Tizanidine is a centrally acting alpha-2 adrenergic agonist commonly used for the management of muscle spasticity. While it can be an effective medication for reducing muscle tone and improving function in select patients, it also carries significant risks that pharmacists should recognize. Understanding the pharmacology, adverse effect profile, drug interactions, and monitoring considerations can help pharmacists optimize therapy and prevent medication-related problems.
Alpha-2 Agonist – Action
If you recall your pharmacology, you’ll understand that tizanidine and clonidine are from the same class. Tizanidine demonstrates greater affinity for alpha-2 receptors involved in spinal motor neuron inhibition. This contributes to its muscle relaxant effects. Clonidine produces a broader reduction in sympathetic outflow throughout the central nervous system, making it more effective for blood pressure lowering. Heart rate may also be lowered.
When you think about the adverse effect profile of tizanidine, you must remember to monitor blood pressure and heart rate. This is particularly true in patients at higher dosages or those at risk for falls and hypotension.
Sedation
Sedation is a very common adverse effect with tizanidine. Make sure we are paying extra special attention to those patients who are already taking medications that may have additive effects. Patients taking opioids, benzodiazepines, alcohol, or other sedative agents should be monitored closely for excessive sedation.
Half-Life
Tizanidine has a very short half-life and duration of action. I typically see this medication dosed every 4-6 hours because of its pharmacokinetics. Tizanidine’s half-life is in the neighborhood of 2-3 hours. This also means that if the patient receives this medication regularly and then abruptly stops it, discontinuation and withdrawal effects may occur, which I will talk more about in an upcoming paragraph.
Interactions – CYP1A2
Medications that inhibit CYP1A2 can increase the concentrations of tizanidine. Ciprofloxacin, fluvoxamine, and cimetidine are three classic CYP1A2 inhibitors that can reduce the metabolism of tizanidine and ultimately increase the risk for tizanidine toxicity.
Discontinuation Risks – Taper Recommended
If you remember that clonidine has a significant risk for rebound hypertension, this should help you remember that tizanidine may have the same issue. Be careful about abrupt discontinuation of tizanidine, as it may precipitate significant rebound hypertension and tachycardia.
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