Phenelzine
Indications
| Indication | Approved Population | Therapy Type | Status |
|---|---|---|---|
| Depression (atypical, nonendogenous, neurotic) | Adults | Second-line or later monotherapy | FDA Approved |
Phenelzine is a non-selective, irreversible monoamine oxidase inhibitor of the hydrazine class, first approved in 1961. The FDA label specifically notes it is effective in patients with depression characterised as “atypical,” “nonendogenous,” or “neurotic” — patients who often present with mixed anxiety and depression, phobic features, or hypochondriacal symptoms. The label explicitly states that phenelzine should rarely be the first antidepressant used and is more suitable for patients who have failed other treatments. Despite its dietary and drug-interaction constraints, phenelzine remains one of the most effective antidepressants available, particularly for atypical depression and social anxiety disorder.
Social anxiety disorder — Six placebo-controlled RCTs demonstrate strong efficacy; effect size among the largest of any pharmacotherapy for social anxiety (ES ~1.02). (Evidence quality: High)
Panic disorder — Effective in multiple controlled trials; generally reserved for SSRI/SNRI failures. (Evidence quality: High)
Treatment-resistant depression — Commonly used when multiple first-line agents have failed; some clinicians regard MAOIs as underutilised in this setting. (Evidence quality: Moderate)
PTSD — Limited controlled data but clinical use in veterans and trauma populations. (Evidence quality: Low)
Bulimia nervosa — Some evidence for efficacy, though SSRIs are preferred. (Evidence quality: Low)
Dosing
Adult Dosing by Clinical Scenario
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| Atypical / treatment-resistant depression | 15 mg TID (45 mg/day) | 60–90 mg/day | 90 mg/day | Increase rapidly to ≥60 mg/day as tolerated; clinical response may take ≥4 weeks at 60 mg After maximum benefit, reduce slowly to maintenance as low as 15 mg/day or QOD |
| Social anxiety disorder (off-label) | 15 mg BID (30 mg/day) | 45–90 mg/day | 90 mg/day | Dose range from clinical trials: 45–90 mg/day; response typically within 6–8 weeks Start lower than for depression if tolerability is a concern |
| Panic disorder (off-label) | 15 mg QD–BID | 45–90 mg/day | 90 mg/day | Some patients respond to lower doses than needed for depression Anxiolytic effects may appear earlier than antidepressant effects |
Special Population Adjustments
| Population | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| Elderly (≥65 years) | 15 mg QD | Titrate cautiously | Individualize (lower doses) | Greater frequency of decreased hepatic, renal, and cardiac function Higher orthostatic hypotension risk; monitor BP closely |
| Hepatic impairment | Contraindicated | History of liver disease or abnormal LFTs is a contraindication per PI | ||
| Severe renal impairment | Contraindicated | Contraindicated in severe renal impairment or renal disease per PI | ||
The plasma elimination half-life of phenelzine is only ~11.6 hours, but this is clinically misleading. Because phenelzine irreversibly inactivates MAO, the duration of biological effect far outlasts the drug’s presence in plasma. After discontinuation, it takes approximately 2–3 weeks for the body to synthesise sufficient new MAO enzyme to restore normal monoamine metabolism. This is why dietary restrictions and drug washout periods must continue for at least 14 days (and up to 3–4 weeks for initiating another antidepressant) after stopping phenelzine.
Phenelzine Pharmacology
Mechanism of Action
Phenelzine is a hydrazine derivative that irreversibly inhibits both isoforms of monoamine oxidase — MAO-A and MAO-B — with a slight preference for MAO-A. By inactivating these enzymes, phenelzine prevents the intraneuronal breakdown of serotonin, norepinephrine, dopamine, epinephrine, and melatonin, leading to increased concentrations of these monoamines in the synaptic cleft. Beyond monoamine oxidase inhibition, phenelzine’s metabolite phenylethylidenehydrazine (PEH) inhibits GABA-transaminase, resulting in elevated brain GABA levels. This GABAergic action is thought to contribute significantly to phenelzine’s anxiolytic and antipanic properties. Phenelzine also inhibits alanine transaminase (ALA-T), which may be related to the rare hepatotoxicity associated with hydrazine MAOIs. Additionally, phenelzine is metabolised to phenylethylamine (PEA), a trace amine that acts as a releasing agent of norepinephrine and dopamine. The antidepressant effect typically requires 2–4 weeks to manifest, reflecting the time needed for cumulative MAO inhibition and downstream neuroadaptive changes.
