N° 39 | November 2009

Garlic supplements for patients with hypertension

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Background

Among its many qualities, garlic’s ability to lower blood pressure is one of the most widely studied. Results from controlled trials however, have varied. Some studies have shown a modest reduction in blood pressure, while others have shown no effect. In addition, a garlic-induced reduction in blood pressure could be a safety concern for those who use it for other reasons. A recent meta-analysis was conducted to A) determine the actual effect of garlic supplements on blood pressure in patients with hypertension and B) determine if blood pressure reductions could occur in patients with normal blood pressure1.

A meta-analysis

A systematic search of the literature was conducted to find randomized controlled trials of garlic compared to placebo which provided data on blood pressure changes. Two authors independently selected trials based on set criteria and extracted data from them including starting average blood pressure in garlic and placebo groups and ending blood pressure in both groups, as well as trial and patient characteristics. Studies were split into those with an average baseline systolic blood pressure (SBP) greater than 140 mmHg (referred to as the hypertensive group) and those with a lower average baseline SBP (referred to as the normotensive group). A random effects model was used to calculate a weighted mean difference in SBP and diastolic blood pressure (DBP) reduction for both sets of trials.

Blood pressure reduced by garlic supplements

Ten studies met the authors’ pre-defined criteria and were included. Three studies were included in the hypertensive group and the other 7 were included in the normotensive group. In the hypertensive group studies, compared to placebo, garlic lowered SBP by 16.3 mmHg (95% confidence interval [CI], 6.2 to 26.5) and DBP by 9.3 mmHg (95% CI, 5.3 to 13.3). There was no such effect in the studies in the normotensive group. An I2 test, which evaluates how different the included studies were from each other, was less than 25% for both analyses indicating only minor differences between the studies.

Garlic supplements or anti-hypertensives prescription ?

Neither SBP nor DBP were reduced in those with a baseline DBP < 140 mmHg, indicating there is likely no safety issue with garlic in regards to blood pressure. These results however, show a rather robust reduction in blood pressure in subjects who have a Baseline SBP greater than 140 mmHg. This level of reduction is similar to those of some prescription anti-hypertensives. These findings warrant further study of garlic in hypertensive patients.

While this data is compelling, there are a few limitations. First is the use of composite data. In the 3 studies included in the analysis of patients with SBP > 140 mmHg, the average SBP of the participants was >140 mmHg, however, it is possible that some participants had much lower baseline SBP. The opposite is true for studies included in the <140 mmHg analysis. Second was that blood pressure was not a primary endpoint for some of the included studies and may have not been obtained with scientific rigor. And finally, there are inherent limitations with the use of meta-analysis including the inability to control for biases of individual trials.

Although data is lacking to support using garlic supplements in place of standard anti-hypertensive medications, this information suggests it may be reasonable for those who refuse or Don't tolerate prescription agents. Close monitoring of blood pressure should be completed on these patients. Garlic has also been found to improve cholesterol levels in some patients and patients who take garlic for this reason are not likely to suffer from hypotension2.

  1. Reinhart K, et al. Effects of Garlic on Blood Pressure in Patients With and Without Systolic Hypertension: A Meta-Analysis. Ann Pharmacother 2008;42:1766-71.
  2. Reinhart K, et al. The Impact of Garlic on Lipid Parameters: A Systematic Review and Meta-Analysis. Nutr Res Rev 2009;22:39-48.
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