Tizanidine

Tizanidine (Zanaflex, Sirdalud) is a centrally acting α-2 adrenergic agonist. It is used to treat the spasms, cramping, and tightness of muscles caused by medical problems such as multiple sclerosis, spastic diplegia, back pain, or certain other injuries to the spine or central nervous system. It is also prescribed off-label as a sleep aid, seizure inhibitor, and for some symptoms of fibromyalgia^[1].

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Tizanidine may cause hypotension so caution is advised when it is used in patients who have a history of orthostatic hypotension.

Tizanidine can come in a white pill with the markings cor 138 and 2 scores on the back that create an X or R179 and a single score through the middle of the back. It is supplied as 2 and 4 mg tablets for oral administration. Tizanidine tablets are composed of the active ingredient, tizanidine hydrochloride (2.288 mg equivalent to 2 mg tizanidine base and 4.576 mg equivalent to 4 mg tizanidine base), and the inactive ingredients, silicon dioxide colloidal, stearic acid, microcrystalline cellulose and anhydrous lactose. They are also in gel cap form with respective doses of 2mg, 4mg, and 6mg.

Use caution with this drug as it can be very strong even at the 2mg dose. Also use caution when switching from gel cap to tablet form and vice versa.

Tizanidine use occasionally causes drug induced liver injury. In controlled clinical studies, approximately 5% of patients treated with Zanaflex had elevations of liver function tests (ALT, AST) to greater than 3 times the upper limit of normal (or 2 times if baseline levels were elevated). Zanaflex use has been associated with hallucinations. Formed, visual hallucinations or delusions have been reported in 5 of 170 patients (3%) in two North American controlled clinical studies.

If therapy needs to be discontinued, especially in patients who have been receiving high doses for long periods, the dose should be decreased slowly to minimize the risk of withdrawal and rebound hypertension, tachycardia, and hypertonia.

Concomitant use of tizanidine and moderate or potent CYP450 1A2 inhibitors is contraindicated. Concomitant use of tizanidine with fluvoxamine, a potent CYP450 1A2 inhibitor in man, resulted in a 33-fold increase in the tizanidine AUC by fluvoxamine.

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