Classic ASMs: Phenytoin, Carbamazepine, Valproate

The “classic” antiseizure medications — phenytoin, carbamazepine, valproate, phenobarbital, and ethosuximide — were developed between the 1930s and 1970s and remain among the most prescribed despite the development of newer agents. They have well-defined efficacy, decades of clinical data, and (in some cases) cost advantages, but they also have substantial interaction profiles, narrow therapeutic windows, and specific safety considerations that demand careful attention. This page covers their pharmacology, clinical use, and the modern positioning of each.

Phenytoin (and Fosphenytoin)

Mechanism

  • Blocks voltage-gated Na⁺ channels in inactivated state.
  • Use-dependent: more effective at high firing rates.

Pharmacokinetics

  • CYP2C9, CYP2C19 metabolism.
  • Highly protein-bound (90%); hypoalbuminemia, renal failure increase free fraction.
  • Saturable (zero-order) kinetics: small dose changes can produce large concentration changes.
  • Half-life variable (12-36 hours).
  • Therapeutic range: 10-20 μg/mL total (~1-2 μg/mL free).

Uses

  • Focal seizures, generalized tonic-clonic.
  • Status epilepticus (loading dose IV).
  • Less commonly used now for maintenance due to side effects.

Side Effects

  • Acute: nystagmus, ataxia, diplopia, sedation at high levels.
  • Chronic: gingival hyperplasia, hirsutism, coarse facies, peripheral neuropathy, osteoporosis, megaloblastic anemia (folate), cerebellar atrophy (long-term high-dose).
  • Idiosyncratic: rash, SJS, hepatitis, lupus-like syndrome.
  • Pregnancy: cleft lip/palate, “fetal phenytoin syndrome.”
  • IV: cardiac arrhythmia, hypotension; max infusion rate 50 mg/min; “purple glove syndrome” with extravasation.

Fosphenytoin

  • Water-soluble prodrug; less infusion site reaction; can give IM.
  • Dose in phenytoin equivalents (PE).
  • Loading: 20 mg/kg PE IV at up to 150 mg/min.
  • Still has cardiac monitoring requirement.

Drug Interactions

  • CYP inducer: reduces levels of many drugs (warfarin, OCs, immunosuppressants, DMTs, antibiotics).
  • CYP2C9 inhibitors (fluconazole, sulfonamides, isoniazid): increase phenytoin levels.
  • Valproate: increases free phenytoin (protein displacement).

Modern Positioning

  • Still used for status epilepticus (IV loading).
  • Less commonly chosen for long-term maintenance — better-tolerated alternatives available.
  • Cheap; widely available globally.

Carbamazepine

Mechanism

  • Na⁺ channel blockade.
  • Possible adenosine receptor effects.

Pharmacokinetics

  • CYP3A4 metabolism.
  • Auto-induction: induces its own metabolism; levels drop over 2-4 weeks despite stable dose.
  • Half-life initially 30 hours, decreasing to 12-17 hours after auto-induction.
  • Active metabolite: carbamazepine-10,11-epoxide (CBZ-E).

Uses

  • Focal seizures.
  • Generalized tonic-clonic (caveat: can worsen absence, myoclonus, in mixed epilepsies).
  • Trigeminal neuralgia (first-line).
  • Mood stabilization (bipolar).
  • Restless legs (off-label).

Side Effects

  • Common: diplopia, dizziness, sedation, nausea.
  • Hyponatremia (SIADH).
  • Leukopenia, aplastic anemia (rare), agranulocytosis.
  • Idiosyncratic: SJS/TEN — strongly associated with HLA-B*15:02 in Asians (FDA screening recommendation).
  • DRESS, hepatitis.
  • Pregnancy: neural tube defects.
  • CYP inducer side effects (osteoporosis, reduced bone density).

Drug Interactions

  • Strong CYP3A4 inducer; reduces many drug levels.
  • CYP3A4 inhibitors increase carbamazepine levels.
  • Grapefruit juice can increase levels.

