Numerous clinical investigations that have focused on the hypotensive effects of garlic-based preparations have led to controversial results that may be partially because of differences in the composition of the preparations and in the biological responses they induce. It is possible that garlic powder tablets with a prolonged mode of action could induce more potent biological effects. In this double-blind, placebo-controlled trial with an active control arm, the hypotensive action of time-released garlic powder tablets (Allicor) was compared with that of regular garlic pills (Kwai) in 84 men with mild or moderate arterial hypertension. After an 8-week placebo treatment run-in phase, patients were randomized either to 600 mg Allicor (n=30) or to placebo (n=20) daily for 8 weeks. In addition, in the open-label branch, patients received either 2400 mg Allicor daily (n=18) or 900 mg Kwai daily (n=16). Allicor treatment (600 mg daily) resulted in a reduction of both systolic and diastolic blood pressures by 7.0 mm Hg (95% confidence interval (95% CI): 5.3-8.7) and 3.8 mm Hg (95% CI: 2.7-4.8), respectively. Increasing the Allicor dosage to 2400 mg daily did not provide any additional benefit. Treatment with Kwai resulted in the same decrease in systolic blood pressure (5.4 mm Hg, 95% CI: 1.9-8.8) as that seen with Allicor, but no decrease in diastolic blood pressure was observed with Kwai. Different effects of Allicor and Kwai on diastolic blood pressure may be because of the prolonged action of Allicor, which allows better bioavailability of the vasoactive constituents of garlic powder. The results of this study show that time-released garlic powder tablets are more effective for the treatment of mild and moderate arterial hypertension than are regular garlic supplements.