Monoamine oxidase inhibitors (MAOIs)

Article written and reviewed by Dr Luca Moretti

Monoamine oxidase inhibitors, or MAOIs, are antidepressants that reduce monoamine breakdown by inhibiting MAO-A and/or MAO-B enzymes. They are effective drugs but require specific knowledge of drug interactions, washout periods and, for classic irreversible nonselective MAOIs, interactions with dietary tyramine. For this reason they are not routinely used as first-line treatment for uncomplicated major depression, but retain a role in difficult-to-treat forms and in carefully selected patients.

The class is heterogeneous. Phenelzine, tranylcypromine and isocarboxazid are classic irreversible, nonselective inhibitors of MAO-A and MAO-B; moclobemide is a reversible and relatively selective MAO-A inhibitor; selegiline is selective for MAO-B at low doses but progressively loses selectivity as exposure increases. These differences affect efficacy, dietary requirements, interactions and safety profile, so the same rules cannot be applied indiscriminately to all MAOIs.

Recent meta-analyses confirm that MAOIs have antidepressant efficacy and that their underuse does not reflect a lack of clinical activity. The main issue is management complexity. Appropriate prescribing requires complete medication reconciliation, awareness of over-the-counter products and substances of misuse, adequate dietary counselling, and an explicit plan for any treatment changes.

MAO-A, MAO-B and pharmacological basis

Monoamine oxidases are mitochondrial enzymes found in numerous tissues, including the central nervous system and gastrointestinal tract. MAO-A preferentially metabolizes serotonin and norepinephrine and also contributes to the metabolism of dopamine and tyramine; MAO-B is particularly involved in the metabolism of phenethylamine and dopamine, with overlap in substrates between the two isoforms. Inhibition therefore alters the availability of several monoamines and cannot be described simply as an increase in a single neurotransmitter.

Classic MAOIs form a functionally irreversible bond with the enzyme. After discontinuation, MAO activity returns mainly through synthesis of new enzyme, a process that takes days and explains why interactions can persist after the last dose. This is the pharmacological basis for the washout periods required before introducing many antidepressants or other incompatible drugs.

Moclobemide inhibits MAO-A reversibly. Its effect can be overcome by high substrate concentrations and resolves more rapidly after discontinuation than with classic MAOIs. This results in a lower, though not zero, risk of tyramine interaction and different management of treatment switches. The lower risk does not, however, justify uncontrolled combinations with serotonergic drugs.

At low oral doses, selegiline predominantly inhibits MAO-B and is used mainly in Parkinson disease. At exposures required for a systemic antidepressant effect, its selectivity decreases. In the United States, a transdermal formulation is approved for major depressive disorder; this indication and its associated dietary rules should not be automatically extrapolated to formulations and authorizations available in Europe or Italy.

Synaptic monoamine levels increase rapidly, but the full antidepressant response requires slower neurobiological adaptations. As with other classes, there is no clinical test that demonstrates an “MAO deficiency” or identifies which patient will respond. Selection is based on treatment history, the overall clinical phenotype, and the balance between potential efficacy and safety complexity.

Tranylcypromine also has a structure related to amphetamines and may have more activating effects, with insomnia or agitation in some patients. Phenelzine may be more commonly associated with sedation, weight gain and sexual dysfunction. These differences can be clinically useful, but they do not replace individual assessment or justify dosing that is inconsistent with product information.

Pharmacokinetics are influenced by formulation, metabolism and individual variability. With irreversible MAOIs, the duration of the pharmacodynamic effect does not correspond to the plasma half-life: the drug may be eliminated while the enzyme remains inhibited. This principle is essential for understanding why interactions persist after discontinuation.

Classic MAOIs: phenelzine, tranylcypromine and isocarboxazid

Phenelzine, tranylcypromine and isocarboxazid are the classic MAOIs most widely represented in the international literature. They inhibit MAO-A and MAO-B nonselectively and can be effective in treatment-resistant depression. Their use varies considerably between countries because availability, regulatory approvals and prescriber familiarity differ substantially.

Historically, MAOIs have been considered particularly effective in depression with atypical features, such as mood reactivity, hypersomnia, hyperphagia, “leaden paralysis” and rejection sensitivity. This association is supported by older studies, but it should not be turned into a rule that every patient with hypersomnia or increased appetite should receive an MAOI. Today, their main role is in specialist treatment after adequate therapeutic failures.

At the most advanced treatment level of the STAR*D study, tranylcypromine produced remission in a minority of patients after multiple treatment failures and did not demonstrate superiority over the venlafaxine-mirtazapine combination. This finding illustrates two points: MAOIs can work even after numerous treatments, but they are not universally more effective and require individualized selection.

