Retrospective Review of Tizanidine (Zanaflex®) Overdose

BACKGROUND: Tizanidine is a centrally acting muscle relaxant with a novel mechanism of action and structurally related to clonidine. There are no large case series of tizanidine exposure. METHODS: Retrospective review of all ingestions involving tizanidine reported to a poison control center from January 2000 through February 2003. Exclusion criteria were polydrug ingestion, no follow-up or lost to follow-up. RESULTS: There were 121 cases of which 45 patients met entrance criteria. Mean age was 32 years (range 1 to 80). Thirty-seven patients were evaluated in a health care facility of which 27 were admitted for medical care. Clinical effects included lethargy (n = 38), bradycardia (n = 14), hypotension (n = 8), agitation (n = 7), confusion (n = 5), vomiting (n = 3), and coma (n = 2). Mean dose ingested by history was 72 mg (S.D. + 86). The lowest dose associated with hypotension was 28mg, which occurred in a 63-year-old female with a BP of 88/52 and a HR of 54. The lowest dose associated with coma was between 60 mg and 120 mg, which occurred in a 30-year-old female with a HR of 30 and BP of 81/48. There were 6 patients < 6 yrs. The lowest dose with bradycardia and drowsiness in a small child was 16 mg in a 2 YO (weight unknown). All other cases in children < 6 yrs involved ingestion of a single tablet (2 or 4 mg) with only mild drowsiness reported. Therapy in this series was primarily supportive and included pressors in 3 cases and intubation in 3 cases. Naloxone was administered to 7 patients. There was no response to naloxone in 5 patients, poor documentation of response in one, and arousal in one patient. All patients recovered without residual complications. CONCLUSION: Clinical manifestations of tizanidine overdose include alterations of mental status, bradycardia, and hypotension. In this series, outcome was good with supportive therapy.