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Anticonvulsant

Carbamazepine

Carbamazepine is a first-generation anticonvulsant that blocks voltage-gated sodium channels, stabilizing neuronal membranes. It is FDA-approved for partial and generalized tonic-clonic seizures, trigeminal neuralgia, and bipolar disorder. The most testable fact: carbamazepine is a potent CYP450 inducer and carries a black box warning for Stevens-Johnson syndrome and toxic epidermal necrolysis, especially in patients with HLA-B*1502 allele (common in Han Chinese and Southeast Asian populations).

Practice questionsHigh-yield facts
Drug classAnticonvulsant, sodium channel blocker
MechanismBlocks voltage-gated sodium channels, stabilizes neuronal membranes
Main usesEpilepsy, trigeminal neuralgia, bipolar disorder
Key adverse effectStevens-Johnson syndrome, aplastic anemia, SIADH, hepatotoxicity
6 min readFor medical studentsUpdated 2026-10-14

Mechanism of action

Carbamazepine blocks voltage-gated sodium channels in their inactive state, preventing repetitive neuronal firing and stabilizing hyperexcitable neural membranes. This action reduces the propagation of abnormal electrical activity in the brain, making it effective for seizure control.

The drug preferentially binds to sodium channels during rapid, sustained firing (use-dependent blockade), which explains its efficacy in treating epilepsy and neuropathic pain conditions like trigeminal neuralgia. In bipolar disorder, the mechanism is less clear but likely involves modulation of neurotransmitter systems and ion channels beyond sodium channels.

Carbamazepine also has mild anticholinergic properties and blocks adenosine receptors, contributing to some of its side effects. Its active metabolite, carbamazepine-10,11-epoxide, contributes to both therapeutic and toxic effects.

High-yield for exams
  • Carbamazepine blocks voltage-gated sodium channels, preventing repetitive neuronal firing (use-dependent blockade).
  • First-line treatment for trigeminal neuralgia; also used for partial seizures, generalized tonic-clonic seizures, and bipolar disorder.
  • Black box warning: Stevens-Johnson syndrome/toxic epidermal necrolysis, especially in patients with HLA-B*1502 allele (screen Asian patients).
  • Black box warning: aplastic anemia and agranulocytosis (monitor CBC periodically).
  • Potent CYP450 inducer: decreases levels of oral contraceptives, warfarin, many anticonvulsants, and numerous other drugs.
  • Autoinduction: induces its own metabolism over 3-5 weeks, requiring dose adjustments.
  • Common cause of SIADH leading to hyponatremia, especially in elderly patients.
  • Structurally similar to tricyclic antidepressants; overdose causes cardiac conduction abnormalities and anticholinergic toxicity.

Indications

FDA-approved indications:

  • Partial seizures (simple and complex)
  • Generalized tonic-clonic seizures (not absence seizures, which it may worsen)
  • Mixed seizure patterns
  • Trigeminal neuralgia (first-line treatment)
  • Acute mania and mixed episodes in bipolar disorder

Off-label uses:

  • Glossopharyngeal neuralgia
  • Neuropathic pain syndromes
  • Bipolar disorder maintenance therapy
  • Alcohol withdrawal syndrome
  • Borderline personality disorder (adjunctive)

Carbamazepine is not effective for absence, myoclonic, or atonic seizures and may exacerbate these seizure types. It has largely been replaced by newer agents (lamotrigine, levetiracetam) for many indications due to its side effect profile and drug interactions, but remains first-line for trigeminal neuralgia.

Pharmacokinetics

Absorption: Well absorbed orally with variable bioavailability (75-85%). Extended-release formulations provide more stable plasma levels and improve tolerability.

Distribution: Highly protein-bound (70-80%, primarily to alpha-1-acid glycoprotein and albumin). Crosses the placenta and enters breast milk. Volume of distribution is approximately 0.8-1.9 L/kg.

Metabolism: Extensively metabolized in the liver via CYP3A4 to the active metabolite carbamazepine-10,11-epoxide. Carbamazepine is a potent inducer of CYP1A2, CYP2C9, CYP2C19, and CYP3A4, and also induces its own metabolism (autoinduction), leading to decreasing plasma levels over 3-5 weeks after initiation. This autoinduction necessitates dose adjustments during the first month of therapy.

