Formulary
Carbamazepine: Uses, Dosing, Side Effects and Indian Brand Names
Carbamazepine is a sodium channel-blocking anticonvulsant that is first-line for focal seizures and the drug of choice for trigeminal neuralgia. It exhibits autoinduction of its own metabolism and carries a risk of SJS/TEN linked to HLA-B*15:02.
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Clinically reviewed by Awaiting clinical review
Carbamazepine: Uses, Dosing, Side Effects and Indian Brand Names
Carbamazepine is a sodium channel-blocking anticonvulsant that is first-line for focal seizures and the drug of choice for trigeminal neuralgia. It exhibits autoinduction of its own metabolism and carries a risk of SJS/TEN linked to HLA-B*15:02.
NEET PG High-Yield: Carbamazepine is the drug of choice for trigeminal neuralgia -- one of the most asked facts in NEET PG. SIADH causing hyponatraemia is the classic distinctive adverse effect. Remember autoinduction (induces its own metabolism). Contraindicated in absence seizures (may worsen). HLA-B*15:02 screening applies to both carbamazepine and phenytoin.
Clinical overview
Carbamazepine is a first-line antiepileptic drug for focal (partial) seizures and is the drug of choice for trigeminal neuralgia. It was introduced in the 1960s initially for trigeminal neuralgia and later found effective in epilepsy. It is structurally related to tricyclic antidepressants (imipramine) but lacks significant antidepressant activity at anticonvulsant doses. Carbamazepine is a potent inducer of hepatic CYP3A4 enzymes and, uniquely, induces its own metabolism (autoinduction), meaning that steady-state levels decrease over the first 3-4 weeks of therapy, necessitating dose escalation. The active metabolite carbamazepine-10,11-epoxide contributes to both therapeutic effect and toxicity. Like phenytoin, carbamazepine is strongly associated with HLA-B*15:02-linked Stevens-Johnson syndrome and toxic epidermal necrolysis, particularly in South and Southeast Asian populations. The Indian Pharmacovigilance Programme recommends HLA testing before initiation in these populations. Carbamazepine is widely prescribed across Indian neurology practice due to its efficacy, established safety profile, and low cost. It is included in the NLEM 2022 and is one of the most commonly tested antiepileptics in NEET PG examinations. Hyponatraemia due to ADH-like (SIADH) activity is a distinctive adverse effect that differentiates it from other antiepileptics in clinical examinations.
Pharmacological class
Carbamazepine belongs to the Iminostilbene anticonvulsant (dibenzazepine derivative) class. Carbamazepine blocks voltage-gated sodium channels in a use-dependent manner, stabilising the inactivated state and preventing repetitive neuronal firing. It also modulates voltage-gated calcium channels and potentiates GABAergic neurotransmission. Its mechanism is similar to phenytoin but with additional mood-stabilising properties attributable to effects on limbic kindling.
Indian brand names and formulations
Available as: Tegretol (Novartis), Zen (Sun Pharma), Mazetol (Intas Pharmaceuticals), Carbatol (Torrent Pharmaceuticals).
Tablets (100 mg, 200 mg, 400 mg), Controlled-release tablets (200 mg, 400 mg CR), Oral suspension (100 mg/5 mL), Chewable tablets (100 mg)
Regulatory status
Carbamazepine is classified under Schedule H in India under the Drugs and Cosmetics Act, 1940. Schedule H prescription drug. NLEM 2022 listed. HLA-B*15:02 screening is recommended before prescribing in Indian patients per pharmacovigilance advisories.
Indications
- Focal (partial) seizures -- first-line
- Generalised tonic-clonic seizures
- Trigeminal neuralgia -- drug of choice
- Glossopharyngeal neuralgia
- Bipolar affective disorder (acute mania and maintenance -- second-line after lithium)
- Neuropathic pain (diabetic neuropathy, post-herpetic neuralgia)
- Alcohol withdrawal (alternative agent)
Dosing
Adults: start 100-200 mg BD, increase by 200 mg/day every 1-2 weeks. Maintenance: 800-1200 mg/day in 2-3 divided doses. Therapeutic range: 4-12 mcg/mL. Trigeminal neuralgia: start 100 mg BD, increase to 200-400 mg TDS. Children: 10-20 mg/kg/day in divided doses. Dose must be increased after 2-4 weeks to compensate for autoinduction.
