Mechanism of action
Amitriptyline inhibits the reuptake of serotonin and norepinephrine at presynaptic nerve terminals, increasing the availability of these monoamines in the synaptic cleft. This action is thought to contribute to its antidepressant and analgesic effects.
In addition to monoamine reuptake inhibition, amitriptyline has significant activity at multiple receptor systems:
- Muscarinic acetylcholine receptors: Potent antagonism produces anticholinergic effects including dry mouth, blurred vision, constipation, urinary retention, and cognitive impairment
- Histamine H1 receptors: Blockade causes sedation and weight gain
- Alpha-1 adrenergic receptors: Antagonism leads to orthostatic hypotension and dizziness
- Sodium channels: Cardiac sodium channel blockade contributes to QRS widening and arrhythmias, especially in overdose
The analgesic effect in neuropathic pain is believed to result from enhanced descending inhibitory pain pathways through increased synaptic norepinephrine and serotonin, independent of antidepressant effects. The migraine prophylaxis mechanism is not fully understood but may involve modulation of central pain pathways and serotonin receptor effects.
- Amitriptyline is a tertiary amine TCA that blocks reuptake of serotonin and norepinephrine and has potent antimuscarinic, antihistaminic, and alpha-blocking effects.
- Most common adverse effects are anticholinergic: dry mouth, constipation, urinary retention, blurred vision, and cognitive impairment in elderly.
- Contraindicated with MAOIs (14-day washout required) due to risk of serotonin syndrome and hypertensive crisis.
- Overdose causes TCA toxidrome: anticholinergic crisis, QRS widening >100 ms, ventricular arrhythmias, seizures, and coma; treated with sodium bicarbonate.
- Black box warning for increased suicidality in children, adolescents, and young adults under 24 with depression.
- Now primarily used for neuropathic pain and migraine prophylaxis at lower doses (25-75 mg) than antidepressant doses (100-300 mg).
- Bedtime dosing recommended due to sedative effects; once-daily dosing possible due to long half-life (10-28 hours).
- Listed in Beers Criteria as potentially inappropriate in elderly due to strong anticholinergic effects causing falls, confusion, and cognitive impairment.
Indications
Amitriptyline is FDA-approved for major depressive disorder, though it is now rarely used as a first-line antidepressant due to its side effect profile and the availability of safer alternatives like SSRIs.
Current common uses include:
Neuropathic pain (off-label but widely accepted):
- Diabetic peripheral neuropathy
- Post-herpetic neuralgia
- Fibromyalgia
- Chronic pain syndromes
Migraine prophylaxis (off-label): Effective for reducing frequency and severity of migraine headaches.
Other psychiatric conditions:
- Major depressive disorder, particularly when sedation is desired
- Anxiety disorders (off-label)
- Insomnia (off-label, low doses)
Other pain conditions (off-label):
- Tension-type headache prophylaxis
- Irritable bowel syndrome
- Interstitial cystitis
For neuropathic pain and migraine prophylaxis, lower doses are typically effective compared to doses used for depression. The analgesic effect may appear within 1-2 weeks, whereas antidepressant effects typically require 4-6 weeks.
Pharmacokinetics
Absorption: Well absorbed from the gastrointestinal tract with peak plasma concentrations occurring 2-12 hours after oral administration. Bioavailability is approximately 30-60% due to significant first-pass metabolism.
Distribution: Highly lipophilic with a large volume of distribution. Extensively bound to plasma proteins (>90%). Crosses the blood-brain barrier and placenta.
Metabolism: Undergoes extensive hepatic metabolism via CYP2D6, CYP2C19, and CYP3A4. The primary metabolite is nortriptyline, an active secondary amine TCA with less anticholinergic activity than the parent compound. Significant inter-individual variability exists due to genetic polymorphisms in CYP2D6.
Elimination: Renal excretion of metabolites. Half-life ranges from 10-28 hours (average ~15 hours), allowing once-daily dosing. Steady state is typically achieved in 4-10 days.
Special populations:
- Elderly patients may have reduced clearance and increased sensitivity to anticholinergic effects
- Hepatic impairment reduces metabolism and prolongs half-life
- Renal impairment has minimal effect on parent drug clearance but may accumulate metabolites
Adverse effects
Amitriptyline has a broad adverse effect profile due to its activity at multiple receptor systems. Adverse effects are dose-dependent and often limit tolerability.
