Mechanism of action
Doxepin is a dibenzoxepin tricyclic compound that exerts its effects through multiple receptor targets:
- Serotonin and norepinephrine reuptake inhibition: Blocks the presynaptic reuptake of serotonin (5-HT) and norepinephrine (NE), increasing synaptic concentrations and enhancing monoaminergic neurotransmission. This mechanism underlies its antidepressant and anxiolytic effects.
- Histamine H1-receptor antagonism: Doxepin has the highest affinity for H1 receptors among all TCAs, approximately 800 times greater than diphenhydramine. This potent antihistaminic action is responsible for its efficacy in treating pruritus and its sedative properties at low doses.
- Muscarinic acetylcholine receptor blockade: Antagonism at M1 receptors produces anticholinergic side effects including dry mouth, constipation, urinary retention, and blurred vision. This effect is dose-dependent and less prominent at low doses used for insomnia.
- Alpha-1 adrenergic receptor antagonism: Blockade of peripheral alpha-1 receptors causes vasodilation and orthostatic hypotension, particularly problematic in elderly patients.
- Sodium channel blockade: At toxic concentrations, doxepin blocks fast sodium channels in cardiac myocytes, causing QRS widening and arrhythmias. This is the primary mechanism of TCA cardiotoxicity in overdose.
At low doses (3–6 mg), used for insomnia, H1 antagonism predominates with minimal effect on monoamine reuptake or other receptors. At antidepressant doses (75–300 mg), all mechanisms contribute to both therapeutic effects and adverse reactions.
- Doxepin has the strongest H1-receptor antagonism of all TCAs, making it highly effective for pruritus and sleep maintenance.
- Low-dose doxepin (3–6 mg) is FDA-approved for insomnia with minimal anticholinergic effects; higher doses (75–300 mg) are used for depression.
- TCA overdose triad: Cardiotoxicity (wide QRS >100 ms), CNS toxicity (seizures, coma), antiCholinergic toxicity (mydriasis, hyperthermia, urinary retention).
- Sodium bicarbonate is first-line treatment for TCA-induced QRS widening and ventricular arrhythmias by reversing sodium channel blockade.
- Contraindicated with MAOIs or within 14 days of MAOI use due to risk of hypertensive crisis and serotonin syndrome.
- Black box warning: Increased suicidal ideation in children, adolescents, and young adults up to age 24.
- Doxepin is metabolized to active metabolite desmethyldoxepin (nordoxepin) via CYP2D6; half-life of metabolite is longer than parent drug.
- Topical doxepin 5% cream for pruritus can cause systemic absorption and drowsiness when applied to large surface areas.
Indications
FDA-approved indications:
- Major depressive disorder: Effective for moderate to severe depression, particularly in patients with prominent anxiety, insomnia, or melancholic features.
- Anxiety disorders: Approved for anxiety associated with depression or alcoholism; useful in generalized anxiety disorder.
- Insomnia: Low-dose formulation (3 mg and 6 mg) approved for sleep maintenance insomnia in adults. Improves total sleep time without next-day residual effects.
- Pruritus: Topical formulation (5% cream) approved for short-term management of pruritus in atopic dermatitis and lichen simplex chronicus. Oral doxepin is used off-label for chronic idiopathic urticaria and other pruritic conditions.
Common off-label uses:
- Chronic pain syndromes (fibromyalgia, neuropathic pain)
- Functional dyspepsia and irritable bowel syndrome
- Migraine prophylaxis
- Chronic urticaria
- Post-herpetic neuralgia
Doxepin is particularly useful when comorbid depression, anxiety, insomnia, or pruritus coexist with the primary condition.
Pharmacokinetics
Absorption: Well absorbed orally with peak plasma concentrations in 2–4 hours. Food does not significantly affect absorption. Bioavailability is approximately 30% due to extensive first-pass metabolism.
Distribution: Highly lipophilic with large volume of distribution (20–30 L/kg). Extensively protein-bound (80–85%). Crosses the blood-brain barrier and placenta; excreted in breast milk.
Metabolism: Hepatic metabolism via CYP2D6 and CYP2C19. Doxepin is N-demethylated to its active metabolite desmethyldoxepin (nordoxepin), which has similar pharmacological activity and a longer half-life than the parent compound. Further metabolism produces inactive glucuronide conjugates.
