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Evidence-Based Medicine

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Garlic supplements show promise for lowering some cardiovascular risk factors, but more studies are needed

Garlic supplements are among the most popular herbal remedies that American consumers pay more than $5 billion for each year. Many animal studies have shown it to reduce blood fats and hypertension.

A recent review of randomized controlled trials in humans found that garlic supplementation (standardized dehydrated garlic powders and various nonstandardized preparations alone or in combination with other ingredients) may reduce levels of total cholesterol, triglycerides, and low-density lipoprotein (LDLs), as well as platelet aggregation (which causes blood thickening) in the short-term compared with placebo or no garlic supplementation. However, the marginal quality and short duration of many trials and inadequate definition of active ingredients in the many types of garlic preparations used limit the clinical usefulness of these studies, cautions principal investigator Cynthia D. Mulrow, M.D., M.Sc.

Dr. Mulrow and her colleagues at the San Antonio Evidence-based Practice Center (EPC) recommend that the major active ingredients of specific garlic supplements and their mechanisms of action be described before more trials are conducted. Their review, which was supported by the Agency for Healthcare Research and Quality (contract 290-97-0012), involved randomized controlled trials lasting at least 4 weeks and conducted between 1966 and December 2000, that compared garlic with placebo, no garlic, or another active agent. Individual studies found insignificant effects of garlic on blood pressure levels and no effects on blood sugar levels.

However, specific pooled results of studies comparing garlic preparations with placebo showed that garlic may lead to small reductions in the total cholesterol level when measured 1 month later and 3 months later, but this trend did not persist to 6 months later. Changes in LDL and triglyceride levels paralleled total cholesterol level results, but there were no significant changes in high-density lipoprotein levels. Trials also reported significant reductions in platelet aggregation. The only proven adverse effects of garlic supplements were bad breath and body odor.

More details are in "Garlic shows promise for improving some cardiovascular risk factors," by Ronald T. Ackermann, M.D., Dr. Mulrow, Gilbert Ramirez, Dr.P.H., and others, in the March 26, 2001, Archives of Internal Medicine 161, pp. 813-824.

Editor's Note: This journal article is based on an evidence report, Effects on Cardiovascular Risks and Disease, Protective Effects Against Cancer, and Clinical Adverse Effects, Evidence Report/Technology Assessment No. 20, that was prepared for AHRQ by the San Antonio EPC based at the University of Texas Health Science Center at San Antonio.

A summary of the report (AHRQ Publication No. 01-E022) and full report (AHRQ Publication No. 01-E023) are available from the AHRQ Publications Clearinghouse.

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