Oxcarbazepine (and Eslicarbazepine)

  • Carbamazepine analog; metabolized to active monohydroxy derivative (MHD).
  • Similar efficacy; less idiosyncratic reactions, less induction.
  • Eslicarbazepine: similar mechanism; once-daily dosing.
  • Hyponatremia risk higher than carbamazepine.
  • Mild CYP3A4 induction; reduces OC efficacy at high doses.
  • Cross-reactivity with carbamazepine for rash (~30%).

Valproate

Mechanism

  • GABA enhancement, Na⁺ channel modulation, T-type Ca²⁺ channel block.
  • Histone deacetylase inhibition (theoretical disease-modifying potential).

Pharmacokinetics

  • Hepatic metabolism (β-oxidation, glucuronidation).
  • Highly protein-bound.
  • Half-life 9-16 hours.
  • Therapeutic range: 50-100 μg/mL.

Uses

  • Broad-spectrum: generalized tonic-clonic, absence, myoclonic, mixed.
  • Migraine prophylaxis.
  • Bipolar disorder.
  • Status epilepticus (IV alternative).

Side Effects

  • GI: nausea, weight gain, anorexia (variable).
  • Tremor.
  • Alopecia, hair texture changes.
  • Thrombocytopenia, mild liver enzyme elevation.
  • Hyperammonemia (with or without encephalopathy).
  • Pancreatitis (rare, can be severe).
  • Polycystic ovarian syndrome (concern in young women).
  • Hepatic failure (rare; highest risk in young children with metabolic disease).
  • Pregnancy: HIGHEST teratogenicity (neural tube defects, cognitive effects, autism risk).

Modern Positioning

  • Avoid in women of childbearing age unless other options inadequate.
  • Most effective for primary generalized epilepsy in men.
  • Most effective for myoclonic epilepsies.
  • Effective for migraine prophylaxis but limited by side effects.

Phenobarbital

Mechanism

  • GABA-A allosteric positive modulator; prolongs Cl⁻ channel opening.
  • At high doses, direct GABA-A activation (lethal in overdose).

Pharmacokinetics

  • Long half-life (~100 hours; longest of all ASMs).
  • CYP2C19 metabolism + glucuronidation.
  • Renal excretion of unchanged drug (~25%); alkalinization enhances clearance in overdose.

Uses

  • Focal and generalized tonic-clonic seizures.
  • Status epilepticus (second-line).
  • Neonatal seizures (historically first-line; now controversial).
  • Alcohol/benzodiazepine withdrawal (some protocols).

Side Effects

  • Sedation, paradoxical hyperactivity in children.
  • Cognitive effects (especially in children).
  • Bone density reduction.
  • Strong CYP inducer.
  • Respiratory depression in overdose.
  • Dependence/withdrawal (seizures).

Modern Positioning

  • Largely supplanted by newer ASMs in developed countries.
  • Still widely used globally (cheap, effective).
  • WHO essential medicine.
  • Reserved for refractory cases or specific populations.

Ethosuximide

Mechanism

  • T-type Ca²⁺ channel block in thalamic neurons.
  • Effective for absence seizures only.

Pharmacokinetics

  • Hepatic metabolism.
  • Half-life 30-60 hours.

Uses

  • Childhood absence epilepsy (first-line).
  • NOT effective for tonic-clonic or focal seizures.

Side Effects

  • GI: nausea, hiccups.
  • Cognitive effects.
  • Rash, SJS (rare).
  • Hepatic enzyme elevation.

Primidone

  • Metabolized to phenobarbital + phenylethylmalonamide.
  • Essential tremor (alternative to propranolol).
  • Rarely used as primary ASM.
  • Side effects similar to phenobarbital + initial sedation/dizziness.

