Learning Objectives
By the end of this session, the learner will be able to identify key MAOIs, explain the consequences of irreversible enzyme inhibition, and recognize the life-threatening drug and food interactions known as the “Cheese Effect” and Serotonin Syndrome.
1. Mechanism and Drugs
MAOIs are powerful antidepressants that prevent the breakdown of monoamine neurotransmitters (NE, 5-HT, and DA) within the presynaptic neuron. Unlike SSRIs that block reuptake, MAOIs stop the destruction of these amines.
| Mechanism | Common MAOIs | Clinical Use |
|---|---|---|
| Irreversible inhibition of MAO-A and MAO-B. | Phenelzine, Tranylcypromine, Selegiline. | Atypical Depression (characterized by mood reactivity, weight gain, and hypersomnia). |
2. Hypertensive Crisis: The “Cheese Effect.”
MAO-A normally degrades Tyramine in the GI tract. When MAO-A is inhibited, Tyramine enters the systemic circulation and acts as a catecholamine releaser, displacing Norepinephrine (NE) into the synapse.
| Trigger | Physiology | Symptoms |
|---|---|---|
| Tyramine (Aged cheese, wine, cured meats) | Massive release of stored NE from vesicles. | Severe hypertension, pounding headache, arrhythmias, and hyperthermia. |
3. Dangerous Drug Interactions
Because MAOIs permanently “turn off” the primary cleanup enzymes, combining them with other drugs can lead to fatal levels of neurotransmitters.
| Interaction Type | Interacting Agents | Clinical Outcome |
|---|---|---|
| Serotonin Syndrome | SSRIs, SNRIs, TCAs, Meperidine, Tramadol, Linezolid, St. John’s Wort. | Hyperthermia, muscle rigidity, myoclonus, and autonomic instability. |
| NE Overload | TCAs,  agonists, Levodopa, Tyramine. |
Hypertensive crisis, severe excitation, arrhythmias. |
Clinical Pearls:
- The Washout Period: You must wait 2 weeks after stopping an MAOI before starting another antidepressant to allow the body to synthesize new MAO enzymes.
- Meperidine Warning: This opioid is absolutely contraindicated with MAOIs, as it can cause a fatal serotonergic reaction.
- Atypical Depression: Remember the classic “leaden paralysis” and overeating/oversleeping profile that makes MAOIs a preferred choice over SSRIs in specific cases.