Derived from the bulb or clove of the plant, garlic is used as a spice and to treat hyperlipidemia, hypertension, atherosclerosis, cancer, and infections. Because processing can have a substantial effect on the chemical content of garlic (the volatile oil components are sensitive to heat and certain enzymes are acid-labile), the best measure of the total activity of garlic is its ability to produce allicin, which in turn, results in the formation of other active constituents (1). Several oral garlic formulations are available, and clinical studies have addressed a variety of the proposed claims.
Placebo-controlled trials on the cholesterol lowering effect of garlic yielded mixed results (2) (3) (4) (5) (6) (7) (31), but a systematic review showed that garlic is effective in lowering total cholesterol and LDL cholesterol levels (37). Studies evaluating the antithrombotic effects repeatedly show modest reduction in platelet aggregation, but varying levels of fibrinolytic activity; and mixed effects with regard to reductions in blood glucose, blood pressure, or risk of cardiovascular disease (8). Garlic supplementation may benefit patients with hepatopulmonary syndrome (32).
More data are needed to determine if garlic is effective against common cold (33) and vaginal candidiasis (47).
Whether garlic is effective in reducing the risk of mortality and cardiovascular morbidity in patients diagnosed with hypertension is also inconclusive (45).
An analysis of several case-controlled studies in Europe suggests an inverse association between garlic consumption and risk of common cancers (9). Specifically, high intake of raw and cooked garlic may be protective against stomach and colorectal cancers (10) (11); however, conflicting data indicate that long-term supplementation with garlic does not significantly reduce gastric cancer incidence nor mortality (43). Garlic intake was inversely associated with cancer of the prostate (12) and endometrium (13). In patients with advanced cancers, aged garlic extract (AGE) improved natural killer (NK) cell number and activity, but not quality of life (14). In patients with a history of adenomas, supplementation with AGE reduced both the number and size of subsequent colorectal adenomas (15). Garlic supplementation may also be associated with reduced risk of hematologic malignancies (40).
Because garlic is known to decrease platelet aggregation and potentially elevate International Normalized Ratio (INR) values, it should not be used with anticoagulants or in patients with platelet dysfunction (17).