Elimination: Elimination half-life of doxepin is 8–24 hours; desmethyldoxepin has a half-life of 28–52 hours. Steady state is reached in 7–10 days. Primarily excreted in urine as metabolites; less than 3% excreted unchanged.
Special populations:
- Elderly: Decreased clearance and increased sensitivity to anticholinergic and cardiovascular effects. Lower doses recommended.
- Hepatic impairment: Reduced metabolism prolongs half-life; dose reduction necessary.
- Renal impairment: No significant dose adjustment needed as excretion of active drug is minimal.
- CYP2D6 poor metabolizers: Higher plasma concentrations of parent drug, increased risk of adverse effects.
Adverse effects
Adverse effects are dose-dependent and related to doxepin's multiple receptor actions.
Common (anticholinergic and antihistaminic):
- Sedation and drowsiness (most common, especially early in treatment)
- Dry mouth (xerostomia)
- Constipation
- Blurred vision
- Weight gain (due to H1 blockade and metabolic effects)
- Increased appetite
- Urinary retention
Cardiovascular:
- Orthostatic hypotension (alpha-1 blockade)
- Tachycardia (reflex response to hypotension; anticholinergic)
- QTc prolongation (arrhythmia risk)
- Conduction abnormalities in overdose
Neuropsychiatric:
- Confusion and delirium (especially in elderly)
- Cognitive impairment
- Tremor
- Seizures (lowers seizure threshold)
- Activation of mania in bipolar disorder
- Increased suicidal ideation in young adults (black box warning)
Dermatologic (topical use):
- Burning or stinging at application site
- Contact dermatitis
- Systemic absorption can occur with widespread application
Serious adverse effects:
- Serotonin syndrome: When combined with other serotonergic agents (SSRIs, MAOIs, linezolid).
- TCA toxicity in overdose: QRS widening (>100 ms), ventricular arrhythmias, seizures, hypotension, anticholinergic delirium, coma. Overdose is potentially fatal.
- Withdrawal syndrome: Abrupt discontinuation causes cholinergic rebound (nausea, vomiting, diarrhea, headache, malaise).
Black box warning: Increased risk of suicidal thinking and behavior in children, adolescents, and young adults (up to age 24) with major depressive disorder and other psychiatric disorders. Close monitoring required during initial treatment and dose changes.
Contraindications and warnings
Absolute contraindications:
- Hypersensitivity to doxepin or other tricyclic compounds
- Concurrent use or within 14 days of MAOIs (risk of hypertensive crisis and serotonin syndrome)
- Acute recovery phase following myocardial infarction
- Untreated narrow-angle glaucoma
- Severe urinary retention or bladder neck obstruction
Warnings and precautions:
- Cardiovascular disease: Use with caution in patients with arrhythmias, heart block, coronary artery disease, or prolonged QT interval. Obtain baseline ECG in at-risk patients.
- Seizure disorders: TCAs lower seizure threshold. Use cautiously in epilepsy or conditions predisposing to seizures.
- Bipolar disorder: Can precipitate manic episodes. Screen for bipolar disorder before initiating treatment.
- Hepatic impairment: Dose reduction required due to decreased metabolism.
- Elderly patients: Increased sensitivity to anticholinergic effects (confusion, delirium, falls) and orthostatic hypotension. Increased risk of cognitive impairment. Consider alternative agents or reduced doses.
- Suicidal risk: Close monitoring during initial weeks of treatment. Provide limited quantities to high-risk patients.
- Angle-closure glaucoma: Anticholinergic effects can precipitate acute angle-closure in susceptible individuals.
- Prostatic hypertrophy: May worsen urinary retention.
- Hyperthyroidism: Increased sensitivity to catecholamines may precipitate arrhythmias.
Pregnancy and lactation:
- Pregnancy category C: Animal studies show adverse effects. Use only if benefit outweighs risk. Some reports of limb deformities with first-trimester exposure, but causality not established.
- Lactation: Excreted in breast milk. Sedation and respiratory depression reported in nursing infants. Avoid use or discontinue nursing.
Topical formulation: Systemic absorption occurs with application to large surface areas. Drowsiness can occur even with topical use.
Drug interactions
Doxepin has numerous clinically significant drug interactions due to its multiple mechanisms and hepatic metabolism.
