)
• Heart concentrations exceed plasma concentrations (explains cardiac toxicity)
• Plasma protein binding: ≥90% (exceptions: hydroxymetabolites have lower binding)
• Cross the blood-brain barrier readily
• P-glycoprotein (P-gp) actively transports several TCAs (amitriptyline, nortriptyline, imipramine, desipramine, doxepin, trimipramine) out of the brain — encoded by ABCB1 gene; polymorphisms affect drug response

• Extensive first-pass hepatic metabolism
• Demethylation: Tertiary amines → secondary amine active metabolites (amitriptyline → nortriptyline; imipramine → desipramine)
• Hydroxylation: CYP2D6 is the primary enzyme (genetic polymorphism critical — poor metabolizers have much higher blood levels)
• CYP1A2, CYP3A4 also involved
• Active hydroxyl metabolites contribute to both efficacy and toxicity

• Half-life generally long: 17–40 hours for most; protriptyline 55–92 hours
• Allows once-daily dosing (usually given at night for sedating TCAs)
• Renal excretion of metabolites

• Routine TDM recommended for nortriptyline, imipramine, desipramine
• Nortriptyline therapeutic window: 50–150 ng/mL (curvilinear response — both low and high levels may be less effective)
• Amitriptyline + nortriptyline combined: 150–250 ng/mL
• Clomipramine: 150–500 ng/mL
• TDM essential in elderly, cardiac patients, poor metabolizers, suspected non-compliance, toxicity

5. Indications

• Major depressive disorder (all TCAs)
• Obsessive-compulsive disorder — clomipramine (only approved antidepressant for OCD before SSRIs; remains first-line)
• Enuresis — imipramine (nocturnal bed-wetting in children >6 years)
• Attention-deficit disorder — imipramine (adults)

• Chronic pain — amitriptyline and nortriptyline are first-line for neuropathic pain (diabetic neuropathy, postherpetic neuralgia), fibromyalgia, headache prophylaxis
• Migraine prophylaxis — amitriptyline
• Irritable bowel syndrome — low-dose TCAs (visceral analgesia)
• Panic disorder — imipramine
• Generalized anxiety disorder
• PTSD
• Low-dose doxepin (3–6 mg) — FDA-approved for insomnia (Silenor)
• Atypical facial pain, complex regional pain syndrome
• Clomipramine for premature ejaculation

6. Dosage and Administration

7. Side Effect Profile

Anticholinergic (most prominent with tertiary amines)

Cardiovascular

• Orthostatic hypotension (α₁ blockade) — most clinically significant in elderly; amitriptyline and imipramine worst; nortriptyline has lowest orthostatic risk among tertiary/secondary amines
• QTc prolongation and QRS widening (Na⁺ channel blockade in His-Purkinje system) — key overdose risk
• T-wave flattening, PR prolongation
• Bundle branch block
• At therapeutic doses: clinically significant only in pre-existing cardiac disease (ischemic heart disease, conduction abnormalities)
• Avoid in post-MI patients and those with significant arrhythmias; nortriptyline preferred if a TCA must be used in cardiac disease

CNS

• Sedation (H₁ blockade) — amitriptyline, doxepin, trimipramine most sedating; useful for insomnia
• Seizures — dose-related; clomipramine 0.5% at ≤250 mg/d, 1.67% at >250 mg/d; maprotiline at >225 mg/d; risk markedly increased in overdose
• Fine rapid tremor — dose-dependent, marker of elevated blood levels (>300 ng/mL)
• Delirium (see anticholinergic above)
• Amoxapine: dopamine blockade via 7-OH metabolite → tardive dyskinesia risk, NMS risk

Metabolic / Endocrine

• Weight gain — significant (antihistaminic mechanism), especially amitriptyline and doxepin
• Sexual dysfunction — particularly prominent with clomipramine
• Elevated liver enzymes (rare acute hepatitis — discontinue if severe)

Other

• Skin rash, photosensitivity (rare)
• Blood dyscrasias (very rare)

8. Contraindications

• Acute recovery phase following myocardial infarction
• Concurrent or recent (≤14 days) MAOI use
• QTc >500 ms or known long QT syndrome (QTc-prolonging TCAs)
• Known hypersensitivity
• Narrow-angle glaucoma (most TCAs)

• Severe hepatic impairment
• Epilepsy (lower seizure threshold)
• Bipolar disorder (can precipitate mania — mood stabilizer cover required)
• Urinary retention/BPH
• Severe constipation / ileus
• Acute porphyria (some agents)
• Pediatric patients under 12 (except imipramine for enuresis)
• Elderly: use with extreme caution (falls, delirium, cardiac effects) — listed on Beers Criteria

9. Special Populations

• TCAs are listed on the Beers Criteria as potentially inappropriate medications for older adults
• Increased sensitivity to anticholinergic, sedating, and orthostatic effects
• If a TCA must be used, nortriptyline or desipramine are preferred (least anticholinergic, lower orthostatic risk)
• Start at 50% of adult dose; titrate slowly

• No definitive teratogenic link established, though isolated case reports exist
• TCAs cross the placenta; neonatal withdrawal syndrome possible (tachypnea, cyanosis, irritability, poor suckling)
• If possible, discontinue 1 week before delivery
• Excretion in breast milk: concentrations similar to plasma, but infant levels usually undetectable/very low due to small absolute quantities
• Risk-benefit discussion required for each patient

