11.2 Antidepressant Pharmacotherapy & Treatment-Resistant Depression
Key Takeaways
- Selective serotonin reuptake inhibitors are first-line, and class efficacy is broadly similar so selection turns on tolerability and interactions.
- An adequate trial requires four to eight weeks at a therapeutic dose before concluding that a drug has failed.
- Bupropion lowers the seizure threshold and is avoided in seizure disorders and in active eating disorders with purging.
- Selective serotonin reuptake inhibitors increase bleeding risk, particularly when combined with antiplatelet or anticoagulant therapy.
- After a first episode remits, treatment continues for at least four to nine months, with longer maintenance after recurrent episodes.
Last updated: August 2026
1. Antidepressant Pharmacotherapy: Drug Classes, Selection & Monitoring
Selecting the optimal antidepressant requires matching the drug's pharmacology and adverse effect profile to the patient's individual clinical presentation and medical comorbidities.
Comprehensive Antidepressant Pharmacotherapy Matrix
| Drug Class & Generic Name | Typical Starting & Target Doses | Mechanism of Action | Clinical Indications & Board Pearls | Key Adverse Effects & Critical Contraindications |
|---|---|---|---|---|
| SSRI: Sertraline | 50 mg/day (start 25 mg in anxiety); target 100-200 mg/day | Selective serotonin reuptake inhibitor | Drug of choice in cardiovascular disease & post-myocardial infarction (proven cardiac safety in SADHART trial); safe in breastfeeding | GI disturbances (nausea, diarrhea), sexual dysfunction (30-40%: delayed ejaculation, anorgasmia), insomnia/agitation |
| SSRI: Escitalopram | 10 mg/day; target 10-20 mg/day | Highly selective 5-HT reuptake inhibitor | Cleanest pharmacokinetic profile; minimal CYP450 drug-drug interactions; highly predictable dose-response | Mild nausea, headache, low risk of QT prolongation compared to citalopram |
| SSRI: Citalopram | 20 mg/day; target 20-40 mg/day | Racemic selective 5-HT reuptake inhibitor | Effective first-line agent | FDA Black Box Warning for QTc Prolongation: Maximum dose is 40 mg/day in adults; Capped at 20 mg/day in patients >=60 years old, hepatic impairment, or CYP2C19 poor metabolizers |
| SSRI: Fluoxetine | 20 mg/day; target 20-60 mg/day | 5-HT reuptake inhibitor + weak 5-HT2C antagonism | Activating profile (useful in psychomotor retardation/fatigue); longest half-life (parent drug 2-4 days, active metabolite norfluoxetine 7-15 days -> lowest risk of discontinuation syndrome) | Insomnia, jitteriness, anorexia; potent CYP2D6 inhibitor; mandates a 5-week washout period before starting an MAOI (vs 2 weeks for other SSRIs) |
| SSRI: Paroxetine | 20 mg/day; target 20-50 mg/day | Potent 5-HT reuptake inhibitor + anticholinergic | Highly sedating; approved for panic disorder and social anxiety | Highest rate of weight gain, sedation, and sexual dysfunction; anticholinergic effects (dry mouth, constipation, urinary retention); highest risk of discontinuation syndrome due to short half-life; teratogenic (cardiac ventricular septal defects) |
| SNRI: Venlafaxine | 375 mg/day (XR start 37.5-75 mg; target 150-225 mg) | 5-HT reuptake inhibitor at low doses; Norepinephrine reuptake inhibitor at >=150 mg/day | Effective for MDD with comorbid severe generalized anxiety disorder, panic disorder, or social phobia | Dose-dependent diastolic hypertension (monitor BP at doses >=225 mg); severe discontinuation syndrome with missed doses (dizziness, electric shock-like "brain zaps", nausea) |
| SNRI: Duloxetine | 30 mg/day for 1 week, then target 60 mg/day (max 120 mg) | Balanced 5-HT and NE reuptake inhibitor | FDA-approved dual indications for: Diabetic peripheral neuropathic pain, Fibromyalgia, and Chronic musculoskeletal low back / osteoarthritis pain | Nausea, dry mouth, constipation, diaphoresis; Avoid in severe hepatic impairment, chronic liver disease, heavy alcohol use (hepatotoxicity risk), or severe renal failure (eGFR <30 mL/min) |
| Atypical (NDRI): Bupropion | 150 mg XL/day; target 300 mg XL/day (max 450 mg) | Norepinephrine-Dopamine Reuptake Inhibitor (NDRI) | No sexual dysfunction; promotes mild weight loss; aids smoking cessation (Zyban); highly activating (ideal for fatigue, hypersomnia, psychomotor slowing, ADHD comorbidity) | ABSOLUTE CONTRAINDICATIONS: History of seizures, active eating disorders (Bulimia nervosa, Anorexia nervosa — high seizure risk from electrolyte shifts), abrupt discontinuation of alcohol or benzodiazepines; can worsen acute anxiety/insomnia |
| Atypical: Mirtazapine | 15 mg at bedtime; target 15-45 mg at bedtime | Central presynaptic alpha-2 adrenergic antagonist + 5-HT2 and 5-HT3 antagonist + potent H1 histamine antagonist | Ideal for depressed geriatric patients with severe insomnia, marked anorexia, and cachexia/weight loss; potent antiemetic properties (via 5-HT3 blockade) | Sedation and significant weight gain / increased appetite (note: lower doses [7.5-15 mg] are more sedating due to unopposed H1 blockade; higher doses [30-45 mg] increase noradrenergic transmission and are less sedating); rare agranulocytosis |
