TCAs, MAOIs & Atypical Antidepressants
Before the SSRIs, two older families ruled depression treatment — and both could kill: one is lethal in overdose with a bottle of pills, the other can trigger a stroke-level blood pressure spike from a slice of aged cheese. They've mostly stepped aside, but they're far from gone, and a third group of 'atypical' drugs fills the gaps SSRIs leave. Knowing all three lets you match the right antidepressant to the right patient.
A teenager is rushed to the emergency department after swallowing a bottle of her grandmother's amitriptyline. At first she seems only drowsy — but the monitor tells a darker story: her heart's electrical tracing is widening dangerously, and soon she's seizing. This is why the old tricyclic antidepressants inspire such caution: the very drugs meant to lift mood are, in overdose, among the deadliest in the medicine cabinet. Understanding why reveals both how they work and why the SSRIs replaced them.
Tricyclics: effective but dirty
TCAs block serotonin and noradrenaline reuptake — plus a lot more. The tricyclic antidepressants (amitriptyline, nortriptyline, imipramine, clomipramine) block the reuptake of serotonin and noradrenaline — so far like an SNRI. The problem is they aren't selective: they also block muscarinic, histamine, and alpha-1 receptors, producing a pile of side effects. Muscarinic block gives dry mouth, constipation, blurred vision, and urinary retention; histamine block gives sedation and weight gain; alpha-1 block gives orthostatic hypotension. Worst of all, in overdose they block cardiac sodium channels, causing the widening heart tracing and lethal arrhythmias. Remember the '3 C's' of TCA overdose: Convulsions, Coma, and Cardiotoxicity — treated with sodium bicarbonate.
Because of that danger they're rarely first-line for depression now — but they remain valuable for other uses where their broad action helps: neuropathic pain, migraine prevention (amitriptyline), OCD (clomipramine), and childhood bedwetting (imipramine). A demanding drug, kept for the jobs it does uniquely well.
MAOIs and the cheese reaction
The monoamine oxidase inhibitors (phenelzine, tranylcypromine) block the enzyme that breaks down monoamines, so serotonin, noradrenaline, and dopamine all rise. They're powerful but reserved, because that same enzyme normally destroys tyramine — a compound in aged cheese, cured meats, and fermented foods. On an MAOI, dietary tyramine floods in, releases a surge of noradrenaline, and can cause a dangerous hypertensive crisis: the classic 'cheese reaction' (from the adrenergic transmission article). MAOIs also can't be combined with serotonergic drugs like SSRIs without a washout, for fear of serotonin syndrome. Effective, but hemmed in by diet and interactions.
- TCAs block serotonin/noradrenaline reuptake but also muscarinic, H1 & α1 → many side effects.
- TCA overdose = the 3 C's: Convulsions, Coma, Cardiotoxicity — treat with sodium bicarbonate.
- MAOIs raise all monoamines; risk the tyramine 'cheese reaction' and serotonin syndrome.
- Both are now reserved; TCAs still used for neuropathic pain, migraine, OCD, enuresis.
The atypicals: filling the gaps
A third group works through different mechanisms and shines where SSRIs fall short. Bupropion raises dopamine and noradrenaline (not serotonin), so it causes NO sexual dysfunction and can even help with energy and smoking cessation — but it lowers the seizure threshold and is avoided in eating disorders. Mirtazapine boosts noradrenaline and serotonin by a different route and strongly blocks histamine, making it sedating and appetite-boosting — ideal for a depressed patient who can't sleep or eat. Trazodone is sedating and often used in low doses just for sleep. The lesson: you pick the antidepressant whose side-effect profile FITS the patient — using sedation, appetite, or a lack of sexual side effects as features, not just faults.
Match the drug's side effects to the patient. Depressed and can't sleep or eat? Mirtazapine's sedation and appetite boost become therapeutic. Bothered by sexual side effects, or trying to quit smoking? Bupropion avoids the first and helps the second. This is the art of prescribing antidepressants: they're roughly equal in efficacy, so you choose based on which side-effect profile helps — or at least harms least — THIS person.
- Giving a TCA to a suicidal patient with easy access to the pills. Overdose can be fatal.
- Combining an MAOI with an SSRI or without a washout. Risk of serotonin syndrome.
- Ignoring the tyramine diet on an MAOI — aged cheese can trigger a hypertensive crisis.
- Using bupropion in a patient with seizures or an eating disorder. It lowers the seizure threshold.
A patient on a monoamine oxidase inhibitor eats aged cheese and develops a severe blood-pressure spike. This is due to:
- TCAs work but block muscarinic/H1/α1 receptors; overdose = the 3 C's (convulsions, coma, cardiotoxicity).
- MAOIs raise all monoamines but risk the tyramine cheese reaction and serotonin syndrome.
- Atypicals fill gaps: bupropion (no sexual effects, aids smoking), mirtazapine (sedation & appetite).
- Efficacy is similar across drugs — choose by side-effect profile to fit the patient.
- Katzung BG. Basic & Clinical Pharmacology — Antidepressant Agents (TCAs, MAOIs & atypicals).
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Drug therapy of depression.
- Rang HP, Dale MM, et al. Rang & Dale's Pharmacology — Antidepressant drugs.
- Whalen K. Lippincott Illustrated Reviews: Pharmacology — Tricyclics, MAOIs & atypical antidepressants.
- Clinical toxicology references — Tricyclic antidepressant overdose & sodium bicarbonate.
