MAOIs

Inhibitors of monoamine oxidase, the enzyme that breaks monoamines down. Remove it and what would have been metabolised is not — dietary tyramine, and anything that releases noradrenaline or serotonin. The distinction that decides the size of the risk is not in the class name: phenelzine, tranylcypromine and isocarboxazid are irreversible and non-selective, moclobemide is reversible and selective for MAO-A, and rasagiline is MAO-B selective only while the dose stays where it was studied.

This is the highest-stakes interaction class on the site. Tyramine and the sympathomimetic releasers — amphetamine, methamphetamine, MDMA — can drive a hypertensive crisis; serotonergic drugs and tramadol can drive serotonin syndrome. Irreversible inhibition outlasts the last dose by however long the body takes to build new enzyme, so having stopped is not the same as being clear. The harmala alkaloids under Psychedelics are MAOIs too, and the same rules apply to them.