| Tricyclics (TCAs): Amitriptyline, Nortriptyline | Nortriptyline: 25 mg qhs; target 50-150 mg (trough level 50-150 ng/mL) | Non-selective 5-HT and NE reuptake inhibitor + alpha-1, H1, and muscarinic blockade | Second/third-line for MDD; neuropathic pain, migraine prophylaxis | Fatal in overdose (3 Cs: Coma, Convulsions, Cardiac arrhythmias from sodium-channel blockade causing QRS widening >100 ms); anticholinergic toxicity; orthostatic hypotension; avoid in elderly and CAD |
| MAOIs: Phenelzine, Tranylcypromine, Selegiline | Phenelzine 15 mg TID; Selegiline transdermal patch 6-12 mg/24h | Irreversible inhibition of Monoamine Oxidase (MAO-A and MAO-B) | Treatment-resistant depression, atypical depression (mood reactivity, leaden paralysis, hypersomnia) | Hypertensive Crisis if combined with dietary tyramine (aged cheeses, cured meats, red wine, draft beer); Serotonin Syndrome if combined with SSRIs/SNRIs/TCAs/meperidine/tramadol/dextromethorphan; requires 14-day washout (5 weeks for fluoxetine) |
Inadequate Response Algorithm & Treatment Duration Rules
- Adequate Trial Definition: An adequate antidepressant trial requires 6 to 8 consecutive weeks at the target therapeutic dose. Evaluating efficacy at 1 to 2 weeks is premature (except to monitor adverse effects and initial behavioral activation).
- Management of Inadequate Response:
- No Response (<25% improvement after 6-8 weeks): Confirm medication adherence; cross-taper and switch to another first-line agent (either within the same class [e.g., switch sertraline to escitalopram] or switch classes [e.g., switch SSRI to SNRI, Bupropion, or Mirtazapine]).
- Partial Response (25% to 50% improvement): Optimize dose to the maximum approved/tolerated level. If still partial response, utilize pharmacologic augmentation:
- Atypical Antipsychotic Augmentation: Low-dose Aripiprazole (2 to 5 mg/day), Quetiapine XR (150 to 300 mg/day), or Brexpiprazole (1 to 2 mg/day) has the strongest randomized trial evidence for augmenting SSRI/SNRIs.
- Bupropion Augmentation: Adding Bupropion (150-300 mg XL) to an SSRI/SNRI targets residual fatigue, low energy, and reverses SSRI-induced sexual dysfunction.
- Lithium Augmentation: Adding Lithium Carbonate (target serum level 0.6 to 0.8 mEq/L) reduces suicide risk and augments serotonergic neurotransmission.
- Liothyronine (T3) Augmentation: Adding T3 (25 to 50 mcg/day) accelerates and enhances antidepressant response even in euthyroid patients.
- Psychotherapy Augmentation: Initiating evidence-based Cognitive Behavioral Therapy (CBT) or Interpersonal Psychotherapy (IPT).
- Treatment Duration Guidelines:
- First Single Unipolar Episode: Continue full-dose antidepressant therapy for 6 to 9 months after achieving full clinical remission (continuation phase) before considering a gradual taper over 1-2 months. Stopping early drastically increases relapse rates.
- Recurrent Depressive Episodes (>=2 to 3 lifetime episodes), Chronic Depression (duration >=2 years), or Severe/Psychotic/Suicidal Episodes: Maintain long-term maintenance pharmacotherapy for at least 2 to 3 years or indefinitely at the full therapeutic dose that induced remission.
- Electroconvulsive Therapy (ECT):
- Efficacy: Most effective acute treatment for severe depression (response rates 70-90%).
- First-Line Board Indications:
- Severe Major Depression with Psychotic Features (delusional guilt, auditory hallucinations).
- Catatonia (stupor, mutism, waxy flexibility, negativism).
- Acute Life-Threatening Suicidal Crisis requiring immediate clinical stabilization.
- Severe Malnutrition / Acute Food and Fluid Refusal causing medical decompensation.
- Severe refractory depression in pregnancy (where pharmacotherapy carries fetal risks).
- Adverse Effects: Post-ictal confusion, retrograde and anterograde amnesia (typically resolves within weeks to months); no absolute medical contraindications (relative contraindication: increased intracranial pressure / intracranial mass).
Test Your Knowledge
A 58-year-old man with a history of hypertension and major depressive disorder presents for a follow-up visit. Eight weeks ago, he was started on Escitalopram 10 mg daily, which was titrated to 20 mg daily at week 3. Today, he reports that while his mood is slightly less depressed, his PHQ-9 score remains elevated at 13 (down from 17). He complains of persistent profound fatigue, lack of energy, hypersomnia, and a 6-lb weight gain. In addition, he reports significant frustration with delayed ejaculation and decreased libido since starting escitalopram. He also mentions that he has smoked 1 pack of cigarettes daily for 30 years and wishes to quit. Blood pressure is 126/78 mmHg, and pulse is 72 bpm. Which of the following is the most appropriate next step in pharmacotherapy?
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D