Antiepileptic Drugs for NEET PG 2026: Mechanisms, DOC, ADRs, Status Algorithm

Antiepileptic drugs are a NEET PG goldmine because the topic combines pharmacology mechanism questions, paediatric neurology vignettes, obstetric drug-safety dilemmas, and emergency-medicine status-epilepticus algorithms in one tightly examinable cluster. The 2017 ILAE classification (focal-onset, generalised-onset, unknown-onset, with motor or non-motor sub-types) and the 2024 NICE epilepsy guideline simplified some drug-of-choice recommendations — examiners now actively ask the updated DOC.

This NEETPGAI deep dive walks through mechanisms by class, DOC by seizure type, the status-epilepticus algorithm, ADRs that recur in vignettes, and the high-yield pregnancy and paediatric considerations. Pair it with the Pharmacology subject hub and the common mistakes in pharmacology guide.

Mechanisms by drug class

Every AED works on one of four principal mechanisms — and many work on more than one. Understanding the mechanism predicts both efficacy and adverse effects.

Sodium channel blockers (use-dependent inactivation)

These bind preferentially to Na channels in the inactivated state, prolonging the refractory period and preventing high-frequency neuronal firing.

• Phenytoin — non-linear (zero-order at therapeutic doses) kinetics. Narrow therapeutic window (10 to 20 mcg/mL).
• Fosphenytoin — water-soluble pro-drug; safer infusion (no purple-glove syndrome).
• Carbamazepine — first-line for focal seizures historically; auto-induces its own metabolism.
• Oxcarbazepine — keto-analogue of carbamazepine; less hepatic enzyme induction; active metabolite is licarbazepine.
• Lamotrigine — also inhibits glutamate release. Broad-spectrum.
• Lacosamide — enhances slow inactivation of Na channels (novel mechanism); useful adjunctive in focal seizures.
• Topiramate — multiple mechanisms including Na channel blockade.

GABA enhancers

• Benzodiazepines (lorazepam, diazepam, midazolam, clobazam) — GABA-A receptor positive allosteric modulators (increase frequency of channel opening).
• Barbiturates (phenobarbitone) — increase duration of GABA-A channel opening.
• Vigabatrin — irreversible GABA transaminase inhibitor; raises CNS GABA. Used in infantile spasms.
• Tiagabine — selective GAT-1 GABA reuptake inhibitor.

Calcium channel modulators

• Ethosuximide — blocks T-type calcium channels in thalamic neurons; specific for absence seizures.
• Gabapentin and pregabalin — bind alpha-2-delta subunit of voltage-gated calcium channels; used for focal seizures and neuropathic pain.

Glutamate antagonists and SV2A binders

• Perampanel — selective non-competitive AMPA glutamate receptor antagonist.
• Felbamate — NMDA antagonist; restricted use due to aplastic anaemia and hepatotoxicity.
• Levetiracetam and brivaracetam — bind synaptic vesicle protein 2A (SV2A), modulating neurotransmitter release. Brivaracetam is more selective and 10 to 30 times more potent than levetiracetam.

Carbonic anhydrase inhibition

• Topiramate, zonisamide, acetazolamide — multiple mechanisms including CA inhibition; cause metabolic acidosis, kidney stones, paraesthesia, weight loss.

Cannabidiol (newer)

• Cannabidiol — approved for Dravet syndrome, Lennox-Gastaut syndrome, and tuberous sclerosis-associated seizures.

Drug of choice by seizure type

The 2024 NICE update made important changes — particularly for women of childbearing age — and these are exam-relevant.

Focal (focal-onset) seizures

First-line — lamotrigine or levetiracetam. Carbamazepine and oxcarbazepine remain widely used; in India, carbamazepine often persists as first-line due to cost.

Second-line — zonisamide, lacosamide, topiramate, eslicarbazepine, perampanel.

Generalised tonic-clonic seizures (GTCS)

Avoid — carbamazepine, phenytoin, oxcarbazepine if myoclonic component or juvenile myoclonic epilepsy is suspected.

Absence seizures (typical, childhood)

• First-line — ethosuximide if pure absence (CAB study showed superiority over valproate and lamotrigine).
• If GTCS coexist — valproate (in males or post-menopausal females) or lamotrigine (women of childbearing age).
• Strictly avoid — carbamazepine, phenytoin, gabapentin, vigabatrin, tiagabine — all worsen absence seizures.