ADME Profile
| Parameter | Value | Clinical Implication |
|---|---|---|
| Absorption | Rapidly absorbed; Cmax 19.8 ng/mL after 30 mg; Tmax ~43 minutes | Fast absorption allows TID dosing schedule; food effects not characterised in the PI |
| Distribution | Widely distributed; protein binding not formally characterised | Broad tissue distribution underlies diverse pharmacological effects throughout the body |
| Metabolism | Primarily by oxidation via MAO to phenylacetic acid and parahydroxyphenylacetic acid (73% urinary recovery); minor acetylation to N2-acetylphenelzine; metabolised to PEH and PEA | Phenelzine is both a substrate and an irreversible inhibitor of MAO — the drug destroys the enzyme it is metabolised by; acetylator phenotype is not a major determinant of clinical effect |
| Elimination | Plasma t½ ~11.6 h (single 30 mg dose); 73% recovered in urine within 96 h as metabolites; multiple-dose PK not studied | Plasma half-life does not reflect duration of biological effect (MAO inhibition is irreversible); dietary and drug restrictions must continue ≥14 days after last dose |
Side Effects
The Nardil PI (approved 1961) lists adverse effects by frequency category without specific percentages from controlled trials. The estimates below are derived from published clinical experience and reviews, with the source noted. Exact incidence figures should be interpreted as approximate ranges rather than precise percentages.
| Adverse Effect | Estimated Incidence | Clinical Note |
|---|---|---|
| Orthostatic hypotension | 20–30% | Most clinically significant common effect; peaks 10–14 days after dose increase; rise slowly from sitting or lying (PI, clinical literature) |
| Weight gain | 15–25% | Can be substantial (average 2–3 kg, some patients much more); increased appetite and carbohydrate craving (PI) |
| Drowsiness / sedation | 15–25% | Often improves with continued treatment; consider dosing adjustment (PI) |
| Sleep disturbance (insomnia / hypersomnia) | 15–25% | Insomnia from serotonergic and GABA effects; hypersomnia less common; take last dose before mid-afternoon (PI) |
| Dizziness | 15–25% | Related to orthostatic and central effects (PI) |
| Sexual dysfunction (anorgasmia, impotence) | 20–40% | Serotonergic effect; major cause of non-adherence; dose reduction may help (PI, clinical literature) |
| Dry mouth | 15–20% | Encourage oral hygiene, sugar-free gum (PI) |
| Constipation | 10–20% | Increase fibre and fluids (PI) |
| Headache | 10–15% | Distinguish from occipital headache of hypertensive crisis — latter is a medical emergency (PI) |
| Fatigue / weakness | 10–15% | Often improves over weeks (PI) |
| Peripheral edema | 10–15% | May respond to compression stockings or dose reduction (PI) |
| Tremor / twitching / myoclonic movements | 5–15% | Dose-related; myoclonus may indicate serotonergic excess (PI) |
| Adverse Effect | Estimated Incidence | Clinical Note |
|---|---|---|
| Paresthesias | ~5% | May relate to vitamin B6 depletion by hydrazine; pyridoxine supplementation can help (PI, clinical literature) |
| Blurred vision | ~5% | Rule out glaucoma if eye pain present (PI) |
| Urinary retention | <5% | Caution in prostatic hypertrophy (PI) |
| Pruritus / skin rash | <5% | Assess for hypersensitivity (PI) |
| Jitteriness / euphoria | <5% | May indicate hypomanic activation (PI) |
| Hypernatremia | <5% | Monitor sodium if symptoms of fluid retention (PI) |
| Elevated transaminases | Infrequent | Usually without clinical symptoms; may rarely precede hepatotoxicity (PI) |
| Adverse Effect | Estimated Frequency | Typical Onset | Required Action |
|---|---|---|---|
| Hypertensive crisis | Rare (with proper diet); potentially fatal | Minutes to hours after tyramine ingestion or drug interaction | Discontinue phenelzine immediately; IV phentolamine 5 mg; external cooling for fever; ED admission |
| Serotonin syndrome | Rare (with contraindicated drug combinations) | Hours after exposure to serotonergic agent | Discontinue all serotonergic agents; supportive care; cyproheptadine; ICU if severe |
| Hepatotoxicity (necrotizing hepatocellular) | Very rare; potentially fatal | Weeks to months | Monitor LFTs; discontinue immediately at first sign of jaundice or significant transaminase elevation |
| Suicidal ideation / behaviour | Uncommon (age-dependent) | First weeks to months | Close monitoring especially <25 years; consider stopping if emergent suicidality |
| Hypomania / mania | Uncommon | First weeks; more common in bipolar spectrum | Discontinue; assess for bipolar disorder; initiate mood stabiliser |
| Seizures | Rare | Variable | Discontinue phenelzine; manage seizures per protocol |
| Withdrawal syndrome (vivid nightmares, agitation, psychosis) | Uncommon (with abrupt discontinuation) | 24–72 hours after abrupt withdrawal | Reinstitute low-dose phenelzine; taper cautiously; never stop abruptly |
| Leukopenia | Very rare | Variable | Check FBC; discontinue if significant cytopaenia |
Hypertensive crises are characterised by sudden-onset occipital headache (may radiate frontally), palpitations, neck stiffness, nausea, vomiting, diaphoresis, dilated pupils, and photophobia. Tachycardia or bradycardia may be present. Intracranial haemorrhage has been reported. Treatment: discontinue phenelzine immediately; administer IV phentolamine 5 mg slowly; manage fever with external cooling. All patients must carry a list of prohibited foods and drugs, and should be instructed to seek emergency care immediately if they develop sudden severe headache.