Anticholinergic effects (most common)
- Dry mouth (xerostomia)
- Constipation
- Urinary retention or hesitancy
- Blurred vision, mydriasis, increased intraocular pressure
- Cognitive impairment, confusion, delirium (especially in elderly)
- Tachycardia
Cardiovascular effects
- Orthostatic hypotension (alpha-1 blockade)
- Tachycardia
- QTc prolongation
- Conduction abnormalities: QRS widening, heart block
- Arrhythmias (especially in overdose or pre-existing cardiac disease)
Central nervous system effects
- Sedation, drowsiness (histamine H1 blockade)
- Dizziness
- Tremor
- Seizures (lowers seizure threshold, especially at high doses)
- Confusion, delirium
Metabolic and endocrine effects
- Weight gain (common, can be significant)
- Increased appetite
- Hyperprolactinemia
- Sexual dysfunction
- Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
Other effects
- Sweating
- Photosensitivity
- Withdrawal symptoms if abruptly discontinued (nausea, headache, malaise)
Black box warning
TCAs including amitriptyline carry a black box warning for increased risk of suicidal thoughts and behavior in children, adolescents, and young adults (under age 24) with major depressive disorder and other psychiatric disorders. Close monitoring is required, especially during initial treatment and dose changes.
Overdose
Amitriptyline overdose is extremely dangerous and can be fatal. Toxicity manifests as the "TCA toxidrome":
- Anticholinergic crisis (hyperthermia, dry skin, mydriasis, delirium, urinary retention)
- Cardiac toxicity: QRS widening (>100 ms indicates severe toxicity), ventricular arrhythmias, hypotension, cardiac arrest
- CNS effects: Seizures, coma, respiratory depression
Treatment includes gastric decontamination if presenting early, sodium bicarbonate for QRS widening and arrhythmias, benzodiazepines for seizures, and supportive care.
Contraindications and warnings
Absolute contraindications
- Acute recovery phase following myocardial infarction
- Concurrent use with or within 14 days of monoamine oxidase inhibitors (MAOIs) — risk of serotonin syndrome and hypertensive crisis
- Known hypersensitivity to amitriptyline or other tricyclic antidepressants
Relative contraindications and warnings
Cardiovascular disease: Use with extreme caution in patients with:
- Conduction abnormalities (bundle branch block, heart block)
- Recent myocardial infarction
- Arrhythmias
- Heart failure
- QT prolongation or risk factors for QT prolongation
TCAs can precipitate arrhythmias and worsen conduction defects. ECG monitoring may be indicated.
Seizure disorders: Lowers seizure threshold; use with caution in patients with epilepsy or conditions predisposing to seizures.
Glaucoma: Anticholinergic effects can precipitate acute angle-closure glaucoma in susceptible individuals. Contraindicated in untreated narrow-angle glaucoma.
Urinary retention: Anticholinergic effects can worsen urinary retention; use with caution in benign prostatic hyperplasia or bladder outlet obstruction.
Hepatic impairment: Reduced metabolism may lead to drug accumulation. Dose adjustment may be necessary.
Elderly patients: Increased sensitivity to anticholinergic and sedative effects. Higher risk of falls, confusion, and cognitive impairment. Listed as potentially inappropriate medication in the Beers Criteria.
Pregnancy and lactation: Pregnancy category C (FDA, now replaced by PPRN). Use only if benefit outweighs risk. Amitriptyline crosses the placenta and is excreted in breast milk. Neonatal withdrawal symptoms have been reported.
Bipolar disorder: May precipitate manic episodes in patients with undiagnosed or poorly controlled bipolar disorder.
Thyroid disease: Use with caution in hyperthyroidism due to increased risk of cardiovascular effects.
Suicide risk: Close monitoring required, especially during treatment initiation and dose changes, due to increased suicidality risk in young adults.
Drug interactions
Amitriptyline has numerous clinically significant drug interactions due to its metabolism and pharmacologic effects.
Contraindicated combinations
MAOIs (monoamine oxidase inhibitors): Risk of serotonin syndrome, hypertensive crisis, hyperpyrexia, seizures, and death. Must allow at least 14 days washout between discontinuing an MAOI and starting amitriptyline, and vice versa.
