Elevation of thioridazine plasma levels by propranolol

As part of a pilot study to assess the efficacy of propranolol in the treatment of aggression in psychiatric patients, drug interactions of propranolol with other agents were estimated by blood level monitoring. In the two patients taking a standard oral dose of thioridazine, the addition of propranolol caused a threefold and fivefold increase of plasma thioridazine levels, which placed them in a potentially toxic range. Although neither patient exhibited a toxic effect, these levels may be associated with an increased risk of thioridazine-induced irreversible pigmentary retinopathy, cardiac arrhythmias, and tardive dyskinesia. The authors suggest precautionary monitoring of thioridazine plasma and other relevant levels and clinical assessments when thioridazine and propranolol are used in combination.