Elimination: Primarily renal excretion of metabolites. Half-life is initially 25-65 hours but decreases to 12-17 hours after autoinduction. Steady state is reached in 2-4 weeks.

Special populations: Dose adjustment may be needed in hepatic impairment. Clearance is increased in pregnancy due to increased metabolism. Geriatric patients may require lower doses.

Adverse effects

Black box warnings:

  • Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN): Risk is strongly associated with HLA-B*1502 allele (prevalence 10-15% in Han Chinese, Thai, and other Southeast Asian populations). Screen patients of Asian ancestry before initiating therapy.
  • Aplastic anemia and agranulocytosis: Rare but potentially fatal. Risk is 5-8 times higher than general population.

Common adverse effects (dose-related):

  • Dizziness, ataxia, drowsiness
  • Diplopia, blurred vision, nystagmus
  • Nausea, vomiting
  • Hyponatremia (SIADH)

Serious adverse effects:

  • Hematologic: Aplastic anemia, agranulocytosis, thrombocytopenia, leukopenia. Obtain baseline CBC and monitor periodically.
  • Dermatologic: SJS/TEN, DRESS syndrome (drug reaction with eosinophilia and systemic symptoms), exfoliative dermatitis
  • Hepatic: Hepatotoxicity, cholestatic and hepatocellular injury, elevated transaminases
  • SIADH: Syndrome of inappropriate antidiuretic hormone secretion leading to hyponatremia, particularly in elderly patients
  • Cardiac: Atrioventricular block, arrhythmias (structurally similar to tricyclic antidepressants)
  • Teratogenicity: Neural tube defects (1-2% risk), craniofacial abnormalities, developmental delay. Pregnancy category D.

Other notable effects:

  • Mild anticholinergic effects (dry mouth, constipation, urinary retention)
  • Weight gain
  • Osteoporosis with chronic use (enzyme induction increases vitamin D metabolism)

Contraindications and warnings

Absolute contraindications:

  • Hypersensitivity to carbamazepine or tricyclic antidepressants
  • Bone marrow suppression
  • Concurrent use of MAO inhibitors (within 14 days)
  • History of AV heart block
  • HLA-B*1502 positive patients (relative contraindication, strong warning)

Warnings and precautions:

  • HLA-B*1502 screening: Recommended for patients of Asian ancestry before initiating therapy due to SJS/TEN risk. HLA-A*3101 allele is associated with increased risk in European and Japanese populations but screening is less consistently recommended.
  • Hematologic monitoring: Obtain baseline CBC, reticulocyte count, iron studies, and liver function tests. Monitor CBC periodically, especially during first 2-3 months. Discontinue if significant bone marrow suppression develops.
  • Hepatic monitoring: Monitor liver enzymes at baseline and periodically. Use with caution in hepatic impairment.
  • Hyponatremia: Monitor sodium levels, especially in elderly patients and those on diuretics. Consider dose reduction or discontinuation if symptomatic hyponatremia develops.
  • Cardiac disease: Use with caution in patients with cardiac conduction abnormalities. Obtain ECG in elderly patients or those with cardiac history.
  • Suicidality: FDA warning applies to all anticonvulsants. Monitor for emergence of suicidal thoughts or behavior.
  • Pregnancy: Increased risk of neural tube defects. Women of childbearing potential should use effective contraception. Folic acid supplementation (5 mg daily) is recommended before conception and during pregnancy.
  • Abrupt withdrawal: Taper gradually to avoid seizure exacerbation or status epilepticus.

Drug interactions

Carbamazepine is a potent CYP450 inducer and has extensive drug interactions.

Interactions due to enzyme induction (carbamazepine decreases levels of):

  • Oral contraceptives: Significantly reduces efficacy; use alternative or additional contraception
  • Warfarin: Decreased anticoagulant effect; monitor INR closely
  • Other anticonvulsants: Decreases levels of phenytoin, valproate, lamotrigine, topiramate
  • Antipsychotics: Reduces levels of haloperidol, risperidone, quetiapine, aripiprazole
  • Immunosuppressants: Decreases cyclosporine, tacrolimus levels
  • Antiretrovirals: Reduces protease inhibitor and NNRTI levels
  • Corticosteroids: Accelerates metabolism of dexamethasone, prednisolone
  • Thyroid hormones: May increase levothyroxine requirements

Drugs that increase carbamazepine levels (CYP3A4 inhibitors):