Adverse effects include orthostatic hypotension, dizziness, insomnia or sedation, edema, weight gain, sexual dysfunction, headache and, with some agents, anticholinergic or hepatic effects. Hypotension can be dose-limiting during titration and requires assessment of hydration, other antihypertensive medications and fall risk.

Insomnia is particularly relevant with activating agents and may be reduced by scheduling doses earlier in the day when the formulation allows. Rapid dose escalation can worsen hypotension and other adverse effects. Titration should therefore be gradual and specific to the individual agent.

The main practical barrier is the risk of tyramine-induced hypertensive crisis. Intestinal and hepatic MAO normally metabolize dietary tyramine; their inhibition allows greater systemic absorption, with norepinephrine release and a possible abrupt rise in blood pressure. Risk depends on the actual tyramine content of food, dose, and the degree of MAO inhibition.

Modern dietary guidance does not require indiscriminate elimination of every food traditionally included on older restriction lists. Tyramine content is particularly high in aged, fermented, spoiled or improperly stored products. Recommendations should be current and practical, avoiding both alarmism and excessive reassurance.

Classic MAOIs require specialist expertise, but they are not “impossible” drugs to use. Modern reviews emphasize that many interactions historically considered absolute are more nuanced; this does not, however, justify experimental combinations without specific expertise. In a monograph intended for general clinical practice, it is safer to recommend case-by-case verification of combinations and specialist input when moving beyond established combinations.

Moclobemide and reversible MAOIs

Moclobemide is a reversible and relatively selective MAO-A inhibitor. By inhibiting mainly MAO-A, it increases the availability of serotonin, norepinephrine and, indirectly, dopamine, while reversibility shortens the duration of inhibition after discontinuation. Moclobemide is used as an antidepressant in several jurisdictions; however, availability, marketing status and approved indications are not uniform across countries and should be checked for the specific product available at the time of prescribing.

Comparative studies and meta-analyses indicate antidepressant efficacy superior to placebo and broadly comparable to that of SSRIs in many studied populations. The adverse-effect profile differs from that of SSRIs and tends to involve less sexual dysfunction and some gastrointestinal adverse effects, but no advantage is universal.

The risk of a pressor reaction to tyramine is much lower than with irreversible nonselective MAOIs because MAO-A inhibition is reversible and tyramine can compete with the drug. This does not mean that any amount of tyramine-rich food is safe under all circumstances. Dose, product characteristics and regulatory recommendations must be respected.

Moclobemide is generally less sedating and less anticholinergic than tricyclic antidepressants. Common adverse effects include insomnia, agitation, nausea, dizziness and headache. In patients with bipolar vulnerability, any antidepressant may promote activation or switching; the emergence of pathological euphoria, reduced need for sleep or a marked increase in activity requires diagnostic reassessment.

Combination with serotonergic antidepressants can cause serotonin syndrome. Reversibility therefore does not make a direct switch without an interval safe. Discontinuation and initiation intervals are generally shorter than those required after an irreversible MAOI, but they must be verified in the product information for the two specific drugs.

Dosage depends on the product information and is generally titrated according to response and tolerability. A single “universal” dose should not be quoted without considering formulation, indication, age, hepatic function and interactions. Exposure may increase in patients with hepatic impairment and dose adjustment may be required.

In terms of treatment positioning, moclobemide may be an option when other classes have not been effective or tolerated and when its safety profile is advantageous. However, there is insufficient evidence to regard it as systematically superior to SSRIs or faster acting in every patient.

Selegiline and the transdermal formulation

Selegiline irreversibly inhibits MAO-B at low doses and is used mainly in Parkinson disease. An antidepressant effect requires broader monoamine oxidase inhibition. In the United States, the transdermal selegiline formulation, known as EMSAM, is approved for major depressive disorder.

The transdermal route reduces intestinal drug exposure and, at the lowest transdermal dose approved in the United States, tyramine dietary restrictions are less stringent than with classic oral MAOIs. At higher transdermal doses, dietary restrictions again become necessary. These rules apply to the specific US formulation and should not be applied to oral selegiline used for Parkinson disease.

Randomized trials and pooled analyses show that the transdermal system has antidepressant efficacy compared with placebo. Adverse effects include application-site reactions, insomnia, headache, hypotension and other effects typical of MAOIs. Important drug interactions also persist with the transdermal formulation.