Drug Interactions
Phenelzine has one of the most extensive and dangerous drug interaction profiles of any medication in clinical use. Because it irreversibly inhibits MAO, interactions persist for 2–3 weeks after discontinuation. The categories below highlight the most critical interactions; prescribers should also consult a comprehensive interaction database.
Monitoring
-
Blood Pressure
Every visit; orthostatic at baseline and during titration
Routine Orthostatic hypotension is the most common clinically significant effect. Measure lying and standing BP. Systolic drop ≥20 mmHg or diastolic drop ≥10 mmHg is significant. Also monitor for hypertensive episodes (headache, palpitations) that may signal dietary non-compliance. -
Liver Function
Baseline; periodically during treatment
Routine Contraindicated with abnormal LFTs or liver disease. Rare but fatal hepatocellular necrosis reported. Monitor transaminases; discontinue immediately if jaundice or significant elevation occurs. -
Weight
Baseline, then every 1–3 months
Routine Weight gain is common and can be substantial. Dietary counselling from treatment initiation. -
Mental State
Every visit for first 3 months
Routine Monitor for suicidality (especially <25 years), hypomania/mania, psychosis, and agitation. Screen for bipolar disorder before initiation. -
Dietary Compliance
Every visit
Routine Review tyramine-restricted diet and medication restrictions at every visit. Ensure patient has written list of prohibited foods and drugs. Assess understanding and adherence. -
Vitamin B6 Status
If paresthesias or neuropathic symptoms develop
Trigger-Based Phenelzine can deplete vitamin B6 via hydrazine metabolism. Pyridoxine supplementation (50–100 mg/day) may be considered if neuropathic symptoms arise. Use pyridoxine form, not pyridoxal. -
Blood Glucose
If diabetic
Trigger-Based MAOIs may increase insulin sensitivity. Monitor glucose closely in diabetic patients; insulin or oral hypoglycaemic dose may need reduction.
Contraindications & Cautions
Absolute Contraindications
- Hypersensitivity to phenelzine or any component
- Pheochromocytoma — catecholamine-secreting tumour; MAO inhibition causes catastrophic hypertension
- Congestive heart failure
- Severe renal impairment or renal disease
- History of liver disease or abnormal liver function tests
- Concurrent or recent use of: sympathomimetic drugs, meperidine, dextromethorphan, alcohol and CNS depressants, serotonergic agents (SSRIs, SNRIs, TCAs, triptans, tramadol, tryptophan), other MAOIs, bupropion, buspirone, linezolid, IV methylene blue, guanethidine, carbamazepine
- Tyramine-containing foods — aged cheeses (cottage cheese and cream cheese are allowed), pickled herring, liver, dry sausage, fava beans, sauerkraut, yogurt, beer, wine, yeast extract; excessive amounts of chocolate and caffeine
- General anaesthesia — discontinue ≥10 days before elective surgery
Relative Contraindications (Specialist Input Recommended)
- Bipolar disorder (unscreened) — risk of manic switching; screen before initiation
- Epilepsy — variable effect on seizure threshold; adequate precautions required
- Poorly controlled diabetes — increased insulin sensitivity may cause hypoglycaemia
Use with Caution
- Elderly — greater susceptibility to orthostatic hypotension; start low, go slow
- Patients at risk of suicide — prescribe smallest feasible quantity; MAOI overdose is frequently fatal
- Patients on antihypertensive therapy — exaggerated hypotension possible
- Asthma patients — effects on sympathetic neurotransmission may alter bronchodilator response
- Patients requiring surgery — discontinue ≥10 days before; avoid cocaine and local anaesthetics with sympathomimetic vasoconstrictors
Antidepressants, including phenelzine, increase the risk of suicidal thinking and behaviour in children, adolescents, and young adults (<25 years). In pooled analyses, the risk was 14 additional cases per 1,000 treated in those under 18 and 5 additional per 1,000 in those aged 18–24. Phenelzine is not approved for use in paediatric patients. All patients should be monitored for clinical worsening and emergence of suicidal thoughts, especially during initial months of therapy and at dose changes.