Major interactions
CNS depressants: Additive sedation and respiratory depression with:
- Alcohol
- Benzodiazepines
- Opioids
- Barbiturates
- Antihistamines
- Other sedating medications
Anticholinergic drugs: Additive anticholinergic effects with antihistamines, antiparkinson drugs, antipsychotics, antispasmodics. Increased risk of anticholinergic toxicity, ileus, urinary retention, and cognitive impairment.
QT-prolonging drugs: Increased risk of cardiac arrhythmias (torsades de pointes) with antiarrhythmics (class IA and III), antipsychotics, macrolide antibiotics, fluoroquinolones, and other QT-prolonging agents.
Sympathomimetics: Potentiation of cardiovascular effects (hypertension, arrhythmias) with direct-acting sympathomimetics (epinephrine, norepinephrine). TCAs block the reuptake of these agents, prolonging and intensifying their effects.
SSRIs and SNRIs: Increased risk of serotonin syndrome. SSRIs that inhibit CYP2D6 (fluoxetine, paroxetine) can significantly increase amitriptyline levels.
Moderate interactions
CYP450 interactions:
- CYP2D6 inhibitors (fluoxetine, paroxetine, bupropion, quinidine, ritonavir) increase amitriptyline levels
- CYP3A4 and CYP2C19 inhibitors may also increase levels
- CYP inducers (carbamazepine, phenytoin, rifampin, St. John's wort) decrease amitriptyline levels
Cimetidine: Inhibits metabolism, increasing TCA levels and toxicity risk.
Thyroid hormones: May increase risk of cardiac arrhythmias and enhance therapeutic and toxic effects of TCAs.
Clonidine: TCAs may antagonize the antihypertensive effect of clonidine. Sudden withdrawal of clonidine with concurrent TCA use can result in hypertensive crisis.
Warfarin: TCAs may potentiate anticoagulant effect; monitor INR closely.
Clinical pearls
- Start low, go slow: Initiate at low doses (10-25 mg at bedtime) and titrate gradually to minimize adverse effects, especially in elderly patients and those using it for pain or migraine prophylaxis rather than depression.
- Bedtime dosing: Administer at bedtime to take advantage of sedative effects and minimize daytime somnolence. The long half-life allows once-daily dosing.
- Analgesic doses lower than antidepressant doses: For neuropathic pain and migraine prophylaxis, effective doses are typically 25-75 mg daily, whereas antidepressant effects usually require 100-300 mg daily.
- Delayed onset of effect: Analgesic effects may appear within 1-2 weeks, but full antidepressant effects require 4-6 weeks at therapeutic doses. Counsel patients about delayed benefit to improve adherence.
- Dry mouth management: The most common reason for discontinuation. Suggest sugar-free gum, frequent sips of water, or saliva substitutes. Dry mouth often improves with continued use but may persist.
- Anticholinergic burden in elderly: Amitriptyline is listed in the Beers Criteria as potentially inappropriate for older adults due to strong anticholinergic properties. Consider alternatives like duloxetine or gabapentin for neuropathic pain in elderly patients.
- Baseline ECG consideration: Obtain baseline ECG in patients with cardiac risk factors, history of arrhythmia, or when prescribing doses >100 mg daily. Monitor for QRS widening and QTc prolongation.
- Overdose lethality: Amitriptyline has a narrow therapeutic index. As little as 10-20 mg/kg can be lethal. Exercise caution when prescribing to patients with suicidal ideation. Dispense limited quantities.
- Nortriptyline as alternative: The active metabolite nortriptyline (a secondary amine TCA) has fewer anticholinergic and sedative effects while maintaining efficacy. Consider switching if side effects are limiting.
- Avoid abrupt discontinuation: Taper gradually over 2-4 weeks to avoid withdrawal symptoms (flu-like symptoms, nausea, insomnia, irritability).
- Weight gain counseling: Weight gain is common and can be significant (several kilograms). Discuss dietary modifications and exercise at initiation.
- Urine color: May cause blue-green discoloration of urine (harmless but may alarm patients if not forewarned).
- Drug-drug interaction vigilance: Always review concurrent medications before prescribing. Pay special attention to other anticholinergics, CNS depressants, and QT-prolonging drugs.
