Drug Interactions between lamotrigine and risperidone

MONITOR: A case report suggests that lamotrigine may increase the plasma concentrations of risperidone. The mechanism of interaction is unknown. The case patient was a 26-year-old woman with schizophrenia who had been treated with clozapine 550 mg daily for 5 years and risperidone 8 mg daily for 4 weeks. Lamotrigine was added due to inadequate response. The patient's risperidone plasma level was 69 ng/mL while receiving lamotrigine 175 mg/day; 284 ng/mL when lamotrigine was increased to 200 mg/day; and 412 ng/mL at lamotrigine 225 mg/day. The patient complained of dizziness and tiredness. Risperidone dosage was reduced to 2 mg/day and discontinued a week later. Pharmacodynamically, additive central nervous system depression may also occur when these drugs are used in combination, resulting in impairment of judgment, thinking, and/or psychomotor skills.

MANAGEMENT: Until more data are available, caution is advised if risperidone must be used in combination with lamotrigine. Pharmacologic response and serum risperidone levels should be monitored more closely whenever lamotrigine is added to or withdrawn from therapy, and the risperidone dosage adjusted as necessary. Patients should be advised to notify their physician if they experience potential signs and symptoms of risperidone toxicity such as excessive sedation, dizziness, tachycardia, seizures, and extrapyramidal symptoms. Ambulatory patients should also be counseled to avoid activities requiring mental alertness (e.g., driving or operating hazardous machinery) until they know how these agents affect them.