Pharmacodynamic interactions:
- MAOIs: Potentially fatal interactions including hypertensive crisis, hyperpyrexia, seizures, and serotonin syndrome. Contraindicated within 14 days of MAOI use.
- SSRIs and SNRIs: Additive serotonergic effects increase risk of serotonin syndrome. CYP2D6 inhibition by fluoxetine, paroxetine, and bupropion increases doxepin levels.
- CNS depressants: Alcohol, benzodiazepines, opioids, and antihistamines potentiate sedation and respiratory depression.
- Anticholinergic agents: Additive anticholinergic effects with antihistamines, antipsychotics, antiparkinsonian drugs, and other TCAs increase risk of delirium, ileus, urinary retention, and hyperthermia.
- Sympathomimetics: Doxepin potentiates pressor effects of norepinephrine and epinephrine. Avoid direct-acting sympathomimetics.
- QT-prolonging drugs: Additive risk of torsades de pointes with antiarrhythmics (class IA and III), antipsychotics, macrolides, and fluoroquinolones.
Pharmacokinetic interactions:
- CYP2D6 inhibitors: Fluoxetine, paroxetine, quinidine, bupropion, and duloxetine increase doxepin and desmethyldoxepin levels, increasing toxicity risk.
- CYP2D6 inducers: Rifampin and carbamazepine decrease doxepin levels, reducing efficacy.
- Cimetidine: Inhibits hepatic metabolism, increasing doxepin concentrations.
Other significant interactions:
- Thyroid hormones: Potentiate therapeutic and toxic effects of both agents; risk of arrhythmias.
- Clonidine: TCAs may block antihypertensive effect of clonidine.
- Warfarin: TCAs may potentiate anticoagulant effects; monitor INR.
- Linezolid: Weak MAOI properties increase serotonin syndrome risk.
Always review complete medication list before initiating doxepin. Consider therapeutic drug monitoring in complex polypharmacy situations.
Clinical pearls
- Low-dose doxepin (3–6 mg) for insomnia: Unlike higher antidepressant doses, low-dose doxepin provides selective H1 antagonism for sleep maintenance without significant anticholinergic burden or morning sedation. It is particularly useful in elderly patients who cannot tolerate benzodiazepines or Z-drugs.
- Most antihistaminic TCA: Doxepin's H1 affinity exceeds all other TCAs and first-generation antihistamines, making it the TCA of choice when pruritus or severe insomnia accompanies depression.
- Therapeutic lag: Antidepressant effects typically require 2–4 weeks, but sedation and antihistaminic effects occur immediately. Counsel patients about delayed onset of mood improvement.
- Overdose management: TCA overdose is a medical emergency. Sodium bicarbonate is the treatment of choice for QRS widening (>100 ms) and ventricular arrhythmias caused by sodium channel blockade. Physostigmine can reverse severe anticholinergic toxicity but is reserved for life-threatening delirium or seizures due to risk of bradycardia and asystole.
- Withdrawal: Taper gradually over 2–4 weeks to avoid cholinergic rebound symptoms. Abrupt discontinuation causes nausea, headache, malaise, and sleep disturbance.
- Topical application: Systemic absorption from 5% cream is significant when applied to large areas (>10% body surface area). Drowsiness can occur even with dermatologic use. Limit application area and duration to 8 days.
- Drug levels: Therapeutic range for combined doxepin plus desmethyldoxepin is 100–250 ng/mL for depression. Levels >300 ng/mL increase toxicity risk without added benefit.
- Exam tip for toxicity triad: TCA overdose presents with the "three Cs": Cardiotoxicity (wide QRS, arrhythmias), CNS toxicity (seizures, coma), and antiCholinergic toxicity (mydriasis, hyperthermia, dry skin, urinary retention).
- Neuropathic pain: Effective for diabetic neuropathy and post-herpetic neuralgia at doses lower than those required for depression (25–100 mg). Analgesic effects are independent of mood effects.
- Elderly prescribing: Doxepin appears on the Beers Criteria as potentially inappropriate in older adults due to strong anticholinergic properties. If used, start at low doses (10–25 mg) and titrate slowly. Low-dose formulation for insomnia (3–6 mg) is considered safer.