Contraindications
- Known HLA-B*15:02 carriers (risk of SJS/TEN)
- AV block or cardiac conduction defects
- History of bone marrow suppression
- Concurrent use of MAO inhibitors (wait 14 days)
- Acute intermittent porphyria
- Absence seizures (may worsen)
Adverse effects
- Diplopia, blurred vision, dizziness, ataxia (dose-related, most common early)
- Hyponatraemia (SIADH-like effect -- distinctive side effect)
- Stevens-Johnson syndrome / Toxic epidermal necrolysis (HLA-B*15:02 linked)
- Aplastic anaemia and agranulocytosis (rare but serious -- requires CBC monitoring)
- Hepatotoxicity (monitor LFTs)
- Teratogenicity (neural tube defects)
- Rash (occurs in 5-10% of patients)
- Drowsiness and cognitive impairment
Drug interactions
- CYP3A4 induction -- reduces levels of oral contraceptives, warfarin, theophylline, corticosteroids, cyclosporine
- Erythromycin, clarithromycin, ketoconazole -- inhibit carbamazepine metabolism, causing toxicity
- Valproate -- inhibits epoxide hydrolase, increasing toxic epoxide metabolite levels
- Phenytoin and phenobarbital -- mutual enzyme induction
- Grapefruit juice -- inhibits CYP3A4 in gut, increases carbamazepine levels
- Lithium -- increased neurotoxicity without change in lithium levels
Pregnancy and lactation
Category D. Associated with neural tube defects (spina bifida, 0.5-1% risk), craniofacial anomalies, and fingernail hypoplasia. High-dose folate (5 mg/day) must be supplemented from preconception through the first trimester. Despite teratogenicity, carbamazepine is considered less teratogenic than valproate and may be continued if seizure control is essential, with appropriate counselling and fetal monitoring.
Exam-style clinical scenario
A 55-year-old man presents with excruciating, lancinating pain in the right cheek triggered by chewing and brushing teeth. Pain occurs in electric shock-like paroxysms lasting seconds. Neurological examination is normal. What is the most appropriate first-line pharmacotherapy? Answer: Carbamazepine is the drug of choice for trigeminal neuralgia. It is started at 100 mg BD and titrated upward. Its sodium channel-blocking mechanism reduces the abnormal neuronal firing in the trigeminal ganglion. Before prescribing, HLA-B*15:02 testing should be considered in Indian patients, and baseline CBC and LFTs should be obtained.
Cost in India
INR 25-60 per strip of 10 tablets (200 mg); CR formulations: INR 40-80 per strip
Clinical governance
Author: MedNext Editorial Team. Clinical reviewer: Awaiting clinical review. Jurisdiction: India (Drugs and Cosmetics Act, 1940). Sources: CIMS India, Indian Pharmacopoeia. Publication state: Awaiting clinical review. Correction: Report errors at support@mednext.academy.
Frequently Asked Questions
What is autoinduction and why is it clinically important?
Autoinduction means carbamazepine induces the CYP3A4 enzymes responsible for its own metabolism. Over the first 3-4 weeks of therapy, its clearance increases and plasma levels fall, even though the patient is taking the same dose. This necessitates planned dose escalation after initiation and makes early therapeutic drug monitoring essential.
Why is carbamazepine contraindicated in absence seizures?
Carbamazepine can paradoxically worsen absence seizures and may precipitate absence status epilepticus. The mechanism is not fully understood but may relate to its effects on thalamocortical circuits that generate absence seizure activity. Ethosuximide or valproate are the appropriate drugs for absence seizures.
How does carbamazepine cause hyponatraemia?
Carbamazepine has an ADH-like effect on the renal collecting ducts (structurally similar to vasopressin) and also stimulates ADH release centrally, causing SIADH-like water retention. This leads to dilutional hyponatraemia, especially in elderly patients. Serum sodium should be monitored, particularly in the first few months and in patients on concomitant diuretics.
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