Combination with SSRIs, SNRIs and other incompatible serotonergic drugs is contraindicated or requires washout according to product information. The fact that the lowest dose permits a less restrictive diet does not imply similar flexibility regarding drug interactions.

In the Italian and European context, selegiline should not be presented as a common antidepressant option equivalent to SSRIs or moclobemide. Approved indications and available formulations differ from those in the United States. The EMSAM literature is useful for understanding MAOI pharmacology but should be clearly identified as applying to a specific jurisdiction.

This regulatory principle is general: when comparing antidepressants internationally, evidence of efficacy and authorization for use are separate issues. An accurate clinical page should state when a use is approved only in certain countries.

Drug interactions and serotonin syndrome

MAOI interactions are the most important safety issue. Combination with drugs that markedly increase serotonergic neurotransmission can cause serotonin syndrome, characterized by changes in mental status, autonomic hyperactivity and neuromuscular signs such as clonus and hyperreflexia. SSRIs, SNRIs and clomipramine are among the combinations that must be avoided according to the required washout rules.

Not all tricyclic antidepressants have the same serotonergic activity. Modern specialist literature discusses the use of some predominantly noradrenergic TCAs in specific circumstances, but these strategies should not be interpreted as routine combinations. Clomipramine is particularly incompatible because of its potent SERT inhibition.

Some analgesics have significant serotonergic activity. Tramadol, meperidine/pethidine and other opioids with serotonergic properties can cause severe reactions with MAOIs. The choice of analgesics, antitussives and perioperative drugs should therefore be planned with the anesthesiologist or prescribing physician, because commonly used products can become hazardous.

Dextromethorphan, found in several over-the-counter cough products, is an example of a drug that can be problematic. Indirect sympathomimetics used as decongestants or stimulants can also cause marked increases in blood pressure. Patients should be instructed not to take over-the-counter medications, supplements or recreational substances without checking for interactions.

Amphetamines and other stimulants are a specialist area. Some modern sources describe combinations used in expert centers, but pressor and serotonergic risks make it inappropriate to present them as generally safe. In routine practice they should be considered high-risk combinations requiring specific expertise.

The pharmacodynamic interaction can persist after the drug is no longer measurable in blood, especially with irreversible MAOIs. For this reason, washout periods are determined by recovery of enzyme activity and the half-life of the preceding drug, not simply by the date of the last tablet.

After fluoxetine, for example, the interval before starting an MAOI must be longer than with most SSRIs because of the long half-lives of fluoxetine and norfluoxetine. Vortioxetine and other antidepressants also have specific intervals. Exact timings should be taken from the current product information for the drugs involved rather than from a single mnemonic rule.

Suspected serotonin syndrome or hypertensive crisis requires urgent assessment. Management should not be left to the patient through generic instructions. Prevention through medication reconciliation, documentation of MAOI treatment in the medical record and communication to all treating clinicians is clearly preferable.

Tyramine, hypertensive crisis and diet

Tyramine is a biogenic amine produced by decarboxylation of tyrosine and can accumulate in aged, fermented or spoiled foods. It is normally metabolized by MAO in the intestinal wall and liver. With an irreversible nonselective MAOI, a greater proportion reaches the circulation and promotes norepinephrine release from sympathetic nerve endings.

The most feared consequence is a hypertensive crisis with severe headache, palpitations, sweating and a marked rise in blood pressure. In more severe cases, cardiovascular or cerebrovascular complications may occur. The risk is dose-dependent and varies with the actual tyramine content, which can differ substantially among apparently similar products.

Modern reviews have shown that many historical food restriction lists were excessively restrictive because they were based on production methods no longer common or on nonquantitative data. Foods of greatest concern remain those with high tyramine content, particularly certain aged cheeses, concentrated yeast extracts, fermented products, and foods that have been improperly stored or spoiled.

Patients should receive an up-to-date, understandable list, preferably produced by the center or service prescribing the MAOI. A simple instruction such as “do not eat cheese or drink wine” is insufficient, as is indiscriminate elimination of large food groups. Explaining the mechanism is important because it improves adherence and helps patients recognize unforeseen situations.

With moclobemide, dietary risk is much lower but depends on dose and product-information recommendations. With US transdermal selegiline, restrictions vary according to patch dose. These differences make it unsafe to use a single dietary table for every MAOI.

The onset of sudden severe headache, chest pain, dyspnea, significant palpitations or neurological symptoms during treatment should be considered an alarm sign and requires urgent assessment. It is not appropriate to advise unsupervised self-treatment with antihypertensive medication at home.

