Antidepressant Types Compared: SSRIs, SNRIs, TCAs, MAOIs

No Single Antidepressant Works for Everyone

The landmark STAR*D trial — the largest antidepressant study ever conducted, enrolling 4,041 patients — found that only 28% of patients achieved remission with their first antidepressant (citalopram). By the fourth treatment step, cumulative remission reached roughly 67%, but each failed trial increased the risk of relapse even after eventual remission. Choosing the right antidepressant is therefore less about finding the "best" drug and more about systematic matching of drug profile to patient characteristics.

Selective Serotonin Reuptake Inhibitors

SSRIs are the default starting point for most depressive episodes. They selectively inhibit the serotonin transporter, increasing synaptic serotonin. All six SSRIs — fluoxetine, sertraline, escitalopram, citalopram, paroxetine, and fluvoxamine — show comparable efficacy. A 2018 Lancet meta-analysis of 522 trials and 116,477 participants found escitalopram and sertraline had the best combination of efficacy and tolerability.

Sexual dysfunction affects 40–65% of SSRI users. Weight gain is modest in the short term (1–2 kg) but accumulates over years. GI effects — nausea, diarrhea — are common early but typically resolve within 2 weeks.

Serotonin-Norepinephrine Reuptake Inhibitors

SNRIs add norepinephrine reuptake inhibition to serotonergic effects. Venlafaxine, duloxetine, and desvenlafaxine are the main agents. The norepinephrine component strengthens efficacy for patients with prominent fatigue, concentration problems, and chronic pain comorbidities. Duloxetine has FDA approval for diabetic neuropathy and fibromyalgia as well as depression. Side effects mirror SSRIs but add dose-dependent hypertension (venlafaxine at doses above 225 mg) and more pronounced discontinuation syndrome.

Antidepressant Class Comparison

Tricyclic Antidepressants — Powerful but Difficult

TCAs predate SSRIs and block the reuptake of both serotonin and norepinephrine, plus multiple other receptors — histamine H1 (sedation), muscarinic (anticholinergic), and alpha-1 adrenergic (orthostatic hypotension). This receptor promiscuity drives both therapeutic effects and side effects. Amitriptyline is useful for chronic pain and migraine prevention. Nortriptyline is the best-tolerated TCA. The critical danger: a 10-day supply of a TCA can be lethal in overdose due to cardiac sodium-channel blockade causing arrhythmia. TCAs require ECG monitoring and are rarely used as first-line agents.

MAOIs — Last Resort, Highest Ceiling

Monoamine oxidase inhibitors — phenelzine, tranylcypromine, isocarboxazid, and the transdermal selegiline patch — inhibit the enzyme that breaks down serotonin, norepinephrine, and dopamine. They show particular efficacy in atypical depression (mood reactivity, hypersomnia, leaden paralysis, rejection sensitivity) and treatment-resistant cases. Irreversible MAOIs require strict dietary tyramine restriction (aged cheese, cured meats, fermented products) to avoid hypertensive crisis. Drug interactions are extensive. The transdermal selegiline patch at the 6 mg/24-hour dose bypasses the gut and requires less dietary restriction.

Atypical Antidepressants — Filling Specific Niches

• Bupropion (NDRI): inhibits norepinephrine and dopamine reuptake. Essentially no sexual dysfunction. Associated with modest weight loss (1–2 kg). FDA-approved for depression and smoking cessation. Contraindicated in eating disorders and seizure risk (lowers seizure threshold at high doses).
• Mirtazapine (NaSSA): blocks alpha-2 autoreceptors and H1 receptors. Potently sedating — useful when insomnia and anorexia dominate the clinical picture. Weight gain is the major long-term liability.
• Vortioxetine: multimodal agent combining SERT inhibition with partial agonism at 5-HT1A and antagonism at 5-HT3/5-HT7. RCT data suggest cognitive benefits beyond mood improvement.

Switching Strategies

When switching antidepressants, three methods apply depending on clinical urgency:

• Cross-taper: gradually reduce the current drug while increasing the new one — the most common approach, minimizes discontinuation symptoms while allowing some overlap.
• Direct switch: stop current drug and start new drug immediately — practical for drugs with long half-lives (fluoxetine) or when side effects require immediate change.
• Washout: essential when switching to or from an MAOI to avoid serotonin syndrome. A 14-day washout is required for most antidepressants; 5 weeks for fluoxetine before starting an MAOI.

Serotonin syndrome — agitation, clonus, hyperthermia, diaphoresis — can be life-threatening and must be distinguished from the milder discontinuation syndrome, which lacks hyperreflexia and clonus.

This article is for informational purposes only. Consult a qualified healthcare professional.