Pharmacodynamics and pharmacokinetics of the oral antispastic agent tizanidine in patients with spinal cord injury.

The efficiency and duration of action of a single oral dose (8 mg) of tizanidine in patients with spinal cord injuries were determined by studying its antispastic, cardiovascular and sedative effects along with its pharmacokinetic profile in five tetraplegic and five paraplegic patients. After the administration of tizanidine, there was a reduction in spasticity in both groups within half an hour, with the effects lasting for 3 to 4 hours. There was no rebound increase in blood pressure. There was a greater increase in sedation in the tetraplegics than in the paraplegics. Plasma tizanidine levels rose within half an hour after dosing and peaked at one hour. The levels had fallen to 15 percent by 6 hours. The plasma half-life was 2.7 +/- 0.06 hours. We conclude that oral tizanidine has antispastic effects in patients with spinal cord injuries without affecting the power of non-involved muscle groups. It has minimal effects on blood pressure and it lowers heart rate. Side effects include sedation and dryness of mouth.