Search Strategy

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: 2-propenesulfenic acid, aglio, ajo, ajoene, alisat, allicin, alliinase, allium sativum, allyl mercaptan, diallyl disulphide, diallyl sulfide, diallyl sulphide, dipropyl disulphide, dipropyl sulphide, garlic extract, garlic oil, garlic, knoblauch, Kwai, Kyolic, S-allylcysteine (SAC), thioallyl derivative, thiosulfinates, and vinyl dithiin. 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.

Selection Criteria

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.

Data Collection and Analysis

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.

Cardiovascular-Related Outcomes

• Thirty-seven randomized trials, all but one in adults, consistently showed that compared with placebo, various garlic preparations led to small, statistically significant reductions in total cholesterol at 1 month (range of average pooled reductions 1.2 to 17.3 milligrams per deciliter [mg/dL]) and 3 months (range of average pooled reductions 12.4 to 25.4 mg/dL). Garlic preparations that were studied included standardized dehydrated tablets (Kwai ^® , Pure-Gar ^® , or noncommercial enteric-coated tablets), dehydrated tablets, "aged garlic extract ^TM ," oil macerates, distillates, raw garlic, and combination tablets. Eight placebo-controlled trials reported total cholesterol outcomes at 6 months; pooled analyses showed no significant reductions of total cholesterol with garlic compared with placebo. It is not clear if statistically significant positive short-term effects-but negative longer term effects-are due to: systematic differences in studies that have longer or shorter followup durations; fewer longer term studies; or time-dependent effects of garlic. Statistically significant reductions in low-density lipoprotein levels (LDL) (range 0 to 13.5 mg/dL) and in triglycerides (range 7.6 to 34.0 mg/dL) also were found in pooled analyses at 3 months. No significant changes in high-density lipoprotein levels (HDL) were seen in pooled analyses at 1 and 3 months. One multicenter trial involving 98 adults with hyperlipidemia found no differences in lipid outcomes at 3 months between persons who were given an antilipidemic agent and persons who were given a standardized dehydrated garlic preparation. Interpreting the lipid results is best tempered by recognizing that trials often had unclear randomization processes, short durations, and no intention-to-treat analyses.
• Twenty-seven small, randomized, placebo-controlled trials, all but one in adults and of short duration, reported mixed but never large effects of various garlic preparations on blood pressure outcomes. Most studies did not find significant differences between persons randomized to garlic compared with those randomized to placebo. The one small trial (n=40) that directly compared a standardized dehydrated garlic preparation with an active antihypertensive agent found no differences in blood pressure between groups. Because of unclear randomization processes, lack of intention-to-treat analyses, missing data, and variability in blood pressure measurement techniques, no firm conclusions can be drawn from these trials.
• Twelve small, randomized trials, all in adults, suggested that various garlic preparations had no clinically significant effect on glucose in persons with or without diabetes. Two small short trials, both in adults, reported no statistically significant effects of garlic compared with placebo on serum insulin or C peptide levels.
• Ten small, randomized trials, all but one in adults and of short duration, showed promising effects of various garlic preparations on platelet aggregation and mixed effects on plasma viscosity and fibrinolytic activity. Because the trials had only 409 participants, short followup periods, unclear randomization processes, no intention-to-treat analyses, missing data, and variability in techniques used to assess outcomes, no firm conclusions can be drawn.
• There were insufficient data to confirm or refute effects of garlic on clinical outcomes such as myocardial infarction and claudication. One 3-year randomized trial with 492 participants found no statistically significant decreases in numbers of myocardial infarctions and deaths when placebo was compared with 6 to 10 grams of garlic ether extract. This trial was not published in peer-reviewed literature; details confirming its randomization process and followup were not obtained, despite requests to the author.
• Two double-blind trials in participants with atherosclerotic lower extremity disease evaluated whether garlic increased pain-free walking distance at 12 to 16 weeks compared with placebo. In one trial, 64 of 80 (80 percent) participants completed followup. Pain-free walking increased by approximately 40 meters with standardized dehydrated garlic (Kwai ^® ) compared with approximately 30 meters with placebo. In the other trial, with 100 participants, the maximum walking distance increased significantly (114 percent) among persons randomized to a combination treatment of garlic oil macerate/soya lecithin/hawthorn oil/wheat germ oil compared with those randomized to placebo (17 percent) (p<0.05).
• RCTs did not establish whether garlic effectiveness varies across preparations or dosages. Limited data not derived from head-to-head comparisons suggest, but do not prove, that standardized dehydrated preparations may result in greater short-term (1- to 3-month) drops in total cholesterol than other preparations.

Associations With Cancer

• Scant data, primarily from case-control studies, suggest, but do not prove, dietary garlic consumption is associated with decreased odds of laryngeal, gastric, colorectal, and endometrial cancer and adenomatous colorectal polyps. Single case-control studies suggest, but do not prove, dietary garlic consumption is not associated with breast or prostate cancer. No epidemiological study has assessed whether using particular types of garlic supplements is associated with reductions in cancer incidence. Preliminary 3-year evidence from a large cohort study suggests consumption of "any" garlic supplement does not reduce risk of breast, lung, colon, or gastric cancer. This study has not reported associations relevant to consumption of fresh or raw garlic, and its data about supplements are limited because information is not available about different types and brands of garlic supplementations.

Adverse Effects

• Adverse effects of oral ingestion of garlic are "smelly" breath and body odor. Other possible, but not proven, adverse effects include flatulence, esophageal and abdominal pain, small intestinal obstruction, contact dermatitis, rhinitis, asthma, bleeding, and myocardial infarction. There are two reports of patients taking warfarin who experienced increases in International Normalized Ratio (INR) when taking garlic pearls or tablets. The content and method of preparation of the pearls and tablets were not given. The frequency of adverse effects with oral ingestion of garlic and whether they vary by particular preparations are not established. Adverse effects of inhaled garlic dust include allergic reactions such as asthma, rhinitis, urticaria, angioedema, and anaphylaxis. Adverse effects of topical exposure to raw garlic include contact dermatitis, skin blisters, and ulcero-necrotic lesions. Frequency of reactions to inhaled garlic dust or topical exposures of garlic is not established.

Conclusions

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.

Cardiovascular-Related Effects

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.

Cancer-Related Associations

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.

Adverse Effects and Interactions

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.