Objectives:

This evidence report summarizes the effects of garlic on cardiovascular-related factors and disease, associations between garlic and cancer, and possible adverse effects of garlic.

Search Strategy:

English and non-English citations were identified through February 2000 from 11 electronic databases, references of pertinent articles and reviews, manufacturers, and technical experts.

Selection Criteria:

We limited review of cardiovascular-related effects to randomized controlled trials in humans that lasted at least 4 weeks and compared garlic with placebo, no garlic, or another active agent. Review of associations with cancer was limited to controlled studies that compared any precancerous or cancerous lesions in humans consuming varying amounts of garlic. All types of studies in humans were used to assess adverse clinical effects.

Data Collection and Analysis:

Two physicians abstracted data from cardiovascular and cancer studies; one physician abstracted data about adverse effects. Lipid outcomes from cardiovascular trials were examined quantitatively using standardized and unstandardized mean differences (adjusted for baseline differences).

Main Results:

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). Eight trials with outcomes at 6 months showed no significant reductions of garlic compared with placebo. Changes in low-density lipoprotein (LDL) levels and triglycerides mirrored total cholesterol results; no significant changes in high-density lipoprotein (HDL) levels were found.

Twenty-seven small, randomized, placebo-controlled trials, all but one in adults, reported mixed but never large effects of various garlic preparations on blood pressure outcomes.

Twelve small, randomized trials suggested that various garlic preparations had no clinically significant effect on glucose in persons with or without diabetes. Two small short trials reported no statistically significant effects of garlic compared with placebo on serum insulin or C peptide levels.

Ten small trials, all but one in adults and of short duration, showed the effects of various garlic preparations on platelet aggregation and mixed effects on plasma viscosity and fibrinolytic activity.

There were insufficient data to confirm or refute garlic's effects on clinical outcomes such as myocardial infarction and claudication.

Scant data, primarily from case-control studies, suggest, but do not prove, that dietary garlic consumption is associated with decreased odds of laryngeal, gastric, colorectal, and endometrial cancer and adenomatous colorectal polyps.

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, dermatitis, rhinitis, asthma, and bleeding.

Conclusions:

Trials show several promising, modest, short-term effects of garlic supplementation on lipid and antithrombotic factors. Effects on clinical outcomes are not established, and effects on glucose and blood pressure are none to minimal. High dietary intake of garlic may be associated with decreased odds of multiple cancers. Ability to interpret existing data is substantively limited by marked variability in types of garlic preparations that have been studied and inadequate definition of active constituents in the various preparations.