Clonidine

In pharmacology, clonidine is a "alpha-2 adrenergic agonist that crosses the blood-brain barrier. Clonidine acts centrally by reducing sympathetic tone, resulting in a fall in diastolic and systolic blood pressure and a reduction in heart rate. It also acts peripherally, and this peripheral activity may be responsible for the transient increase in blood pressure seen during rapid intravenous administration. (From Martindale, the Extra Pharmacopoeia, 30th ed, p350)."

Its main use is for hypertension, but it has also been studied for alcohol withdrawal.