Pharm 101: Carbamazepine

Class

Anticonvulsant
Mood stabiliser


Pharmacodynamics
  • Structurally similar to tricyclic antidepressants such as imipramine
  • Works by sodium channel blockade in a manner similar to phenytoin:
    • Presynaptic blockade of synaptic transmission
    • Inhibits high frequency repetitive firing of neurons
  • Also acts at adenosine receptors and as an anti-cholinergic

Pharmacokinetics
  • Bioavailability ~99%
  • Peak levels 6-8 hours after administration, food slows absorption
  • Volume of distribution 1L/kg
  • 70% plasma protein bound
  • Liver metabolism to active metabolites (carbamazepine epoxide), only 5% excreted unchanged
  • Half life initially 36 hours then shortens over time to 12 hours
  • CYP enzyme inducer:
    • Results in increased clearance of anticonvulsant drugs including itself, as well as phenytoin, valproate, lamotrigine, diazepam
    • Can result in breakthrough seizures
    • Increases metabolism of OCP reducing its effectiveness
    • Note valproate and phenytoin may inhibit carbamazepine elimination

Clinical uses
  • Anticonvulsant for partial and generalised tonic-clonic seizures
  • Bipolar mood disorder
  • Trigeminal neuralgia

Adverse effects
  • CNS:
    • Nystagmus, diplopia
    • Ataxia
    • Sedation occurs only at high doses
  • GI: upset, hepatic dysfunction
  • Haematological: benign leukopenia
  • Dermatological: erythematous skin rash, rarely Stevens-Johnson Syndrome
  • Metabolic: hyponatraemia
  • Overdose causes seizures and cardiac toxicity. Activated charcoal may assist in elimination of drug

Further Reading

Pharm 101 700

Pharmacology 101

Top 200 drugs

MBBS (UWA) CCPU Emergency Medicine Trainee with interests in medical education, ECG interpretation, and the use of point-of-care ultrasound in the undifferentiated patient. Co-author of the LITFL ECG Library | Twitter

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