  • Macrolide antibiotics (erythromycin, clarithromycin)
  • Azole antifungals (ketoconazole, fluconazole, itraconazole)
  • Calcium channel blockers (diltiazem, verapamil)
  • Grapefruit juice
  • Cimetidine
  • SSRIs (fluoxetine, fluvoxamine)
  • Valproate (also inhibits epoxide hydrolase, increasing active metabolite)

Drugs that decrease carbamazepine levels:

  • Phenobarbital, phenytoin, primidone (enzyme inducers)
  • Rifampin
  • St. John's wort

Pharmacodynamic interactions:

  • MAO inhibitors: Hypertensive crisis, hyperpyrexia. Contraindicated within 14 days.
  • Lithium: Increased neurotoxicity risk
  • CNS depressants: Additive sedation with alcohol, benzodiazepines, opioids
  • Diuretics: Increased risk of hyponatremia
  • Tricyclic antidepressants: Additive anticholinergic and cardiac effects

Clinical pearls

Exam high-yield points:

  • Carbamazepine is first-line for trigeminal neuralgia; if a patient has lancinating facial pain triggered by chewing or touching the face, think carbamazepine.
  • Always screen Asian patients for HLA-B*1502 before starting carbamazepine due to SJS/TEN risk.
  • Autoinduction means plasma levels fall over the first 3-5 weeks; dose adjustments are often needed.
  • Carbamazepine can worsen absence, myoclonic, and atonic seizures.
  • Monitor for hyponatremia, especially in elderly patients (SIADH).
  • Structurally related to tricyclic antidepressants; similar toxicity profile in overdose (cardiac conduction abnormalities, anticholinergic effects, CNS depression).

Practical tips:

  • Start low and titrate slowly to minimize dose-related adverse effects (dizziness, diplopia, ataxia).
  • Extended-release formulations improve tolerability and compliance.
  • Therapeutic drug monitoring is useful; target range 4-12 mcg/mL for epilepsy (lower for trigeminal neuralgia).
  • Women taking carbamazepine need alternative contraception due to reduced oral contraceptive efficacy.
  • In overdose, consider multiple-dose activated charcoal due to enterohepatic recirculation.

Exam scenarios:

  • Patient with new-onset seizures started on carbamazepine develops a rash within 2-4 weeks → think SJS/TEN, especially if Asian ancestry.
  • Elderly patient on carbamazepine presents with confusion, lethargy, seizures → check sodium (SIADH-induced hyponatremia).
  • Patient on carbamazepine has unplanned pregnancy → counsel about neural tube defect risk, start high-dose folate.
  • Bipolar patient on carbamazepine and oral contraceptive becomes pregnant → carbamazepine reduced contraceptive efficacy.
  • Patient taking warfarin starts carbamazepine and INR drops → enzyme induction decreased warfarin levels.
Mnemonic

HLA-B*1502 and Carbamazepine: "Screen ASIANS Before Carbamazepine" (Asian patients need HLA-B*1502 screening before starting carbamazepine due to SJS/TEN risk).

Practice questions

1. A 55-year-old man with a history of epilepsy controlled on carbamazepine for the past 5 years presents to the emergency department with confusion and nausea. His wife reports he has been more lethargic over the past week. Vital signs are normal. Physical examination shows no focal neurological deficits. Laboratory studies show: Sodium 118 mEq/L, Potassium 4.2 mEq/L, serum osmolality 250 mOsm/kg, urine osmolality 450 mOsm/kg, urine sodium 40 mEq/L. Which of the following is the most likely cause of this patient's presentation?

Answer: B. This patient has hyponatremia with low serum osmolality and inappropriately concentrated urine (high urine osmolality and urine sodium), consistent with SIADH. Carbamazepine is a well-known cause of SIADH, particularly in elderly patients. The clinical picture and laboratory findings (euvolemic hyponatremia with urine osmolality >100 mOsm/kg despite low serum osmolality) are classic for SIADH. Primary polydipsia would show dilute urine, adrenal insufficiency would present with hyperkalemia, and cerebral salt wasting occurs after CNS injury with volume depletion.

2. A 35-year-old woman of Han Chinese ancestry with newly diagnosed focal epilepsy is started on carbamazepine. Three weeks later, she develops fever, malaise, and a painful rash on her face and trunk with mucosal involvement. Physical examination reveals erythematous macules with central dusky discoloration and sloughing epidermis covering approximately 15% of her body surface area. Which of the following genetic screening tests should have been performed before initiating this medication?