Benzodiazepines as ASMs

  • Lorazepam, diazepam, midazolam: status epilepticus first-line.
  • Clobazam: less sedation than other benzos; Lennox-Gastaut, refractory.
  • Clonazepam: myoclonic, restless legs, paroxysmal dyskinesias.
  • Issues: tolerance, sedation, withdrawal.

🔍 Did You Know?

The recognition that valproate is the highest-teratogenicity ASM has dramatically changed clinical practice over the past two decades. Long-term cohort studies have shown that valproate exposure in utero is associated with: neural tube defects (5-10× population baseline), cardiac defects, oral clefts, and — most strikingly — significant cognitive effects on offspring, including reduced IQ (~9 points average) and increased risk of autism spectrum disorder. The cognitive risk persists even when major malformations are absent. The clinical implications have transformed prescribing: valproate is now AVOIDED in women of reproductive age in many jurisdictions (UK MHRA, FDA warnings), and many regions require specific consent forms before prescription. Even women who appear to have no pregnancy plans must be counseled extensively, with effective contraception emphasized. The alternative ASMs for primary generalized epilepsy in young women are levetiracetam and lamotrigine, both substantially better tolerated and lower-teratogenicity, though valproate may still be needed for refractory cases — with the highest priority on family planning counseling. The lesson generalizes: drug teratogenicity is not just about birth defects but about cognitive trajectories of children exposed in utero, and pharmacologic risk-benefit must include both the patient and future children. The case has also informed broader thinking about how teratogenicity studies should incorporate cognitive endpoints, not just structural malformations.

Pitfalls and Pearls

  • Phenytoin: saturable kinetics; small dose changes → large level changes; TDM essential.
  • Phenytoin free level: hypoalbuminemia, renal failure increase free fraction; measure if concerned.
  • IV fosphenytoin preferred over phenytoin: less infusion reactions; same monitoring.
  • Carbamazepine auto-induction: levels drop over 2-4 weeks; expect dose increase.
  • HLA-B*15:02 + carbamazepine: screen in Asians (SJS risk).
  • Carbamazepine hyponatremia: check Na⁺ if symptoms.
  • Oxcarbazepine: similar efficacy to CBZ, less idiosyncratic; more hyponatremia.
  • Valproate broad-spectrum: GTC, absence, myoclonic; preferred for primary generalized epilepsy in men.
  • Valproate AVOID in women of childbearing age: teratogenic.
  • Valproate + lamotrigine: rash risk; slow titration.
  • Phenobarbital: cheap, effective globally; sedation limits use; long half-life.
  • Ethosuximide: absence ONLY; first-line for childhood absence.
  • Enzyme inducers (CBZ, phenytoin, phenobarbital, primidone): reduce OC, warfarin, immunosuppressants, DMTs.
  • Primidone: essential tremor alternative.
  • IV phenytoin contains propylene glycol: hypotension, arrhythmia at fast infusion rate; max 50 mg/min.
  • Fetal hydantoin syndrome: cleft palate, midfacial hypoplasia, finger abnormalities.

References

  1. Brodie MJ, Sills GJ. Combining antiepileptic drugs—rational polytherapy? Seizure. 2011;20(5):369-375.
  2. Patsalos PN, Berry DJ, Bourgeois BF, et al. Antiepileptic drugs—best practice guidelines for therapeutic drug monitoring. Epilepsia. 2008;49(7):1239-1276.
  3. Tomson T, Battino D, Bonizzoni E, et al. Comparative risk of major congenital malformations with eight different antiepileptic drugs. Lancet Neurol. 2018;17(6):530-538.
  4. Meador KJ, Baker GA, Browning N, et al. Cognitive function at 3 years of age after fetal exposure to antiepileptic drugs. N Engl J Med. 2009;360(16):1597-1605.
  5. Glauser T, Ben-Menachem E, Bourgeois B, et al. Updated ILAE evidence review of antiepileptic drug efficacy and effectiveness as initial monotherapy. Epilepsia. 2013;54(3):551-563.