Myoclonic seizures and juvenile myoclonic epilepsy

• First-line — valproate (men); levetiracetam (women of childbearing age).
• Second-line — lamotrigine, topiramate, zonisamide.
• Avoid — carbamazepine, phenytoin, oxcarbazepine, gabapentin, pregabalin (worsen myoclonus).

Lennox-Gastaut syndrome

Valproate, lamotrigine, topiramate, rufinamide, clobazam, cannabidiol, vagus nerve stimulation.

Infantile spasms (West syndrome)

• First-line — vigabatrin (especially in tuberous sclerosis) or ACTH.
• Second-line — high-dose oral steroids.

Dravet syndrome

Valproate plus clobazam first; stiripentol, fenfluramine, cannabidiol added. Avoid sodium channel blockers (carbamazepine, phenytoin, lamotrigine) — they worsen Dravet seizures.

Key adverse drug reactions

Cutaneous and hypersensitivity

• Stevens-Johnson syndrome and toxic epidermal necrolysis — carbamazepine (HLA-B*1502 in Han Chinese, Thai, and South Indian populations), lamotrigine (especially with rapid titration or co-valproate), phenytoin, phenobarbitone.
• Anticonvulsant hypersensitivity syndrome (DRESS) — fever, rash, lymphadenopathy, hepatitis. Cross-reactivity between carbamazepine, phenytoin, oxcarbazepine, phenobarbitone.

Hepatic

• Valproate — hepatotoxicity (highest risk in children under 2 years on polytherapy with mitochondrial disease — especially POLG mutation). Hyperammonaemia even with normal LFTs.
• Felbamate — fulminant hepatic failure (restricted use).

Haematologic

• Carbamazepine — aplastic anaemia (rare), agranulocytosis, hyponatraemia (SIADH-like).
• Felbamate — aplastic anaemia.
• Valproate — thrombocytopenia, platelet dysfunction.

Cognitive and behavioural

• Topiramate — cognitive slowing (the "dopamax" colloquialism), word-finding difficulty.
• Levetiracetam — irritability, aggression, depression — often dose-limiting.
• Phenobarbitone — sedation, paradoxical hyperactivity in children.

Neurological and visual

• Vigabatrin — irreversible peripheral visual field constriction (concentric); requires regular Goldmann perimetry.
• Phenytoin — cerebellar atrophy with chronic toxicity, gingival hyperplasia, hirsutism, coarse facies, megaloblastic anaemia (folate inhibition), peripheral neuropathy.
• Carbamazepine — diplopia, ataxia, drowsiness.

Endocrine and metabolic

• Valproate — weight gain, polycystic ovary syndrome, alopecia, tremor, pancreatitis.
• Topiramate, zonisamide, acetazolamide — metabolic acidosis, nephrolithiasis, paraesthesias.
• Phenytoin and phenobarbitone — vitamin D deficiency, osteomalacia, osteoporosis.

Teratogenicity (high-yield for OBG)

All women on AEDs need preconceptional folate 4 to 5 mg daily and active monotherapy planning.

Status epilepticus algorithm

Status epilepticus is defined as continuous seizure activity above 5 minutes or recurrent seizures without recovery between episodes. Mortality rises with duration. The 2016 NCS and 2024 ILAE algorithms agree on a stepwise approach.

Stage 1 — Stabilisation phase (0 to 5 minutes)

ABCs, IV access, glucose check (give 25 g dextrose if hypoglycaemic plus thiamine 100 mg in malnourished), ECG, basic labs.

Stage 2 — Initial therapy (5 to 20 minutes)

First-line — benzodiazepine:

• IV lorazepam 0.1 mg/kg (max 4 mg) — preferred IV agent.
• IM midazolam 10 mg — preferred when IV access not yet available (RAMPART trial).
• IV diazepam 0.15 to 0.2 mg/kg — alternative.
• PR diazepam 0.2 to 0.5 mg/kg — used in paediatrics at home or in transit.

Stage 3 — Second-line therapy (20 to 40 minutes)

Established status epilepticus. Choose ONE:

• Fosphenytoin 20 mg PE/kg IV at up to 150 mg PE/min.
• Valproate 40 mg/kg IV (max 3 g) over 10 minutes.
• Levetiracetam 60 mg/kg IV (max 4500 mg) over 15 minutes.

ESETT trial (2019) showed all three are equivalent for second-line therapy. Levetiracetam is increasingly preferred for safety.

Stage 4 — Third-line therapy (above 40 minutes)

Refractory status epilepticus — give continuous-infusion anaesthesia and intubate:

• Propofol — bolus 1 to 2 mg/kg, then 2 to 10 mg/kg/hr.
• Midazolam infusion — 0.05 to 0.4 mg/kg/hr.
• Pentobarbital — last-line; deep coma with burst suppression on EEG.

