How Does Tranylcypromine Work? Mechanism of Action Explained in Plain English

If you or someone you know takes tranylcypromine, you might wonder how exactly it works. MAOIs are some of the oldest antidepressants in existence, but the science behind them is fascinating — and understanding the mechanism can help you make sense of why the dietary and drug restrictions are so important.

What Is Monoamine Oxidase?

Monoamine oxidase (MAO) is an enzyme — a protein that performs a specific chemical job in the body. MAO's job is to break down monoamine neurotransmitters. These are the brain chemicals responsible for regulating mood, motivation, energy, and anxiety, including:

Serotonin — Regulates mood, sleep, and appetite

Dopamine — Controls motivation, reward, and pleasure

Norepinephrine — Involved in alertness, energy, and the stress response

MAO comes in two forms: MAO-A and MAO-B. MAO-A primarily breaks down serotonin, norepinephrine, and dopamine. MAO-B is more selective for dopamine and trace amines (like phenethylamine).

How Tranylcypromine Inhibits MAO

Tranylcypromine is a nonselective, irreversible MAO inhibitor. Let's unpack what that means:

Nonselective: It inhibits both MAO-A and MAO-B (with a slight preference for MAO-B). This leads to increased levels of all three major monoamines — serotonin, dopamine, and norepinephrine — plus trace amines like phenethylamine.

Irreversible: Tranylcypromine permanently binds to and inactivates MAO. The enzyme cannot be turned back on — your body has to synthesize new MAO enzymes to restore normal function, which takes approximately 10 days after stopping the drug. This is why the dietary restrictions continue for 2 weeks after stopping.

How This Produces an Antidepressant Effect

Depression is associated with reduced availability of monoamine neurotransmitters in key brain regions. By blocking the enzyme that breaks these chemicals down, tranylcypromine allows serotonin, dopamine, and norepinephrine to accumulate — staying in the synaptic cleft longer and exerting more prolonged effects on mood-regulating circuits.

Tranylcypromine's antidepressant mechanism is broader than SSRIs (which only boost serotonin) or SNRIs (serotonin and norepinephrine). The simultaneous increase in all three monoamines, plus trace amines, may explain why it works in patients who have not responded to more selective drugs.

Research also shows that tranylcypromine may act as a norepinephrine reuptake inhibitor at higher therapeutic doses — similar to how stimulants work — which may contribute to its more activating properties compared to phenelzine.

Why the Tyramine Restriction Exists: The Chemistry Explained

Tyramine is a compound found naturally in aged, fermented, and processed foods. Under normal circumstances, MAO in the gut and liver breaks down dietary tyramine before it can reach the bloodstream in significant amounts.

When MAO is inhibited by tranylcypromine, this defense is gone. Dietary tyramine passes into the bloodstream, where it triggers the release of large amounts of norepinephrine from nerve endings. This causes a sudden, massive increase in blood pressure — a hypertensive crisis. This is why the dietary restrictions are not optional or casual guidance; they are a hard clinical requirement.

Tranylcypromine vs. SSRIs and SNRIs: What Makes It Different?

Most modern antidepressants work differently from MAOIs:

SSRIs — Block the reuptake of serotonin after it's released, keeping it available at the synapse longer; they do NOT prevent its breakdown by MAO

SNRIs — Block reuptake of both serotonin and norepinephrine; again, do not affect MAO

Tranylcypromine (MAOI) — Blocks the breakdown of monoamines at the source; increases baseline levels of serotonin, dopamine, AND norepinephrine simultaneously

This different mechanism explains both why MAOIs can work when SSRIs and SNRIs haven't — and why the two classes cannot be combined. If you take an SSRI while on an MAOI, serotonin builds up uncontrollably in both directions (not being broken down AND being blocked from reuptake), leading to potentially fatal serotonin syndrome.

An Interesting Historical Note

Tranylcypromine was originally developed as an amphetamine analog in 1948 — intended as a nasal decongestant. Its MAOI activity was only discovered in 1959, and it was approved as an antidepressant in 1961. Like amphetamines, it has a cyclopropyl ring structure that may contribute to its slightly more stimulating profile compared to other MAOIs.

For a broader overview of the medication, including dosing and who it's for, see: What Is Tranylcypromine? Uses, Dosage, and What You Need to Know in 2026.

To understand the clinical implications of the mechanism — specifically drug interactions — see: Tranylcypromine Drug Interactions: What to Avoid and What to Tell Your Doctor.