Arkamin H is a combination of clonidine 100 mcg and hydrochlorothiazide 20 mg, the drugs that have been found clinically effective at lowering blood pressure. Clonidine belongs to the drug called alpha-2 agonists. It works by dilating and relaxing blood vessels, allowing the heart to function properly when it comes to pumping blood across the body. Hydrochlorothiazide is a water pill, a diuretic that lowers blood pressure by getting rid of extra water and sodium from the body. Hydrochlorothiazide relaxes arteries and improves blood circulation.