The Dirtiest Drug That Still Works
IUPAC: 3-(10,11-dihydro-5H-dibenzo[a,d][7]annulen-5-ylidene)-N,N-dimethylpropan-1-amine · Elavil / Laroxyl / Tryptizol · MW 277.40 g/mol (free base) · CAS 50-48-6 · ChEMBL629 · ATC N06AA09
Amitriptyline (Elavil). A first-generation tricyclic antidepressant from the dibenzocycloheptadiene series. Where the SSRIs are precision-guided at SERT, amitriptyline is a shotgun: it blocks SERT and NET reuptake and slams into histamine, muscarinic, α-adrenergic and cardiac sodium channels with near-equal force. That promiscuity is the whole story — it is simultaneously why the drug sedates, dries, dizzies and kills in overdose, and why sub-antidepressant doses treat neuropathic pain, migraine and insomnia. Its own N-demethylated metabolite, nortriptyline, is the molecule captured inside the transporter in PDB 4M48.
Amitriptyline's therapeutic intent is monoamine reuptake inhibition: like cocaine, it binds the orthosteric S1 site of the serotonin and norepinephrine transporters (SERT / NET) and occludes the pore, blocking recapture of 5-HT and NE without being transported itself. Chronic elevation of synaptic monoamines drives the slow (2–6 week) downstream adaptations — receptor downregulation, BDNF signaling, hippocampal plasticity — that constitute the antidepressant effect.
The problem, and the defining feature of the whole tricyclic class, is that the same flat, lipophilic three-ring scaffold fits comfortably into a half-dozen unrelated pockets. Amitriptyline binds histamine H1, muscarinic M1–M5, and α1/α2-adrenergic receptors at single-digit to low-double-digit nanomolar affinity — comparable to or tighter than its action at its "real" targets. At toxic concentrations it also blocks voltage-gated cardiac Nav1.5 and hERG K+ channels. There is no clean separation between "mechanism" and "side effect" here: the receptor promiscuity is the drug.
Binds human SERT (SLC6A4) with Ki ≈ 0.9 nM (DrugMatrix binding; functional uptake IC50 higher, ~30–100 nM). Elevates synaptic 5-HT — the serotonergic arm of the antidepressant effect, and the reason amitriptyline carries full serotonin-syndrome and MAOI-contraindication risk.
Blocks the norepinephrine transporter (SLC6A2), Ki ≈ 22 nM. The noradrenergic arm — mood, analgesia in neuropathic pain, and part of the descending pain-modulation effect. Nortriptyline, its metabolite, is markedly more NET-selective than the parent.
Inverse agonist at H1, Ki ≈ 1.1 nM — one of the most potent H1 antagonists in clinical use. Drives profound sedation, weight gain and daytime somnolence. This is the basis of amitriptyline's off-label use as a hypnotic at 10–25 mg.
Antagonist at M1, Ki ≈ 2.7 nM (potent at M1–M5). The anticholinergic load: dry mouth, blurred vision, constipation, urinary retention, tachycardia, and — especially in the elderly — confusion and delirium. A major reason TCAs are Beers-criteria "avoid" drugs in geriatrics.
Antagonist at α1A/1B/1D, Ki ≈ 5–50 nM. Causes vasodilation → orthostatic hypotension, dizziness and falls. In overdose, α1 blockade compounds cardiotoxic hypotension that is refractory to standard pressors.
At supratherapeutic levels amitriptyline blocks the cardiac fast Na+ channel (SCN5A) use-dependently, IC50 ≈ low µM, plus hERG K+ block. This is a class-I-antiarrhythmic-like action — inconsequential at therapeutic dose, lethal in overdose (QRS widening, arrhythmia). See §04.
Amitriptyline is highly lipophilic and extensively tissue-bound — a huge volume of distribution (~6–16 L/kg) and ~95% plasma protein binding. This matters clinically two ways: it accumulates in myocardium and CNS (worsening overdose), and it is not dialyzable, so hemodialysis is useless in poisoning. Oral bioavailability is a modest 30–60% because of substantial first-pass metabolism.
Metabolism cascade: CYP2C19 N-demethylates amitriptyline to nortriptyline — itself an approved, NET-preferring TCA (Aventyl/Pamelor) and the active species crystallized in PDB 4M48. Both parent and metabolite are then E-10-hydroxylated by the highly polymorphic CYP2D6; 10-hydroxy metabolites are weakly active and glucuronidated for renal excretion. Poor CYP2D6 metabolizers (~7–10% of Europeans) accumulate higher levels and are more prone to anticholinergic and cardiac toxicity at standard doses.
Nortriptyline ★ (marked) is pharmacologically active and clinically monitored: therapeutic drug monitoring for amitriptyline sums parent + nortriptyline, target ~80–200 ng/mL. Above ~500 ng/mL, toxicity is expected; above ~1000 ng/mL, severe cardiotoxicity and death. The gap between a therapeutic and a fatal plasma level is small — the defining hazard of the class.
In modern psychiatry amitriptyline is rarely a first-line antidepressant — the SSRIs and SNRIs match its efficacy with a fraction of the toxicity. Yet it remains one of the most prescribed drugs in the world, because the exact receptor promiscuity that disqualifies it as a clean antidepressant makes it superb at sub-antidepressant doses for three other problems.
The best-evidenced use. NET blockade enhances descending noradrenergic pain inhibition in the dorsal horn; additional Nav and NMDA-adjacent actions dampen ectopic firing in sensitized nociceptors. Effective at doses far below those needed for mood (NNT ~3–4 for diabetic neuropathy, post-herpetic neuralgia, fibromyalgia) — analgesia here is independent of any antidepressant effect.
