Garlic constituents may affect blood vessel elasticity and blood pressure. Several mechanisms have been proposed. Twenty placebo-controlled studies using single-ingredient preparations of GP (13 studies), AGE (5 studies), or other preparations (2 studies) were included in a meta-analysis. Significant reductions in systolic blood pressure (SBP) and diastolic blood pressure (DBP) were present when all trials were considered. Benefits were most pronounced in subjects with baseline hypertension (mean SBP reduction of 8.6 ±2.2 mm Hg and DBP reduction of 6.1 ±1.3 mm Hg), and no significant effect was observed in subjects who had normal or prehypertensive blood pressures (<140/90 mm Hg) at baseline. A Cochrane review on the effect of garlic monotherapy for prevention of cardiovascular morbidity and mortality in hypertensive patients also identified a significant reduction in systolic and diastolic pressure compared with placebo. A separate Cochrane review of the effect of garlic on peripheral occlusive disease found insufficient support for this indication.
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Endocrine effects—The effect of garlic on glucose homeostasis does not appear to be significant in persons with diabetes. Certain organosulfur constituents in garlic, however, have demonstrated hypoglycemic effects in nondiabetic animal models.
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Antimicrobial effects—The antimicrobial effect of garlic has not been extensively studied in clinical trials. Allicin has been reported to have in vitro activity against some gram-positive and gram-negative bacteria as well as fungi (Candida albicans), protozoa (Entamoeba histolytica), and certain viruses. The primary mechanism involves the inhibition of thiol-containing enzymes needed by these microbes. A Cochrane review studying the effect of garlic on cold prevention and treatment found a significant reduction in total number of colds using a garlic supplement (with 180 mg allicin content) once daily for 12 weeks. Limited conclusions can be drawn regarding the effects observed, however, because only one trial met inclusion criteria. Given the availability of safe and effective prescription antimicrobials, the usefulness of garlic in this area appears limited.
4. Antineoplastic effects—In rodent studies, garlic inhibits procarcinogens for colon, esophageal, lung, breast, and stomach cancer, possibly by detoxification of carcinogens and reduced carcinogen activation. Several epidemiologic case-control studies demonstrate a reduced incidence of stomach, esophageal, and colorectal cancers in persons with high dietary garlic consumption. Current anticancer studies are focused on specific organosulfur garlic compounds in in vivo animal models of cancer and in vitro effects on human cancer cell lines.
Following oral ingestion, adverse effects of garlic products may include nausea (6%), hypotension (1.3%), allergy (1.1%), and bleeding (rare). Breath and body odor have been reported with an incidence of 20–40% at recommended doses using enteric-coated powdered garlic formulations. Contact dermatitis may occur with the handling of raw garlic.
Because of reported antiplatelet effects, patients using anticlotting medications (eg, warfarin, aspirin, ibuprofen) should use garlic cautiously. Additional monitoring of blood pressure and signs and symptoms of bleeding is warranted. Garlic may reduce the bioavailability of saquinavir, an antiviral protease inhibitor, but it does not appear to affect the bioavailability of ritonavir.