Elevated serum carbamazepine concentrations following diltiazem initiation

Area of Science:

• Pharmacology
• Clinical Toxicology
• Drug Interactions

Background:

• Carbamazepine is an anticonvulsant and mood-stabilizing drug with a narrow therapeutic index.
• Drug interactions are common and can lead to altered pharmacokinetics and toxicity.
• Atrial fibrillation is a common cardiac arrhythmia requiring treatment.

Observation:

• A patient on stable carbamazepine therapy developed neurological symptoms after initiating diltiazem for atrial fibrillation.
• The patient experienced neurotoxicity, necessitating hospitalization and medication review.

Findings:

• A significant drug interaction between diltiazem and carbamazepine was identified.
• Diltiazem likely inhibited the metabolism or elimination of carbamazepine, leading to increased serum concentrations and toxicity.
• A 62% reduction in carbamazepine dosage resulted in the resolution of neurotoxicity.

Implications:

• Clinicians should be aware of the potential for diltiazem to increase carbamazepine levels.
• Careful monitoring of carbamazepine serum concentrations and neurological status is crucial when co-prescribing these medications.
• Dosage adjustments may be necessary to prevent or manage carbamazepine neurotoxicity in patients receiving diltiazem.