Under its Evidence-based Practice Program, the Agency for Healthcare Research and Quality (AHRQ) is developing scientific information for other agencies and organizations on which to base clinical guidelines, performance measures, and other quality improvement tools. Contractor institutions review all relevant scientific literature on assigned clinical care topics and produce evidence reports and technology assessments, conduct research on methodologies and the effectiveness of their implementation, and participate in technical assistance activities.
Overview / Reporting the Evidence / Methodology / Findings / Conclusions / Limitations / Future Research / Availability of Full Report
This evidence report is a systematic review that summarizes clinical studies of garlic in humans. It addresses three areas:
The report was requested by the National Center for Complementary and Alternative Medicine, a component of the National Institutes of Health, and sponsored by the Agency for Healthcare Research and Quality. The following are the rationale for this report:
The report is intended primarily for agencies interested in funding clinical garlic studies, clinicians, and researchers, and secondarily for consumers.
The report addresses the following topics:
Eleven electronic databases, including AMED, CISCOM, the Cochrane Library (including DARE and the Cochrane Controlled Trials Registry), EMBASE, MEDLINE, and NAPRALERT, were searched using the following terms:
English and non-English citations were identified through July 1999 from these electronic databases, references in pertinent articles and reviews, manufacturers, and technical experts. Finally, an electronic update search using PubMed was conducted in February 2000.
Reports of garlic's effects on cardiovascular factors and outcomes were limited to randomized controlled trials (RCTs) lasting at least 4 weeks that compared garlic with placebo, no garlic, or another active agent. Reports of preventive effects on occurrence of precancerous lesions and cancer were limited to case-control and cohort studies that compared varying levels of garlic consumption. All types of studies in humans were used to assess adverse clinical effects.
Two independent physicians abstracted data from trials, and one physician abstracted data about adverse effects. Data were synthesized descriptively, emphasizing methodological characteristics of the studies such as populations enrolled, definitions of selection and outcome criteria, sample sizes, adequacy of randomization process, interventions and comparisons, cointerventions, biases in outcome assessment or intervention administration, and study designs. Relationships among clinical outcomes, participant characteristics, and methodological characteristics were examined in evidence tables and graphical summaries. Lipid outcomes of trials were examined quantitatively using standardized and unstandardized mean differences (adjusted for baseline differences). Hedges' g was used to compute the standardized mean difference for each trial.
There are insufficient data to draw conclusions regarding garlic's effects on clinical cardiovascular outcomes such as claudication and myocardial infarction. Garlic preparations may have small, positive, short-term effects on lipids; whether effects are sustainable beyond 3 months is unclear. Consistent reductions in blood pressure with garlic were not found, and no effects on glucose or insulin sensitivity were found. Some promising effects on antithrombotic activity were reported, but few data are available for definitive conclusion.
Using "any" garlic supplement for less than 3 to 5 years was not associated with decreased risks of breast, lung, gastric, colon, or rectal cancer. Some case-control studies suggest that high dietary garlic consumption may be associated with decreased risks of laryngeal, gastric, colorectal, and endometrial cancers, and adenomatous colorectal polyps.
Multiple adverse effects, including smelly breath and body odor, dermatitis, bleeding, abdominal symptoms, and flatulence, have been reported. Whether adverse effects occur more commonly with certain preparations than others was not established. Furthermore, the causality of the adverse effects was not clear, except for breath and body odor, and the expected frequency of adverse effects was not determined.
Notable limitations in summarizing findings from garlic research include the substantial variability in types of garlic and garlic preparations that have been studied and an inadequate definition of the active, biologically available constituents in the various preparations. In addition, many trials that evaluated the effects of garlic on cardiovascular-related endpoints are limited by short durations; inadequate randomization and blinding procedures; lack of clear specification of contents of garlic preparations—including their constituents and dissolution properties; lack of intention-to-treat analyses; and incomplete reporting of data. The meta-analysis we performed is limited by some missing data at different time points and by the need to impute variability data from some trials.
We found few studies assessing associations between garlic consumption and cancer. Some pertinent studies may have been missed because they addressed associations with multiple foods and either did not report or analyze findings specific to garlic. Studies that were found sometimes failed to distinguish the type of garlic exposure (raw, cooked, or specific supplement), used subject recall to assess different frequencies of use over varying time periods, and adjusted for various potential confounders in different ways. Although we believe that we found most reported adverse-effect literature regarding garlic, adverse effects in general are frequently underreported or reported in ways that do not allow causality and frequency to be determined.
Before undertaking future trials that evaluate the efficacy of garlic, the equivalency and the amount of release of the main constituents of various garlic preparations must be established. Placebos designed to simulate garlic odor should be developed, and adequacy of blinding should be assessed in trials. Well-designed randomized trials that are longer than 6 months in duration and that are powered to assess morbidity and mortality outcomes, as well as lipid and thrombotic outcomes, are needed. Appropriate analyses that are intention-to-treat and two-tailed should be used.
Additional cohort and case-control studies that assess associations between garlic and precancerous and cancerous lesions are likely to be helpful only if the frequency, types, and formulations of garlic that are consumed are specified clearly. Such studies should use sampling techniques that allow multiple levels of garlic consumption to be represented. Consideration should be given to mounting more trials, such as the ongoing Chinese trial, that evaluate the protective effects of different garlic preparations in persons with very high risk of cancer or precancerous lesions. Future reviews in this area should search more broadly for diet-related population studies and aim to place findings specific to garlic in a broader context that takes into account findings regarding other Allium-containing vegetables as well as other foods.
The frequency and severity of adverse effects related to garlic should be quantified. Whether adverse effects are specific to particular preparations, constituents, or doses should be elucidated. In particular, adverse effects related to bleeding and interactions with other drugs such as aspirin and warfarin warrant study.
The full evidence report from which this summary was derived was prepared for the Agency for Healthcare Research and Quality by the San Antonio Evidence-based Practice Center based at The University of Texas Health Science Center at San Antonio and the Veterans Evidence-based Research, Dissemination, and Implementation Center (VERDICT), a Veterans Affairs Health Services Research and Development Center of Excellence under contract No. 290-97-0012. Printed copies may be obtained free of charge from the AHRQ Publications Clearinghouse by calling 800-358-9295. Requesters should ask for Evidence Report/Technology Assessment Number 20, Garlic: Effects on Cardiovascular Risks and Disease, Protective Effects Against Cancer, and Clinical Adverse Effects (AHRQ Publication No. 01-E023).
The Evidence Report is also online on the National Library of Medicine Bookshelf, or can be downloaded as a zipped file.
AHRQ Publication Number 01-E022
Current as of October 2000
Internet Citation:
Garlic: Effects on Cardiovascular Risks and Disease, Protective Effects Against Cancer, and Clinical Adverse Effects. Summary, Evidence Report/Technology Assessment: Number 20. AHRQ Publication No. 01-E022, October 2000. Agency for Healthcare Research and Quality, Rockville, MD. http://www.ahrq.gov/clinic/epcsums/garlicsum.htm
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