Clinical evidence,mechanism, importance and management
An isolated report describes a 59-year-old man whose phenytoin levels rose by one-third,from about 19 to 25.5 micrograms/mL,and who experienced drowsiness within a week of starting tizanidine 6mg daily. The phenytoin was stopped for 3 days and restarted at a reduced dose, but drowsiness recurred in 3 weeks (phenytoin level 20.5 micrograms/mL). Therefore, tizanidine was withdrawn (See reference number 1). The general importance of this interaction is unclear,but it would seem prudent to remain aware of this interaction in case of an unusual response to treatment.
1. Ueno K,Miyai K, Mitsuzane K. Phenytoin-tizanidine interaction. DICP Ann Pharmacother (1991) 25, 1273.