|Trade names||Zanaflex, Sirdalud, and others|
|By mouth (tablets, capsules)|
|Metabolism||Liver (CYP1A2, 95%)|
|Elimination half-life||2.54 hours (tizanidine), 20–40 hours (inactive metabolites)|
|Excretion||Urine (60%), feces (20%)|
|CompTox Dashboard (EPA)|
|Chemical and physical data|
|Molar mass||253.71 g·mol−1|
|3D model (JSmol)|
Tizanidine, sold under the brand name Zanaflex among others, is a medication that is used to treat muscle spasticity due to spinal cord injury or multiple sclerosis. Effectiveness appears similar to baclofen or diazepam. It is taken by mouth.
Common side effects include dry mouth, sleepiness, weakness, and dizziness. Serious side effects may include low blood pressure, liver problems, psychosis, and QT prolongation. It is unclear if use in pregnancy and breastfeeding is safe. It is an α2-adrenergic agonist and how it works is not entirely clear.
Tizanidine was approved for medical use in the United States in 1996. It is available as a generic medication. In 2017, it was the 89th most commonly prescribed medication in the United States, with more than nine million prescriptions.
Side effects include dizziness, drowsiness, weakness, nervousness, hallucinations, depression, vomiting, dry mouth, constipation, diarrhea, stomach pain, heartburn, increased muscle spasms, back pain, rash, sweating, and a tingling sensation in the arms, legs, hands, and feet.
Concomitant use of tizanidine and moderate or potent CYP1A2 inhibitors (such as zileuton, certain antiarrhythmics (amiodarone, mexiletine, propafenone, verapamil), cimetidine, famotidine, aciclovir, ticlopidine and oral contraceptives) is contraindicated. Concomitant use of tizanidine with fluvoxamine, a potent CYP1A2 inhibitor in humans, resulted in a 33-fold increase in the tizanidine AUC (plasma drug concentration-time curve). For this reason both fluvoxamine and tizanidine should not be taken at the same time. Fluoroquinolone antibiotics such as moxifloxacin, levofloxacin, and ciprofloxacin should also be avoided due to an increased serum concentration of tizanidine when administered concomitantly. Tizanidine has the potential to interact with other central nervous system depressants. Alcohol should be avoided, particularly as it can upset the stomach. The CNS-depressant effects of tizanidine and alcohol are additive. Caution with the following interactions:
- antibiotics such as enoxacin, gatifloxacin, levofloxacin, lomefloxacin, moxifloxacin, ofloxacin, sparfloxacin, trovafloxacin, or norfloxacin;
- blood pressure medications such as clonidine, guanabenz, guanfacine (Tenex), or methyldopa;
- heart rhythm medications such as amiodarone (Cordarone, Pacerone), mexiletine (Mexitil), propafenone (Rhythmol), and verapamil (Calan, Covera, Isoptin).
Tizanidine is an α2 receptor agonist closely related to clonidine. It has approximately one tenth to one fifteenth of the pressure lowering effect of clonidine. The relation between the α2 receptor agonism and the spasmolytic action is still not fully understood.
Route of administration
Tizanidine is available as a tablet or capsule. Capsules may be opened and sprinkled on food. However, this may change the absorption of the medication compared to taking the capsule whole.
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