TCA side effects: 3 Cs and an A: Cardiac (arrhythmias, QRS widening), Convulsions (seizures), Coma, Anticholinergic (dry mouth, urinary retention, mydriasis, confusion)
Practice questions
1. A 68-year-old man with diabetic peripheral neuropathy is started on amitriptyline 25 mg at bedtime. Two weeks later, his wife reports that he has become increasingly confused and had difficulty urinating. On examination, he is disoriented to time, has dry mucous membranes, dilated pupils, and his bladder is palpable above the pubic symphysis. His temperature is 37.8°C (100°F). Which of the following receptors is most likely responsible for these findings?
2. A 28-year-old woman is brought to the emergency department 3 hours after ingesting approximately 2 grams of amitriptyline in a suicide attempt. She is drowsy but arousable. Vital signs show blood pressure 95/60 mmHg, heart rate 118/min, and temperature 37.2°C (99°F). ECG shows sinus tachycardia with QRS duration of 125 milliseconds and QTc of 485 milliseconds. Which of the following is the most appropriate immediate intervention?
3. A 55-year-old man with major depressive disorder has been taking amitriptyline 150 mg daily for 6 weeks with good response. His psychiatrist plans to add phenelzine for treatment-resistant symptoms. The patient asks when he can start the new medication. What is the minimum waiting period after discontinuing amitriptyline before starting phenelzine?
Frequently asked questions
How does amitriptyline work?
Amitriptyline works primarily by inhibiting the reuptake of serotonin and norepinephrine in the brain, increasing the availability of these neurotransmitters in the synaptic cleft. It also blocks muscarinic acetylcholine receptors (causing anticholinergic side effects), histamine H1 receptors (causing sedation), and alpha-1 adrenergic receptors (causing orthostatic hypotension). For neuropathic pain, the increased synaptic norepinephrine and serotonin enhance descending pain inhibitory pathways. The mechanism for migraine prophylaxis is not fully understood but involves modulation of central pain pathways.
What are the side effects of amitriptyline?
The most common side effects are anticholinergic: dry mouth, constipation, urinary retention, blurred vision, and confusion especially in elderly patients. Sedation and drowsiness are very common due to antihistamine effects. Orthostatic hypotension and dizziness occur from alpha-1 receptor blockade. Weight gain is common and can be significant. Cardiovascular effects include tachycardia, QT prolongation, and conduction abnormalities. In overdose, amitriptyline causes life-threatening cardiac arrhythmias (QRS widening, ventricular arrhythmias), seizures, and anticholinergic crisis. A black box warning exists for increased suicidality in patients under age 24.
Is amitriptyline a controlled substance?
No, amitriptyline is not a controlled substance in the United States or most other countries. It is not classified as a drug of abuse and does not have significant potential for dependence or recreational use. However, it should not be stopped abruptly as withdrawal symptoms (nausea, headache, malaise, insomnia) can occur. Gradual tapering over 2-4 weeks is recommended when discontinuing. While not controlled, amitriptyline does require a prescription due to its significant adverse effect profile and potential toxicity, particularly in overdose.
Amitriptyline vs nortriptyline — what is the difference?
Nortriptyline is the active metabolite of amitriptyline and a secondary amine TCA, while amitriptyline is a tertiary amine TCA. The key difference is that nortriptyline has fewer anticholinergic side effects (less dry mouth, constipation, urinary retention, cognitive impairment) and less sedation compared to amitriptyline, making it better tolerated especially in elderly patients. Both drugs are effective for depression and neuropathic pain at similar doses. Nortriptyline also causes less orthostatic hypotension. However, both have similar cardiac effects and overdose risks. Nortriptyline may be preferred when anticholinergic side effects limit amitriptyline tolerability.
Can you take amitriptyline while pregnant?
Amitriptyline is pregnancy category C, meaning risk cannot be ruled out. It crosses the placenta and should only be used during pregnancy if the potential benefit justifies the potential risk to the fetus. There is limited human data, but no clear evidence of major congenital malformations. However, neonatal withdrawal symptoms and temporary adverse effects have been reported in newborns exposed late in pregnancy. If amitriptyline is essential during pregnancy, use the lowest effective dose and avoid use in the first trimester if possible. The decision should involve careful discussion between the patient and healthcare provider weighing the risks of untreated maternal condition against potential fetal risks.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: N06AA09
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.