Patient Counselling
Purpose of Therapy
Phenelzine works by preventing the breakdown of important brain chemicals (serotonin, norepinephrine, and dopamine), allowing them to build up and relieve depression and anxiety. It is typically prescribed when other antidepressants have not worked well enough. The medication requires strict dietary restrictions because certain foods and drugs can cause a dangerous spike in blood pressure when combined with phenelzine.
How to Take
Take phenelzine exactly as prescribed, typically three times daily. It may take 2–4 weeks to notice benefit, and up to 6–8 weeks for full effect. Do not stop taking phenelzine abruptly — this can cause withdrawal symptoms. When your doctor is ready to stop the medication, the dose will be reduced gradually over several weeks.
Sources
- Nardil (phenelzine sulfate) Tablets — Full Prescribing Information. Parke-Davis Division of Pfizer Inc. DailyMed Primary source for FDA-approved indication, dosing, pharmacokinetics, adverse reactions, contraindications, and drug/food interactions.
- Liebowitz MR, Quitkin FM, Stewart JW, et al. Phenelzine v imipramine in atypical depression: a preliminary report. Arch Gen Psychiatry. 1984;41(7):669–677. DOI Landmark Columbia University trial establishing phenelzine superiority over imipramine and placebo in atypical depression (response rate 67% vs 43% vs 29%).
- Heimberg RG, Liebowitz MR, Hope DA, et al. Cognitive behavioral group therapy vs phenelzine therapy for social phobia: 12-week outcome. Arch Gen Psychiatry. 1998;55(12):1133–1141. DOI RCT comparing phenelzine with CBGT for social anxiety disorder, showing both superior to placebo with phenelzine having broader response.
- Blanco C, Heimberg RG, Schneier FR, et al. A placebo-controlled trial of phenelzine, cognitive behavioral group therapy, and their combination for social anxiety disorder. Arch Gen Psychiatry. 2010;67(3):286–295. DOI Follow-up RCT demonstrating combined phenelzine + CBGT is superior to either monotherapy for social anxiety disorder.
- Quitkin FM, Stewart JW, McGrath PJ, et al. Phenelzine versus imipramine in the treatment of probable atypical depression: defining syndrome boundaries of selective MAOI responders. Am J Psychiatry. 1988;145(3):306–311. DOI Further defined the atypical depression phenotype responsive to phenelzine, confirming superiority over imipramine and placebo.
- American Psychiatric Association. Practice guideline for the treatment of patients with major depressive disorder, 3rd ed. Am J Psychiatry. 2010;167(10 Suppl):1–152. DOI APA guideline recommending MAOIs for treatment-resistant depression after failure of multiple first-line agents.
- Bandelow B, Michaelis S, Wedekind D. Treatment of anxiety disorders. Dialogues Clin Neurosci. 2017;19(2):93–107. DOI Review of pharmacotherapy for anxiety disorders positioning phenelzine as a highly effective but second-line option for social anxiety.
- Baker GB, Coutts RT, McKenna KF, Sherry-McKenna RL. Insights into the mechanisms of action of the MAO inhibitors phenelzine and tranylcypromine: a review. J Psychiatry Neurosci. 1992;17(5):206–214. PubMed Review of phenelzine’s mechanisms beyond MAO inhibition, including GABA-transaminase inhibition and metabolite pharmacology.
- Bortolato M, Chen K, Shih JC. Monoamine oxidase inactivation: from pathophysiology to therapeutics. Adv Drug Deliv Rev. 2008;60(13–14):1527–1533. DOI Comprehensive review of MAO-A and MAO-B biochemistry and the pharmacological basis of irreversible MAO inhibition.
- Robinson DS, Nies A, Ravaris CL, Lamborn KR, Korson L. A multiple-dose controlled study of phenelzine in depression-anxiety states. Arch Gen Psychiatry. 1976;33(3):347–350. DOI Early controlled dose-response study establishing therapeutic dose range and side-effect profile of phenelzine.
- Blanco C, Schneier FR, Schmidt A, et al. Pharmacological treatment of social anxiety disorder: a meta-analysis. Depress Anxiety. 2003;18(1):29–40. DOI Meta-analysis showing phenelzine had the largest effect size (1.02) among medications studied for social anxiety disorder.
- Sunderland T, Mueller EA, Cohen RM, et al. Tyramine pressor sensitivity changes during deprenyl treatment. Psychopharmacology (Berl). 1985;86(4):432–437. DOI Study characterising tyramine pressor sensitivity during MAO inhibition, informing dietary restriction guidelines.
- Gillman PK. Monoamine oxidase inhibitors, opioid analgesics and serotonin toxicity. Br J Anaesth. 2005;95(4):434–441. DOI Definitive review of MAOI-opioid interactions, clarifying which opioids are safe and which are contraindicated with phenelzine.