Answer: B. This patient has Stevens-Johnson syndrome/toxic epidermal necrolysis (SJS/TEN) due to carbamazepine. The HLA-B*1502 allele strongly predisposes to carbamazepine-induced SJS/TEN and is present in 10-15% of Han Chinese, Thai, and other Southeast Asian populations. The FDA recommends screening patients of Asian ancestry for this allele before starting carbamazepine. HLA-A*3101 is associated with carbamazepine hypersensitivity in European populations but is less strongly predictive. CYP enzyme polymorphisms affect drug metabolism but not immune-mediated severe cutaneous reactions.

3. A 42-year-old woman with bipolar disorder well-controlled on carbamazepine for 2 years presents for contraceptive counseling. She is currently taking combined oral contraceptive pills and wishes to continue this method. Her menstrual cycles have been irregular for the past 6 months, and she had one episode of breakthrough bleeding. Which of the following is the most appropriate recommendation?

Answer: C. Carbamazepine is a potent CYP450 inducer that significantly reduces the efficacy of hormonal contraceptives by accelerating their metabolism. This patient's irregular cycles and breakthrough bleeding suggest contraceptive failure. The most appropriate recommendation is to switch to a non-hormonal method (copper IUD) or use a highly effective alternative that is not affected by enzyme induction (levonorgestrel IUD may also have reduced efficacy). Simply increasing the estrogen dose or adding barrier methods is inadequate. Progesterone-only pills are also affected by enzyme induction and are not recommended.

Frequently asked questions

How does carbamazepine work?

Carbamazepine works by blocking voltage-gated sodium channels in neurons, which prevents repetitive firing and stabilizes hyperexcitable neuronal membranes. This use-dependent blockade preferentially affects rapidly firing neurons, making it effective for controlling seizures and neuropathic pain. The mechanism reduces the spread of abnormal electrical activity in conditions like epilepsy and trigeminal neuralgia.

What are the side effects of carbamazepine?

Common side effects include dizziness, drowsiness, blurred vision, nausea, and unsteadiness. Serious adverse effects include Stevens-Johnson syndrome (especially in patients with HLA-B*1502 allele), aplastic anemia, agranulocytosis, liver toxicity, and hyponatremia due to SIADH. Carbamazepine can also cause cardiac conduction abnormalities and is teratogenic, increasing the risk of neural tube defects in pregnancy. Regular monitoring of blood counts, liver function, and sodium levels is important.

Is carbamazepine a controlled substance?

No, carbamazepine is not a controlled substance. It is a prescription anticonvulsant medication that does not have abuse potential and is not regulated under the Controlled Substances Act. However, it does require a prescription and medical supervision due to its potential for serious adverse effects and numerous drug interactions.

Carbamazepine vs lamotrigine — what is the difference?

Both are anticonvulsants that block sodium channels, but carbamazepine is a potent CYP450 inducer with extensive drug interactions, while lamotrigine is not. Carbamazepine carries a higher risk of serious side effects like aplastic anemia, SJS/TEN (especially in HLA-B*1502 positive patients), and SIADH. Lamotrigine has a lower side effect burden but requires slow titration to minimize rash risk. Carbamazepine is first-line for trigeminal neuralgia, while lamotrigine is used for partial and generalized seizures and is preferred for bipolar depression. Ethosuximide, not lamotrigine, is the first-line treatment for typical absence seizures.

Can carbamazepine cause low sodium?

Yes, carbamazepine commonly causes hyponatremia through the syndrome of inappropriate antidiuretic hormone secretion (SIADH). This occurs in approximately 5-40% of patients and is more frequent in elderly individuals. SIADH leads to water retention and dilutional hyponatremia, which can cause confusion, lethargy, nausea, headaches, and in severe cases, seizures. Sodium levels should be monitored regularly, especially during the first few months of treatment and in high-risk patients.

Conditions it is used for

EpilepsyTrigeminal neuralgiaBipolar disorderGlossopharyngeal neuralgiaNeuropathic painAcute maniaMixed episodes
OxcarbazepineLamotriginePhenytoinValproateLevetiracetamGabapentinLithiumTopiramate

Sources and further reading

DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: N03AF01

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.