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Hypertension affects about 30% of adults worldwide. Garlic has blood pressure lowering properties and the mechanism of action is biologically plausible. Our trial assessed the effect, dose-response, tolerability and acceptability of different doses of aged garlic extract as an adjunct treatment to existing antihypertensive medication in patients with uncontrolled hypertension. Methods A total of 79 general practice patients with uncontrolled systolic hypertension participated in a double-blind randomised placebo-controlled dose-response trial of 12 weeks. Participants were allocated to one of three garlic groups with either of one, two, or four capsules daily of aged garlic extract (240/480/960 mg containing 0.6/1.2/2.4 mg of S-allylcysteine) or placebo. Blood pressure was assessed at 4, 8 and 12 weeks and compared with baseline using a mixed model approach. Tolerability was monitored throughout the trial and acceptability was assessed at 12 weeks by questionnaire. Results Systolic blood pressure was significantly reduced by -11.8±5.4 mm Hg on average in the garlic-2-capsule group over 12 weeks compared with placebo (p=0.006), and reached borderline significant reduction in the garlic-4-group at 8 weeks (-7.4±4.1 mm Hg, p=0.07). Changes in systolic blood pressure in the garlic-1-capsule group and diastolic blood pressure were not significantly different to placebo. Tolerability, compliance, and acceptability were high in all garlic groups (93%) and highest in the groups taking one or two capsules daily. Conclusions Our trial suggests aged garlic extract to be an effective and tolerable treatment in uncontrolled hypertension, and may be considered as a safe adjunct treatment to conventional antihypertensive therapy.