Tizanidine: Pharmacological properties and action mechanism

Tizanidine is a centrally acting muscle relaxant, the exact action mechanism of which has not yet been clarified. The most recent studies support the hypothesis that its myotonolytic activity derives from inhibition at spinal level and through the involvement of supraspinal structures of reflex motor activity. This effect seems to be mediated by the imidazolic receptors and only marginally by alpha-2 adrenergics. This drug is commonly employed in the treatment of spasticity secondary to lesions of the bone marrow and of numerous disease conditions related to painful muscular spasms but currently tizanidine seems to find rational employment in the treatment of other pain conditions that afflect minor muscle groups (such as chronic cephalea of musculo-tensive type, in addition to the management of the symptomatology of certain muscle forms of temporomandibular disorders) and a markedly lower dose than that currently employed up to now in clinical practice. In spite of the undoubted spread of the drug and the numerous pharmacological properties that the molecule seems to exhibit over and above muscle relaxant activity alone, it has not yet been possible to pin-point its precise action mechanism. The present paper reviews available information and examines the various hypotheses proposed to justify the use of tizanidine in the management of chronic pain. From the present study we evince the need to further study the interaction of the molecule with the imidazolic receptors which would justify the rare unwanted side-effects which the drug presents compared to other alpha-2 agonists with which tizanidine has been associated by virtue of its structural resemblance.