Pharmacology  ·  Anti-Seizure Drugs

Sodium Channel Blockers

Phenytoin, carbamazepine, and lamotrigine at a glance

• All three drugs share a single mechanism: they stabilize the inactivated state of voltage-gated sodium channels, slowing recovery and reducing the ability of neurons to fire at high frequency. The shared mechanism explains the shared spectrum — they are effective for focal seizures and generalized tonic-clonic seizures, and none of them works for absence.
• What differentiates the three is not their seizure indication but their pharmacokinetic behavior and adverse effect profile. Phenytoin's zero-order kinetics is what makes it clinically hazardous at high doses: a small dose increase produces a disproportionate concentration increase that can cross the toxic threshold without warning.
• Carbamazepine's autoinduction is the pharmacokinetic trap that most often catches prescribers. Levels achieved in the first week of treatment can be halved by the fourth week as the drug induces its own metabolism — patients who seem well-controlled early may lose efficacy without any change in dose.
• Lamotrigine's Stevens-Johnson syndrome risk is titration-rate dependent, not dose dependent. The mandatory slow titration schedule exists because the immune mechanism requires time to develop — rushing through it dramatically increases the risk even at doses that would ultimately be safe at steady state.
• The interaction section pairs the two red cards because they share a common clinical consequence: both the enzyme-inducing pair (phenytoin, carbamazepine) and the valproate-lamotrigine combination are sources of clinically significant concentration changes that are not predictable from doses alone and require active monitoring.