Question

What is the effectiveness of using garlic monotherapy in lowering the risk of cardiovascular morbidity and mortality compared to placebo among hypertensive patients?

Relevance to nursing care

Garlic is widely used by patients for its blood pressure lowering effects and nurses are frequently approached by patients regarding this. It is important to review the current available evidence to determine whether garlic may have a beneficial role in the reduction of these cardiovascular events and/or whether it may reduce mortality rates in patients with hypertension.

Study characteristics

This summary is based on the results of a Cochrane Systematic Review.1 Two randomized controlled trials (RCTs) with 87 participants were included. Participants were persons aged above 18 years with a diagnosis of primary hypertension (defined as systolic blood pressure >=140 mmHg or diastolic blood pressure >=90 mmHg or both), who were not on any therapy for hypertension. The intervention of interest was the use of garlic (allium sativum) monotherapy in a preparation without any other ingredients known or thought to have an antihypertensive effect comparing with placebo. The garlic could be raw, cooked or manufactured. No restrictions were placed on the quantity and frequency of garlic administration. The primary outcome was mortality, serious adverse events such as life threatening events and cardiovascular events including myocardial infarction and stroke. The secondary outcomes were blood pressure reduction as well as breath and body odour, mouth and gastrointestinal burning or irritation, or other severe adverse events. The duration of follow up was 12 weeks.

The methodological quality of both trials was largely unclear due to both of the studies not providing information on sequence generation, allocation concealment and blinding. Pooling of studies could not be undertaken.

The major review findings were reported as follows:

* Neither trial reported data on mortality.

* One trial reported a significant reduction in mean supine systolic blood pressure by 12 mmHg (95% confidence intervals [CI] 0.56 to 23.44 mmHg, P = 0.04) and mean supine diastolic blood pressure by 9 mmHg (95% CI 2.49 to 15.51 mmHg, P = 0.007) in the intervention group after 12 weeks of treatment compared to those receiving placebo.

* The other trial reported that in those receiving garlic powder there was a reduction in systolic blood pressure by 10-11 mmHg and diastolic blood pressure by 6-8 mmHg with additional baseline anti-hypertensive therapy after 12 weeks compared to those receiving placebo. This trial could not undergo meta-analysis as the numbers of participants randomised was not provided.

* Both trials provided insufficient data on adverse events.

Implications for nursing care

Currently there is insufficient evidence to determine the effectiveness of garlic in reducing mortality and cardiovascular morbidity in hypertensive patients compared to a placebo. Therefore, garlic cannot be recommended as monotherapy to reduce hypertension.

Implications for research

Garlic seems to lower blood pressure but the precise magnitude of this effect is unknown. More importantly, if garlic is to be used in hypertensive patients, randomized controlled trials adequately powered to detect its effects on serious adverse events, morbidity and mortality are required. These additional trials should also measure blood pressure lowering effects in order to provide a more precise estimate of the effect of garlic on this outcome.

Reference