Drug interactions between Lithobid and Topamax

MONITOR: Coadministration with topiramate may increase the serum concentrations of lithium. The exact mechanism of interaction is unknown but may involve competition for renal excretion and/or sodium depletion induced by topiramate, which is a weak carbonic anhydrase inhibitor. There have been isolated case reports of patients treated with lithium who developed symptoms of toxicity (e.g., impaired concentration, confusion, lethargy, tremors, bradycardia, nystagmus) in association with elevated serum lithium levels following the addition or increase in dosage of topiramate. A reduction in the lithium dosage was necessary to maintain therapeutic levels during coadministration with topiramate. However, topiramate at a daily dosage of 200 mg has been reported by some investigators to exhibit little or no pharmacokinetic interaction with lithium.

MANAGEMENT: Caution is advised if topiramate must be used concomitantly with lithium. Pharmacologic response and serum lithium levels should be monitored closely and the dosage adjusted accordingly, particularly following initiation, discontinuation or change of dosage of topiramate in patients who are stabilized on their lithium regimen. Patients should be advised to notify their physician if they experience early symptoms of lithium toxicity such as drowsiness, dizziness, diarrhea, vomiting, muscle weakness, incoordination, ataxia, tremor, blurred vision, tinnitus, excessive thirst, or increased urination.