First-line preventive for chronic migraine and tension-type headache. Mechanism is multimodal — serotonergic modulation of trigeminovascular pain, NE reuptake block, and downregulation of central sensitization. Taken nightly, the H1-driven sedation is a feature, not a bug, for patients with comorbid insomnia.
Potent H1 inverse agonism (Ki ~1.1 nM) plus α1 and 5-HT2A antagonism produce reliable sedation and increased slow-wave sleep. Widely used off-label for sleep, though the same anticholinergic load causes morning grogginess, dry mouth, and cognitive dulling — and is a poor choice in older adults.
Every therapeutic use above is inseparable from the side-effect burden: anticholinergic dry mouth/constipation/retention, antihistaminergic sedation and weight gain, α1 orthostasis. The receptors that deliver the benefit are the receptors that deliver the harm — dose is titrated to the narrow band where the two are tolerable, and that band is where overdose danger begins.
This is the single most important fact about amitriptyline: it has a narrow therapeutic index, and TCA overdose is a leading cause of drug-poisoning death. A supply of tricyclic — an amount routinely dispensed — can be lethal in a single ingestion, historically ~10–20 mg/kg is potentially fatal. Unlike an SSRI overdose (usually benign), a TCA overdose is a time-critical cardiotoxic emergency.
The lethal mechanism is use-dependent blockade of the cardiac fast sodium channel Nav1.5. As heart rate and drug level rise, progressively more Nav1.5 channels are blocked, slowing phase-0 depolarization and intraventricular conduction. On the ECG this appears as QRS widening and a rightward terminal QRS axis (tall R in aVR) — the classic, prognostic sign. QRS > 100 ms predicts seizures; QRS > 160 ms predicts ventricular arrhythmia. Simultaneous hERG K+ block prolongs QT, and the toxidrome adds anticholinergic tachycardia, α1-mediated hypotension, H1-driven coma and lowered seizure threshold. The mnemonic is the "three C's": Coma, Convulsions, Cardiotoxicity.
Sodium bicarbonate is the cornerstone antidote. A sodium bolus increases the electrochemical gradient across Nav1.5 and outcompetes the drug, while serum alkalinization (target pH 7.45–7.55) reduces the fraction of ionized, channel-blocking amitriptyline. Give for QRS > 100 ms, hypotension, or ventricular dysrhythmia.
Supportive: airway protection (coma), benzodiazepines for seizures, IV fluids and norepinephrine for hypotension. Avoid class-Ia/Ic and class-III antiarrhythmics — they add sodium/potassium-channel block and worsen conduction. Lipid emulsion and mechanical circulatory support are salvage options. Hemodialysis does not work (high Vd, high protein binding). Any tricyclic ingestion warrants continuous ECG and a minimum 6-hour observation window.
Amitriptyline's dibenzocycloheptadiene core is a semi-rigid, near-planar tricycle with only a short
dimethylaminopropylidene tail carrying real rotational freedom. In FlexAID∆S entropy modeling the fused ring system contributes
a small conformational-entropy penalty (ΔS_conf) on binding — the scaffold is largely pre-organized — while the flexible
side chain pays the entropic cost of ordering its terminal amine against the transporter's Asp/aromatic cage. The nortriptyline
complex in PDB 4M48 shows exactly this: the tricyclic wedged between TM1, TM3, TM6 and TM8, protonated amine
anchored at the central S1 aspartate.
The transporter pocket itself is conformationally hot — dDAT/SERT/NET cycle through outward-open, occluded and
inward-open states in their alternating-access transport mechanism. Shannon entropy of the unbound pocket, H_pocket,
is high (many accessible conformers); amitriptyline binding collapses that ensemble by trapping the outward-open state,
the thermodynamic signature of a competitive, non-transported inhibitor. This entropy-collapse fingerprint is the same one
cocaine leaves at DAT — TCAs and cocaine are mechanistic cousins at the transporter, differing mainly in which of SERT/NET/DAT
they prefer.
The deeper lesson for polypharmacology: a flat, lipophilic, moderately rigid scaffold pays a low, roughly constant entropic
entry fee into many different pockets. Because ΔG_bind = ΔH − TΔS, a low-ΔS penalty lets even modest enthalpic
complementarity yield nanomolar affinity — at H1, M1, α1 and the transporters alike. FlexAID∆S ensemble
docking against the 4M48 pose estimates ΔG_bind ≈ −11 to −12 kcal/mol at SERT (consistent with Ki ≈ 0.9 nM;
theoretical −RT·ln Ka ≈ −12.4 kcal/mol at 298 K). Promiscuity is not sloppiness — it is the predictable consequence of a
pre-organized ligand whose entropy budget is cheap everywhere.
Amitriptyline is a prescription medicine, but it is one of the deadliest common drugs in overdose and carries real interaction and discontinuation hazards. Clinical, evidence-based, dose- and context-dependent.
| Target | Affinity | Rel. | Mechanism |
|---|---|---|---|
|
SERT
Serotonin transporter (SLC6A4)
|
Ki ≈ 0.9 nM
Primary · antidepressant
|
Reuptake block | |
|
H1
Histamine H1 receptor
|
Ki = 1.12 nM
Sedation · weight gain
|
Inverse agonist | |
|
M1
Muscarinic ACh receptor M1
|
Ki = 2.72 nM
Anticholinergic load
|
Antagonist | |
|
α1A
α1A-adrenergic receptor
|
Ki = 5.6 nM
Orthostatic hypotension
|
Antagonist | |
|
NET
Norepinephrine transporter (SLC6A2)
|
Ki = 22 nM
Analgesia · mood
|
Reuptake block | |
|
Nav1.5
Cardiac Na⁺ channel (SCN5A)
|
IC50 ≈ low µM
Overdose lethality
|
Use-dep. block |