Reviewed: Rohner A, Ried K, Sobenin IA, Bucher HC, Nordmann AJ. A systematic review and metaanalysis on the effects of garlic preparations on blood pressure in individuals with hypertension. Am J Hypertens. March 2015;28(3):414-423.
One in three adults worldwide has high blood pressure.1 The adverse effects of conventional antihypertensive medications often result in low compliance, poor blood pressure (BP) control,2 and dissatisfied patients seeking alternative therapies. Garlic (Allium sativum, Amaryllidaceae) is one of the most popular herbal treatments for hypertension. The authors of this systematic review evaluated randomized clinical trials to assess the effect of garlic on BP in patients with hypertension and identified the risk of bias in the reported evidence.
The authors searched clinical trial registries of ongoing studies in addition to PubMed, Embase, Cochrane Library, and the Web of Science from inception through March 2014 using the terms “garlic” and “blood pressure” or “garlic” and “hypertension.” Included in the systematic review were full-text, randomized, controlled trials of garlic preparations for the treatment of hypertension that reported BP values at baseline and after four or more weeks of treatment. There was no language restriction.
The authors identified nine trials (n=577 patients in total) that met the inclusion criteria. From the two trials that included both normal and hypertensive subjects, only data for the hypertensive subgroup (n=482) were included in the meta-analysis.
The included trials were heterogeneous in terms of subject naivety to hypertensive medication, medical diagnoses (hypertension or dyslipidemia), BP measurement methods (sitting or supine), type of garlic preparation (raw or aged dried extracts), dosage (240-2,400 mg/day), type of control (placebo or active treatment), and study duration (8-26 weeks).
The analysis of bias indicated that randomization and concealment of group allocation were adequate in five trials and unclear in four trials. The risk of performance bias was low in all trials, while detection bias was low in four trials and unclear in five trials. Only four trials included an intention-to-treat analysis. Although none of the trials explicitly reported industry funding, at least one study author in two of the trials was identified as an employee of the company producing the garlic preparation being investigated. The authors rated the risk of selective reporting bias for the trials as unclear because only two trials included study protocols.
The meta-analysis revealed that there was a significant reduction in both systolic BP (SBP) and diastolic BP (DBP) in patients treated with garlic compared to the controls; however, there was significant heterogeneity in the results. After restricting the analysis to higher-quality trials, the treatment effects were less pronounced but remained significant for garlic preparations compared to controls, with lower heterogeneity for SBP but not for DBP.
Addressing the heterogeneity of the study findings, the authors state, “It must be noted that different garlic preparations have variable effectiveness on BP.” The different garlic preparations used in the trials, they continue, “may have contributed to the heterogeneous study findings and preclude an appropriate analysis of a dose relationship.”
The adverse side effects associated with the garlic preparations were rare and mild. In two trials, patients taking garlic reported a higher incidence of bloating, flatulence, and reflux compared to control. Only three trials reported drop-outs in the garlic groups due to adverse gastrointestinal symptoms (a total of five out of 105 patients, or less than 5%).
Some of the study’s strengths, according to the authors, were the comprehensive literature search and robust meta-analyses across sub-group and sensitivity assessments. Acknowledged limitations were as follows: the highly heterogeneous estimates of BP with large effect sizes and confidence intervals; small sample size of the included trials; overall quality of the included trials; lower significance of the results from trials with more rigorous methodologies; inconsistencies in the DBP data despite various sensitivity analyses; varied trial durations; and heterogeneity in the type and dosage of garlic preparations used. As one peer reviewer of this summary wrote, it is difficult to conduct a meta-analysis of trials using different commercial garlic products because each preparation contains different chemical constituents; for example, one product may contain more oil-soluble compounds and another may contain more water-soluble compounds (e.g., S-acetyl cysteine).
The authors conclude that the observed improvements are clinically relevant and that garlic preparations offer promise as an alternative treatment for reducing high BP. However, they note: “As of now, there is insufficient evidence to have confidence that garlic preparations are an effective alternative or complementary/adjunct herbal medication to conventional antihypertensive drugs.”
Additional research is needed to understand the mechanisms of garlic’s BP-lowering effects. As the authors conclude, after more than 25 years since the publication of the first randomized controlled trial of garlic for the treatment of hypertension,3 “We still do not know whether garlic preparations lower BP in the long term. … A well-conducted, sufficiently powered long-term trial is needed to assess the BP-lowering capacities of a standardized form of a garlic preparation.”
- World Health Organization. World Health Statistics: A Snapshot of Global Health. Geneva, Switzerland: World Health Organization (WHO); 2012. Available at: http://apps.who.int/iris/bitstream/10665/70889/1/WHO_IER_HSI_12.1_eng.pdf. Accessed February 16, 2015.
- Krousel-Wood MA, Muntner P, Islam T, Morisky DE, Webber LS. Barriers to and determinants of medication adherence in hypertension management: perspective of the cohort study of medication adherence among older adults. Med Clin North Am. 2009;93(3):753-769.
- Kandziora J. Blood pressure and lipid-lowering effect of garlic – preparation in combination with a diuretic [in German]. Arztl Forsch. 1988;35(3):1-8.