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Garlic for blood pressure

Clinical bottom line: Based on trials of generally moderate to poor quality, garlic supplement of 600-900 mg/day for one to three months is associated with a modest reduction in systolic and diastolic blood pressure. This reduction is likely to be clinically relevant. Not all patients studied were hypertensive, and more information is required to assess the effect of garlic specifically in hypertensives, and to establish a dose-response relationship and the duration of benefit.


Garlic (Allium sativum) has been used for a wide variety of ailments. An association between garlic consumption and decreased cardiovascular morbidity and mortality has been suggested as well as antihypertensive properties.

Systematic review

Silagy CA, Neil HAW. A meta-analysis of the effect of garlic on blood pressure. Journal of Hypertension, 1994; 12 (4):463-468

Date review completed: pre-1994

Number of trials included: 8

Number of patients: 415

Control group: placebo and active

Main outcomes: reduction in systolic and diastolic blood pressure

Inclusion criteria were randomised controlled trials of garlic supplement for reducing blood pressure; intervention of at least four weeks; measurement of systolic and diastolic blood pressure.

Reviewers calculated the variance of the difference between baseline mean and final mean blood pressure measurements for each treatment group. Data on paired means were not always available in original trials, and a conservative assumption was made that reported means were unpaired. A pooled effect size weighted by the average of the individual effects was calculated with 95% confidence intervals. Information on heart rate was extracted where possible.

Findings

Included trials used standardised doses of dried garlic preparation in the dose range of 600-900 mg per day. Interventions ranged from 4 weeks to 10 months (median 12 weeks). Patient groups were predominantly hypertensives or hyperlipidaemics, with one trial on patients with increased spontaneous aggregation and one trial did not state.

Reviewers note that overall trial quality was poor, with limited monitoring of treatment compliance, unconvincing descriptions of blinding, and non-automated measurements of blood pressure. However, heterogeneity was not detected in analyses.

Seven double-blind trials compared garlic with placebo in 383 patients. Of these, three of seven trials showed a significantly greater reduction of systolic blood pressure from baseline with garlic compared with placebo, with an overall pooled difference of a 7.7mmHg (95% CI, 4.3 to 11) advantage with garlic. Four of the seven trials showed a significantly greater reduction of diastolic blood pressure from baseline with garlic compared with placebo, with an overall advantage of 5 mmHg (95% CI 2.9 to 7.1).

Two of these trials looked at hypertensives specifically. Here improvements in systolic and diastolic blood pressure were slightly better with garlic, but not significantly.

One single-blind trial compared garlic with a combined diuretic-reserpine preparation, and reported no difference in blood pressure. Insufficient information was available to assess dose-response or duration of improvement.

Three trials reported on change in heart rate. None reported a difference between garlic and placebo patients.

Adverse effects

Seven of eight trials reported that there was no significant difference in incidence of adverse effects. The most commonly reported adverse effect was odour. Pooled data produced an odds ratio of 7.7 (95% CI 1.0 to 56).

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