Garlic and high blood pressure

The studies looked at garlic powder, garlic extract and garlic oil

 “A daily dose of garlic can save your life”, is the headline in the Daily Express. People with high blood pressure who took garlic supplements daily for up to five months “saw their blood pressure levels drop significantly. In some cases, the drop was as much as that seen in patients taking drugs such as beta blockers and ACE inhibitors”, the newspaper says. It goes on to point out that “researchers have yet to establish whether garlic supplements are as effective as prescribed medicines when used for many years.”

Although the research is reliable, the newspaper’s claims for garlic are somewhat overblown. The research involved pooling results from 11 studies of garlic preparations (mainly garlic powder), and found that they reduced blood pressure more than inactive placebo pills in people with high blood pressure (hypertension). The review did not assess whether garlic preparations were as effective as high blood pressure medications (antihypertensives such as beta blockers or ACE inhibitors) or whether they reduced deaths from high blood pressure-related illnesses. In the absence of trials directly comparing garlic preparations versus blood pressure medication, people taking them should not be tempted to switch to garlic preparations.

Where did the story come from?

Dr Karin Reid and colleagues from the University of Adelaide carried out this research. The study was funded by the Royal Australian College of General Practitioners, and the Australian Government Primary Health Care Research Evaluation Development Program. It was published in the open-access peer-reviewed medical journal: BMC Cardiovascular Disorders.

What kind of scientific study was this?

This was a systematic review and meta-analysis looking at the effects of garlic preparations on blood pressure. The researchers searched electronic databases of scientific and medical literature in October 2007 to identify any relevant studies. They also looked at other systematic reviews and meta-analyses to identify other potentially relevant studies. From the studies they identified, they selected only randomised controlled trials (RCTs) looking at the effects of garlic on blood pressure for inclusion in their review. They included studies written in English or German. The researchers only included studies that compared garlic-only preparations with inactive dummy pills (placebo). Blood pressure measurements consist of two readings: systolic blood pressure (SBP) and diastolic blood pressure (DBP), and studies that gave either or both of these readings were included.
"Our meta-analysis suggests that garlic preparations are superior to placebo in reducing blood pressure in individuals with hypertension."

The researchers extracted data from studies that met these criteria, and if studies did not provide appropriate data, the researchers tried to obtain it by contacting the study authors. They judged the quality of these studies using standard criteria, and then used statistical methods to pool all of the data from studies of sufficient quality (meta-analysis). They split the studies into those that included people with normal blood pressure (normotension: SBP less than 140mmHg or DBP less than 90mmHg), and those that included people who had high blood pressure (hypertension: SBP 140mmHg or above or DBP 90mmHg or above). They conducted separate analyses of these two groups of studies. They also used statistical methods to look at the relationship between the size of the effects and how long treatment was given for, the dosage, initial blood pressure and who funded the study (industry or other source).

What were the results of the study?

The authors identified 25 relevant randomised controlled trials, and 11 of these (including more than 500 people) provided enough information to be included in the meta-analysis. Nine of these studies compared garlic preparation alone versus placebo alone, and two compared garlic preparation plus another drug versus placebo plus the same drug (either a blood pressure or cholesterol-lowering drug). Nine of the studies used garlic powder, one used garlic extract and one used distilled garlic oil. Garlic powder was used in doses from 600mg to 900mg daily, and treatment lasted from 12 to 23 weeks. Average blood pressure in participants was in the high range in seven of these studies.
Overall, they found that garlic preparations reduced systolic blood pressure (SBP) by 4.6mmHg more than placebo. Although garlic reduced diastolic blood pressure (DBP) compared with placebo overall, this reduction was not large enough to be statistically significant. When the researchers looked only at the studies that included people with high blood pressure, they found that garlic preparations reduced SBP by 8.4mmHg, and DBP by 7.3mmHg. There was no significant reduction in either blood pressure measure with garlic preparations in studies including people with normal blood pressure.

Further statistical analyses confirmed that the higher a person’s blood pressure was at the start of the study, the greater their reduction in blood pressure with garlic preparations. The dosage of treatment, length of treatment or source of study funding did not affect the results.
What interpretations did the researchers draw from these results?

The researchers concluded that garlic preparations are better than placebo at reducing blood pressure in people with high blood pressure. They say that further trials are needed to assess whether they could provide an alternative to or supplement existing blood pressure medications.
What does the NHS Knowledge Service make of this study?

This well-conducted review pooled all known studies of the effects of garlic on blood pressure, as of October 2007. This type of study can help to clarify conflicts between individual studies. There are a number of points to consider when interpreting this study:
Most importantly, although the authors note that the reductions in blood pressure seen with garlic preparations are similar to those seen with commonly prescribed blood pressure medications in other studies, this type of indirect comparison (comparison of results from different studies) should be interpreted cautiously. This is because the people treated in the different trials may have had different characteristics which could affect the results. A direct comparison between garlic preparations and blood pressure-lowering medication in a randomised controlled trial would be needed to determine whether their effects are truly similar.

This research does suggest that garlic preparations can reduce blood pressure, but it remains to be seen whether this reduction is sufficient to lead to reductions in cardiovascular events such as heart attack (one of the reasons to treat high blood pressure in the first place).

This review did not look at any adverse effects of taking garlic preparations.

As garlic preparations are not subject to the same regulations as drugs, different garlic preparations may contain differing concentrations of the active components, as well as other compounds, and this may mean that their effectiveness varies. This study also did not look at the effects of consuming garlic as part of the diet and the effect of cooked garlic may not be the same as the garlic preparations, as heating it could reduce its effectiveness.

In the absence of evidence comparing the effects of garlic preparations with those of existing blood pressure medication, it is too early to suggest that people taking them could switch to garlic preparations.