These agents have been shown to be effective in the management of acute postoperative pain and in chronic pain states, (including disorders involving spasticity or myofascial pain, neuropathic pain, and chronic daily headaches).
Clonidine (Catapres®, Duraclon®): 0.1, 0.2, 0.3 mg tablets; 0.1, 0.5 mg/ml injectable solution. A centrally acting, direct a-adrenergic receptor agonist (a2 > a1) which can be given PO, IV, epidural injection, and intrathecal injection. The analgesic effect of clonidine is felt to occur at presynaptic and postjunctional a2-adrenergic receptors in the spinal cord.
Dexmedetomidine (Precedex®): 100 mcg/ml injectable solution. Dexmedetomidine is a relatively selective a2-adrenergic agonist. It is currently approved by the FDA for ICU sedation (continuous infusion not to exceed 24 h) and sedation of nonintubated patients prior to and during surgical procedures. New studies have shown that dexmedetomidine has significant opioid sparing effects and can be effective when included in PCA formulations containing opioids (Lin et al. 2009).
Tizanidine (Zanaflex®): a2-adrenergic agonist often used as an antispasmodic agent. Initial dose may have to be as low as 0.25 mg because of potential significant sedation. Titrate up to 2-4 mg every 6-8 h until relief or excessive side effects occur; hypotension, sedation, asthenia, and dry mouth (dose related) are frequent; need to monitor for elevated liver function enzyme levels and hepatotoxicity. Maximum dose recommended is 36 mg/day. Zanaflex has 1/10 to 1/50 the potency of clonidine in lowering blood pressure. Concomitant use of tizanidine with fluvoxamine or with ciprofloxacin (potent inhibitors of CYP1A2) is contraindicated.
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