Appropriate dietary management reduces risk without unnecessarily impairing quality of life. Dietary education is therefore an integral part of prescribing classic MAOIs, alongside dose selection and management of drug interactions.

Clinical selection, monitoring and discontinuation

Before starting an MAOI, the diagnosis should be confirmed, previous treatments reconstructed, and apparent treatment failures checked for adequate dose, duration and adherence. Bipolar disorder, substance use, cardiovascular disease, hypotension, liver disease, concomitant medications and over-the-counter products should be assessed. An MAOI is particularly unsuitable when reliable follow-up cannot be ensured or the patient cannot understand and follow the safety rules.

Blood pressure, including the orthostatic component, is an important clinical parameter. Weight and liver function may be monitored when relevant to the specific drug and the patient’s profile. An ECG is indicated when history, age, cardiac disease or concomitant medications make it appropriate.

Titration should follow product information and tolerability. Increasing the dose too rapidly raises the risk of hypotension and other adverse effects without guaranteeing a faster response. Efficacy is assessed after an adequate period at a therapeutic dose, taking severity and the history of treatment resistance into account.

Stopping an MAOI does not immediately end the risk of interactions. After irreversible MAOIs, recovery of enzyme activity must be allowed before many incompatible antidepressants are introduced. After moclobemide, the interval may be shorter but remains specific to the subsequent drug.

Switching from an antidepressant to an MAOI is one situation in which a written plan is particularly useful. It should specify the date of the last dose, any tapering, washout duration, planned MAOI start date, drugs to avoid and warning signs. This reduces errors between different prescribers.

Clinical response should be reassessed together with functioning, sleep, weight, blood pressure, sexual adverse effects and dietary adherence. If a patient achieves remission after previous treatment failures, long-term continuation may be justified provided safety and preferences remain favorable.

MAOIs should be regarded neither as obsolete drugs to be excluded a priori nor as treatments to be “rediscovered” without caution. Their role is that of effective antidepressants with high management complexity, particularly useful when the likelihood of benefit justifies the burden of monitoring and interaction prevention.

Summary table

Bibliography
1. Giménez-Palomo A et al. Efficacy and tolerability of monoamine oxidase inhibitors for the treatment of depressive episodes in mood disorders: A systematic review and network meta-analysis. Acta Psychiatr Scand. 150(6), 2024: 500-515.
2. Van den Eynde V et al. The prescriber’s guide to classic monoamine oxidase inhibitors (phenelzine, tranylcypromine, isocarboxazid) for treatment-resistant depression. CNS Spectr. 28(4), 2023: 427-440.
3. Lam RW et al. Canadian Network for Mood and Anxiety Treatments (CANMAT) 2023 Update on Clinical Guidelines for Management of Major Depressive Disorder in Adults. Can J Psychiatry. 69(9), 2024: 641-687.
4. Cipriani A et al. Comparative efficacy and acceptability of 21 antidepressant drugs for the acute treatment of adults with major depressive disorder: a systematic review and network meta-analysis. Lancet. 391(10128), 2018: 1357-1366.
5. Gillman PK. A reassessment of the safety profile of monoamine oxidase inhibitors: elucidating tired old tyramine myths. J Neural Transm. 125(11), 2018: 1707-1717.
6. Gillman PK. Much ado about nothing: monoamine oxidase inhibitors, drug interactions, and dietary tyramine. CNS Spectr. 22(5), 2017: 385-387.
7. Papakostas GI et al. A metaanalysis of clinical trials comparing moclobemide with selective serotonin reuptake inhibitors for the treatment of major depressive disorder. Can J Psychiatry. 51(12), 2006: 783-790.
8. McGrath PJ et al. Tranylcypromine versus venlafaxine plus mirtazapine following three failed antidepressant medication trials for depression: a STAR*D report. Am J Psychiatry. 163(9), 2006: 1531-1541.
9. Robinson DS et al. Treatment effects of selegiline transdermal system on symptoms of major depressive disorder: a meta-analysis of short-term, placebo-controlled, efficacy trials. Psychopharmacol Bull. 40(3), 2007: 15-28.
10. Robinson DS et al. The selegiline transdermal system in major depressive disorder: a systematic review of safety and tolerability. J Affect Disord. 105(1-3), 2008: 15-23.
11. Gillman PK et al. Revitalizing monoamine oxidase inhibitors: a call for action. CNS Spectr. 25(4), 2020: 452-454.
12. National Institute for Health and Care Excellence. Depression in adults: treatment and management. NICE guideline NG222, 2022, current revision accessed August 19, 2026.