Clonidine and Related Drugs
Clonidine and the related centrally acting adrenergic inhibitors guanabenz, guanfacine, and methyldopa are commonly used for the treatment of hypertension. Clonidine also has been used to alleviate opioid and nicotine withdrawal symptoms. Clonidine overdose may occur after ingestion of pills or ingestion of the long-acting skin patches. Oxymetazoline, nephazoline, and tetrahydrozoline are nasal and conjunctival decongestants that may cause toxicity identical to that of clonidine. Tizanidine is a chemically related agent used for the treatment of muscle spasticity. Apraclonidine and brimonidine, ophthalmic preparations for the treatment of glaucoma and ocular hypertension, may cause poisoning from ingestion and from systemic absorption after topical administration.
Mechanism of toxicity. All these agents decrease central sympathetic outflow by stimulating alpha2-adrenergic presynaptic (inhibitory) receptors in the brain.
Clonidine, oxymetazoline, and tetrahydrozoline may also stimulate peripheral alpha1 receptors, resulting in vasoconstriction and transient hypertension.
Guanabenz is structurally similar to guanethidine, a ganglionic blocker. Guanfacine is related closely to guanabenz and has more selective alpha2 agonist activity than does clonidine.
Methyldopa may further decrease sympathetic outflow by metabolism to a false neurotransmitter (alpha-methylnorepinephrine) or by decreasing plasma renin activity.
Tizanidine is structurally related to clonidine but has low affinity for alpha1 receptors.
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