Antiepileptic drug interactions

Enzyme Inducers - Metabolic Accelerators

• Certain AEDs ↑ the synthesis of hepatic cytochrome P450 (CYP) enzymes, accelerating the metabolism of themselves (auto-induction) and other drugs.
• Key Inducers:
  • Phenytoin
  • Carbamazepine
  • Phenobarbital
  • Primidone
  • 📌 Mnemonic: Phen-Phen Carbs Go.
• Clinical Impact:
  • ↓ serum levels & efficacy of co-administered drugs (e.g., other AEDs, warfarin, oral contraceptives).
  • Can lead to therapeutic failure.

⭐ High-Yield Interaction: Enzyme-inducing AEDs significantly reduce the effectiveness of hormonal contraceptives. This can result in unintended pregnancy; counsel patients on alternative, non-hormonal contraception methods.

Enzyme Inhibitors - Metabolic Brakes

• Certain antiepileptics act as "brakes" on hepatic enzymes (cytochrome P450), slowing the metabolism of other drugs, leading to ↑ serum concentrations and potential toxicity.
• Primary Inhibitor: Valproic Acid is a broad-spectrum inhibitor.
• Key Interactions (Valproate + ...):
  • Lamotrigine: ↑ lamotrigine levels → ↑ risk of Stevens-Johnson Syndrome (SJS). Requires significant lamotrigine dose reduction.
  • Phenytoin: ↑ free phenytoin levels.
  • Carbamazepine: ↑ toxic metabolite (carbamazepine-epoxide).
  • Phenobarbital: ↑ phenobarbital levels → sedation/coma.

⭐ Valproic acid significantly inhibits the glucuronidation of lamotrigine, doubling its half-life. This dramatically increases the risk for life-threatening rashes like SJS/TEN, especially during initial titration.

Clinically Key Pairs - High-Stakes Duos

• Enzyme Inducers vs. The World

  • Inducers: Phenytoin, Carbamazepine, Phenobarbital, Primidone.
  • Mechanism: ↑ CYP450 enzyme activity, accelerating metabolism of co-administered drugs.
  • Victims: ↓ levels of Warfarin, oral contraceptives, and other AEDs.
  • 📌 Mnemonic: Phenytoin, Carbamazepine, Phenobarbital Primidone Induce Cytochrome P450. (PCP PIC)

• Valproic Acid vs. Lamotrigine

  • Mechanism: Valproate (an inhibitor) ↓ UGT glucuronidation, significantly ↑ lamotrigine levels.
  • Result: Potentially fatal Stevens-Johnson Syndrome (SJS) or Toxic Epidermal Necrolysis (TEN).
  • 💡 Pearl: When combining, start lamotrigine at a very low dose and titrate slowly ("start low, go slow").

⭐ Valproate + Lamotrigine Interaction: This combination can double lamotrigine levels. The increased risk of life-threatening rash (SJS/TEN) is a classic board question. Always requires dose reduction of lamotrigine.

High‑Yield Points - ⚡ Biggest Takeaways

• Enzyme-inducing AEDs (Carbamazepine, Phenytoin, Phenobarbital) ↓ levels of other drugs, including oral contraceptives.
• Valproic acid is a broad-spectrum enzyme inhibitor, which ↑ levels of other drugs, notably lamotrigine.
• Coadministration of valproate and lamotrigine significantly ↑ the risk of life-threatening Stevens-Johnson syndrome.
• Phenytoin exhibits zero-order kinetics; its metabolism is easily saturated, leading to toxicity.
• Many AEDs are teratogenic, requiring careful management in pregnancy.