• Imipramine approved for enuresis (>6 years, 10–25 mg QHS)
• Clomipramine approved for OCD in children ≥10 years
• Black box warning: Antidepressants (including TCAs) increase suicidal thinking/behavior in children, adolescents, and young adults — monitor closely
• Cardiac monitoring recommended (baseline and periodic ECG in children)

10. Drug Interactions

11. Discontinuation

• Cholinergic rebound: Nausea, vomiting, diarrhea, diaphoresis, excessive salivation (most pronounced with tertiary amines)
• GI cramping, flu-like symptoms
• Anxiety, agitation, insomnia, vivid dreams
• Rarely: parkinsonism-like symptoms, akathisia

12. Toxicity / Overdose

1. Na⁺ channel blockade (His-Purkinje and ventricular myocardium) → QRS widening → ventricular tachycardia/fibrillation
2. Anticholinergic toxidrome → tachycardia, hyperthermia, dry skin, urinary retention, delirium
3. α₁ blockade → hypotension
4. GABA-A blockade → seizures
5. Respiratory depression

1. Sodium bicarbonate IV (50–100 mEq bolus, 1–2 mEq/kg) — key antidote for QRS widening; raises extracellular Na⁺ and alkalinizes to overcome Na⁺ channel blockade; target serum pH 7.45–7.55
2. Benzodiazepines — for seizures (first-line, NOT phenytoin)
3. Lipid emulsion therapy — intralipid for severe refractory toxicity
4. Norepinephrine — for hypotension (phenylephrine is also used; avoid dopamine)
5. Avoid physostigmine (seizure risk)
6. Avoid flumazenil (lowers seizure threshold)
7. Continuous cardiac monitoring; ICU admission
8. Gastric lavage if early presentation (<1 hour); activated charcoal

PART III: MAOI + TCA INTERACTIONS AND COMBINED USE

• Tranylcypromine + trimipramine (least serotonergic TCA)
• Phenelzine + amitriptyline (historical use)

• Start both together at low doses (do NOT add a MAOI to an established TCA regimen)
• Avoid clomipramine and imipramine (most serotonergic TCAs)
• Monitor intensively
• This should only be done by experienced psychopharmacologists

PART IV: RECENT ADVANCES (2023–2026)

1. MAOI rehabilitation: A landmark 2026 review in J Clin Psychiatry ("MAOI Antidepressants: A History Being Rewritten," Van den Eynde et al.) argues that fear of MAOIs is based on historical misinformation and outdated beliefs, and under modern dietary guidance serious hypertensive events are virtually absent. Expert consensus calls for their revival in treatment-resistant depression.
2. Network meta-analysis (2024): Giménez-Palomo et al. (Acta Psychiatr Scand 2024, PMID 39001570) confirmed MAOIs' efficacy and tolerability for depressive episodes in mood disorders, strengthening the evidence base.
3. Atypical depression NMA (2025): Fornaro et al. (Eur Neuropsychopharmacol 2025, PMID 40412292) showed MAOIs remain among the most effective treatments for atypical depression.
4. Tranylcypromine pharmacotherapy review (2025): Ulrich & Lewitzka (Fortschr Neurol Psychiatr 2025) reviewed key aspects and trends in tranylcypromine prescribing practice.
5. Transdermal delivery advances: Extended-release MAO-B inhibitor formulations maintain therapeutic levels for up to 18 hours vs. 8–10 hours with immediate-release. Nanoparticle-based delivery for enhanced blood-brain barrier penetration is under research.
6. Selegiline transdermal (Emsam) data: The 6 mg patch maintains antidepressant efficacy while sparing gut MAO-A, eliminating the need for dietary restrictions at the lowest dose.
7. TAAR1 mechanism: Emerging understanding that tranylcypromine activates trace amine-associated receptor 1 (TAAR1) — this may contribute to efficacy in melancholic depression beyond simple MAO inhibition.
8. TCA in pain medicine: Growing evidence for low-dose TCAs in IBS, central sensitization, and fibromyalgia — with mechanistic understanding of their visceral analgesic properties increasingly being elucidated.
9. Pharmacogenomics: CYP2D6 genotyping increasingly used to predict TCA and MAOI dose requirements, particularly for personalized dosing of nortriptyline, desipramine, and amitriptyline.

Quick Reference: Class Comparison

• Historical discovery from iproniazid (TB drug, 1952) through selegiline
• Full biochemistry of MAO-A vs MAO-B enzyme subtypes
• Classification table (irreversible/reversible × selective/non-selective)
• Pharmacokinetics of each agent individually (phenelzine, isocarboxazid, tranylcypromine, oral/transdermal selegiline)
• Detailed dosing table with formulations
• Indications (FDA-approved + off-label, especially treatment-resistant depression and atypical depression)
• Complete side effect profile (CV, CNS, metabolic, hepatic)
• Drug and food interactions with mechanisms (tyramine reaction, serotonin syndrome)
• Contraindications, special populations, discontinuation, and overdose management

• History from imipramine (1957, Kuhn) through the full class development
• Chemistry (tertiary amines vs secondary amines vs tetracyclics)
• Multi-receptor pharmacodynamics table with clinical correlates
• Detailed pharmacokinetics including P-glycoprotein, CYP2D6 genetics, and TDM targets
• Full dosing table for all 9+ agents
• Side effects by system (anticholinergic, cardiac, CNS, metabolic)
• Overdose management with sodium bicarbonate as the key antidote
• All special populations including pregnancy, elderly (Beers Criteria), children