Chapter 19  ·  Module 3
Valproate
Mechanisms, adverse effects, teratogenicity, and key interactions
Three Concurrent Mechanisms
Mechanism 1

Sodium Channel Blockade

Reduces high-frequency firing → focal and tonic-clonic seizures

Mechanism 2

T-Type Calcium Channel Blockade

Interrupts thalamic rhythm → absence seizures

Mechanism 3

Gamma-Aminobutyric Acid Transaminase Inhibition

Raises gamma-aminobutyric acid levels → enhanced inhibition

Adverse Effects
Figure 1 — Valproate Adverse Effects (Gemini)
Black Box Hepatotoxicity
  • Fatal cases reported
  • Highest risk: children under 2 on polypharmacy
  • Monitor liver function first 6 months
Black Box Teratogenicity
  • Highest teratogenic risk among anti-seizure drugs
  • Neural tube defects — 10–20x background rate
  • Neurodevelopmental impairment (lower IQ, autism risk)
  • Folic acid mandatory — reduces but does not eliminate risk
Teratogenicity Ranking (Highest to Lowest)
Figure 2 — Teratogenicity Ranking (Gemini)
Key Drug Interactions
Valproate Is an Inhibitor Raises Levels of Other Drugs
  • + Lamotrigine → doubles lamotrigine level → doubles Stevens-Johnson syndrome risk; halve lamotrigine dose
  • + Phenytoin → displaces from protein binding + inhibits metabolism; monitor levels
  • Enzyme inducers (carbamazepine, phenytoin) → lower valproate levels; may need dose increase
Clinical Priority — Women of Childbearing Potential
Valproate should not be first-line in women of childbearing potential. Lamotrigine or levetiracetam are preferred. If valproate cannot be avoided, folic acid supplementation is mandatory and the patient must receive explicit counseling about teratogenic risk.