Tizanidine-Induced Bradycardia Without Concomitant Medications: A Case Report.

A 37-year-old woman was admitted to our hospital due to a loss of consciousness. She had been taking 2 mg of tizanidine for two months to manage shoulder muscle pain at night. On admission, an electrocardiogram showed sinus bradycardia with a heart rate of 30 bpm and QT prolongation (QTc 495 msec). She had a temporary pacemaker inserted in the catheterization room, after which an improvement in her level of consciousness was observed. There were no apparent endocrine disorders or structural heart diseases. The administration was discontinued after admission, and 12 hours after admission, her heart rate normalized to a sinus rhythm of 70-100 bpm, and QTc improved to 431 msec. Therefore, she was diagnosed with tizanidine-induced bradycardia. Although reports of tizanidine-induced bradycardia are rare, tizanidine's central α2 agonistic effects can cause bradycardia, necessitating caution.