Continuous EEG monitoring is mandatory once anaesthesia is started. Goal — burst suppression for 24 to 48 hours, then taper.

Stage 5 — Super-refractory status (above 24 hours despite anaesthesia)

Ketamine, isoflurane, hypothermia, ketogenic diet, immunotherapy if autoimmune cause suspected.

NEET PG MCQ traps

1. Absence seizure with carbamazepine — paradoxical worsening; switch to ethosuximide or valproate. Classic vignette stem.
2. Phenytoin zero-order kinetics — small dose increase from 300 to 400 mg can cause toxicity. Plot levels.
3. Purple-glove syndrome — IV phenytoin extravasation; use fosphenytoin instead.
4. Pseudoseizures (PNES) — bilateral asynchronous limb movements, pelvic thrusting, eyes forced closed, post-ictal lactate normal. EEG normal during episode.
5. Antiepileptic enzyme induction — phenytoin, carbamazepine, phenobarbitone reduce OCP efficacy and warfarin levels. Valproate is an inhibitor (raises lamotrigine levels — start low, go slow).
6. Lamotrigine plus valproate — valproate inhibits lamotrigine glucuronidation; halve the lamotrigine dose to avoid SJS.
7. HLA-B*1502 — Han Chinese, Thai, South Indian; test before starting carbamazepine.
8. Vigabatrin VF defect — irreversible concentric peripheral constriction; mandatory perimetry.
9. Tiagabine and gabapentin — worsen absence and myoclonic seizures.
10. Valproate in pregnancy — avoid in women of childbearing age unless no alternative; risk-mitigation programs require dual contraception.

Recent updates and Indian context

• 2024 NICE epilepsy guideline — lamotrigine and levetiracetam are first-line in women of childbearing age for GTCS and focal seizures; valproate avoided.
• ESETT trial (2019) — fosphenytoin, valproate, and levetiracetam are equivalent in established status epilepticus.
• Cannabidiol — approved for Dravet, Lennox-Gastaut, and tuberous sclerosis-associated seizures.
• HLA-B*1502 screening before carbamazepine is now formally recommended in South Asian populations — relevant FMGE and NEET PG vignette.
• Indian context — phenytoin and carbamazepine remain widely prescribed due to cost; levetiracetam generics have increased access. Valproate is still over-prescribed in young women in district practice — this is the single most common drug-related teratogenicity question in NEET PG.

Frequently asked questions

What is the drug of choice for absence seizures?

Ethosuximide is first-line for typical childhood absence seizures (3 Hz spike-and-wave on EEG) when only absence seizures are present. Valproate is preferred when GTCS coexist. Lamotrigine is third-line. Carbamazepine, phenytoin, and gabapentin can paradoxically worsen absence seizures and are contraindicated.

Which AED is safest in pregnancy?

Lamotrigine and levetiracetam have the lowest teratogenicity and are preferred in pregnancy. Valproate has the highest risk (neural tube defects up to 10 percent, IQ reduction) and should be avoided in women of childbearing age unless no alternative exists. All women on AEDs need pre-conceptional folate 4 to 5 mg daily.

What is the status epilepticus management ladder?

First line is IV lorazepam 0.1 mg/kg (or IV diazepam, IM midazolam if no IV access) within 5 minutes. Second line at 20 to 40 minutes is IV fosphenytoin, valproate, or levetiracetam. Refractory status (above 40 minutes) needs anesthetic infusion — propofol, midazolam, or pentobarbital with continuous EEG.

Which AED causes Stevens-Johnson syndrome most commonly?

Carbamazepine has the strongest association, especially in patients with HLA-B1502 allele (common in Han Chinese, Thai, and South Indian populations). Lamotrigine causes SJS particularly with rapid titration or co-administration with valproate. Phenytoin and phenobarbitone are also implicated. HLA-B1502 testing is recommended before starting carbamazepine in at-risk populations.

What is the drug of choice for focal seizures?

First-line options are lamotrigine, levetiracetam, oxcarbazepine, and carbamazepine. Levetiracetam is increasingly preferred due to favourable pharmacokinetics, no significant drug interactions, and rapid titration. Carbamazepine remains widely used in India given cost. Phenytoin has fallen out of favour due to non-linear kinetics.

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Written by: NEETPGAI Editorial Team Reviewed by: Pending SME Review Last reviewed: May 2026