Hypertensive urgency induced by an interaction of mirtazapine and clonidine

Mirtazapine is a new antidepressant with a tetracyclic chemical structure that is not related to selective serotonin reuptake inhibitors, tricyclic antidepressants, or monoamine oxidase inhibitors. The antidepressant effect results from stimulation of the noradrenergic system through antagonism at central (alpha2-inhibitory receptors. Clonidine exerts its antihypertensive effect by stimulating these receptors to cause a reduction in endogenous release of norepinephrine. Therefore, the two agents have mechanisms of action that potentially oppose one another. We report a case of hypertensive urgency that ensued after a patient stabilized on clonidine began taking mirtazapine.