TCA toxicity triad = "3 Cs": Cardiotoxicity (wide QRS, arrhythmias), CNS toxicity (Coma, seizures), antiCholinergic toxicity (hot as a hare, blind as a bat, dry as a bone, red as a beet, mad as a hatter)
Practice questions
1. A 68-year-old woman with major depressive disorder is started on doxepin 75 mg at bedtime. Three days later, she presents to the emergency department after a fall. She reports dizziness upon standing and nearly fainting. Vital signs show blood pressure 145/85 mmHg supine and 95/60 mmHg standing, heart rate 105 bpm. Physical examination reveals dry mucous membranes and dilated pupils. ECG shows sinus tachycardia with QTc of 475 ms. Which receptor blockade is primarily responsible for her orthostatic hypotension?
2. A 34-year-old man is brought to the emergency department by ambulance after ingesting 3 grams of doxepin in a suicide attempt 2 hours ago. He is confused and agitated. Vital signs show temperature 38.9°C, heart rate 128 bpm, blood pressure 90/55 mmHg. Physical examination reveals dry flushed skin, mydriasis, absent bowel sounds, and a distended bladder. ECG shows QRS duration of 145 ms. Which of the following is the most appropriate immediate treatment?
3. A 42-year-old woman with chronic idiopathic urticaria reports persistent pruritus despite maximum-dose cetirizine. Her physician considers adding doxepin for its antipruritic effects. The patient is currently taking fluoxetine 40 mg daily for depression. Which of the following is the most important reason to avoid adding doxepin in this patient?
Frequently asked questions
How does doxepin work?
Doxepin blocks the reuptake of serotonin and norepinephrine, increasing their synaptic concentrations to improve mood. It also has potent histamine H1-receptor antagonism (strongest among all TCAs), which accounts for its effectiveness in treating insomnia and pruritus. Additionally, doxepin blocks muscarinic acetylcholine receptors (causing anticholinergic side effects) and alpha-1 adrenergic receptors (causing orthostatic hypotension). At low doses (3–6 mg), H1 antagonism predominates, providing sedation without significant antidepressant or anticholinergic effects.
What are the side effects of doxepin?
Common side effects include sedation, dry mouth, constipation, blurred vision, weight gain, and orthostatic hypotension. Anticholinergic effects like urinary retention and confusion are more pronounced in elderly patients. Cardiovascular effects include tachycardia and QTc prolongation. Serious adverse effects include seizures (due to lowered seizure threshold), cardiac arrhythmias, serotonin syndrome when combined with other serotonergic drugs, and potentially fatal toxicity in overdose. Abrupt discontinuation can cause withdrawal symptoms including nausea, headache, and malaise.
Is doxepin a controlled substance?
No, doxepin is not a controlled substance in the United States or most other countries. It is a prescription medication but does not have abuse potential and is not classified under the Controlled Substances Act. However, it should still be used with caution due to its side effect profile and potential for serious toxicity in overdose. The low-dose formulation (3 mg and 6 mg) for insomnia is particularly safe regarding abuse potential.
Can doxepin be used for anxiety?
Yes, doxepin is FDA-approved for anxiety, particularly anxiety associated with depression or alcoholism. It is effective for generalized anxiety disorder due to its serotonin and norepinephrine reuptake inhibition combined with anxiolytic effects from H1 antagonism. However, doxepin is not typically a first-line agent for anxiety due to its side effect profile. Modern SSRIs or SNRIs are generally preferred, but doxepin remains useful when comorbid insomnia, pruritus, or treatment-resistant anxiety is present. Therapeutic effects take 2–4 weeks to develop.
Doxepin vs amitriptyline — what is the difference?
Both are tricyclic antidepressants with similar mechanisms (serotonin/norepinephrine reuptake inhibition, anticholinergic, antihistaminic, and alpha-blocking effects), but doxepin has significantly stronger H1-receptor antagonism than amitriptyline, making it more sedating and more effective for pruritus and insomnia. Doxepin is available in a low-dose formulation specifically for insomnia (3–6 mg), while amitriptyline is more commonly used for neuropathic pain and migraine prophylaxis. Both have similar cardiovascular and anticholinergic side effect profiles. Amitriptyline may have slightly greater efficacy for neuropathic pain in some studies. Both are equally dangerous in overdose.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: D04AX01
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.