Known for their pungent odor, garlic bulbs have been revered as both a food and medicine in many cultures for millennia. Construction workers who built the Egyptian pyramids were supposedly given huge rations of garlic to sustain their resistance against fevers. Legend has it that gravediggers in early eighteenth-century France drank a concoction of macerated garlic in wine to protect themselves against a plague. And during the two world wars, military physicians gave garlic to their patients as a preventive against gangrene.

The primary active compound in garlic is alliin, an odorless substance derived from the sulfur-containing amino acid, cysteine. However, alliin is found only within the intact cells of garlic. When garlic bulbs are crushed, the cell walls are broken, and an enzyme, allinase, converts alliin into a degradation product called allicin (diallyldisulfide-S-oxide). Allicin is an unstable compound that gives garlic its characteristic odor. Allicin is more active than alliin, and it readily forms other odorous sulfur-containing active constituents.

Native to central Asia, garlic now grows worldwide as a cultivated plant. This perennial reaches a height of 25 to 70 cm (10 to 28 in.). Its stem is either erect or crook-like, and its leaves are flat and broad. Topping the stalks are five to seven pale flowers with reddish or greenish white petals arranged in a loose globular cluster. The subterranean compound bulb has 4 to 20 cloves, or secondary bulbs, each one weighing about 1 g. Each clove is covered by a silky white or green skin.

• Bulbs

On average, 0.35% sulfur (1% of the dry weight). Alliin rapidly decomposes to form allicin which comprises 0.25% to 1.15% of garlic cloves. Alliin content is 0.7% to 1.7% in dried bulbous garlic. Other sulfur-containing constituents comprise about 25% to 35% of compounds in garlic after the cells have been damaged.

Commercial preparations are manufactured from whole fresh bulbous garlic, dried bulbous garlic, or oil of garlic. The quantity of active principles in commercial products varies depending on the method of preparation and percentage of active compounds in fresh garlic cloves. This percentage reportedly varies by a factor of 10. Aged garlic products (garlic fermentation products) are odor-free. However, aged garlic products have limited therapeutic benefits because the active principles in them are usually converted into inert substances. Consumers should use standardized garlic products containing a specified concentration of allicin.

Historical uses: all infections both internally and as a poultice. Used as a warming herb and as preventive for colds and flu, menstrual pain, mouthwash, and as a douche. Anthelmintic (expels worms).

Traditional actions: antihypertensive, anticholesterolemic, antilipidemic, reduces platelet aggregration, vasodilator, expectorant, antihistaminic, antimicrobial

Clinical applications: treatment and prevention of atherosclerosis, elevated blood lipids, and thrombosis. Also used to stabilize blood sugar level, and for gastrointestinal infections by positively affecting intestinal flora.

Numerous in vitro and in vivo investigations show that garlic has broad spectrum antimicrobial activity against bacteria, viruses, fungi, and intestinal parasites (helminths). Garlic also has immune-enhancing, antioxidant, and vasodilating activity. In both in vitro and in vivo studies, garlic produces anti-inflammatory, blood-sugar lowering (antidiabetic), and anticancer effects.

Research on garlic shows unequivocally that it can help prevent atherosclerosis through its effects on elevated lipids and blood pressure. Studies on both animals and humans indicate that garlic favorably shifted the high-density lipoprotein:low-density lipoprotein ratio toward lowered LDL and higher HDL values. It also lowered plasma viscosity and improved both blood fluidity and capillary blood flow. Garlic increased fibrinolytic activity, prolonged bleeding and clotting time, and inhibited platelet aggregations. Garlic consumption reduced blood pressure in hypertensive patients.

Double-blind clinical studies reveal that garlic lowered cholesterol and triglyceride levels in hyperlipidemic patients, and reduced blood pressure. In a 16-week, placebo-controlled trial involving 261 patients, the treatment group had a significant reduction in total cholesterol when compared with the placebo group. This trend has been confirmed by two meta-analyses of investigations on the influence of garlic on blood lipids. Garlic powder administered to patients for a minimum of four weeks resulted in an average decline of either 9% or 12% in total blood cholesterol. The average reduction in triglycerides was 13%.

Findings on the therapeutic effects of commercial garlic preparations were contradictory, presumably due to variable levels of active constituents in the commercial products used in the clinical trials. Garlic preparations can be dried (heated or freeze-dried), distilled, extracted with garlic oils, aged, or deodorized by unspecified processes. Allicin and ajoene (a self-condensation product of allicin) are absent in dried garlic preparations. Furthermore, allinase is unstable in the presence of gastric acids in the stomach. In order to be efficacious, dried garlic products must take the form of enteric-coated capsules or tablets.

Garlic has antioxidant properties, and antitoxic activity against carbon tetrachloride, isoproterenol, and heavy metal poisoning. It inhibits tumor proliferation in sarcoma, bladder tumors, isolated colon carcinoma cells, and liver cell carcinomas. In population studies in Asia, the incidence of stomach cancer deaths was lower in people who ate large quantities of garlic. The active principles in garlic may exert anticancer effects by stimulating the immune system to inhibit carcinogenesis.

Allicin probably accounts for antibiotic and antiplatelet activity. Allicin also lowers cholesterol levels by blocking lipid synthesis and by increasing the excretion of neutral and acidic sterols. Ajoene prevents blood clots by inhibiting platelet aggregation in vitro and in vivo in a dose-dependent and reversible manner. By inhibiting platelet aggregation, ajeone has a protective effect against atherosclerosis, coronary thrombosis, and stroke.

Recommended dosage (lower doses for prevention, higher doses for infection):

• 1,000 to 3,000 mg daily usually taken in encapsulated form (500 mg capsules)
• Oil: 0.03 to 0.12 ml tid

Excessive dietary intake can cause stomach upset, and topical use of garlic can result in both burn-like skin lesions and allergic contact dermatitis.

Individuals prone to slow blood clotting should not take therapeutic doses of garlic. Excessive intake of either dietary or nondietary sources of garlic can increase the risk of hemorrhagic complications during surgery and postoperative bleeding. Pregnant and lactating women should also avoid consuming garlic in large quantities since it has abortifacient and uteroactive properties. Garlic can alter the menstrual cycle.

Ajoene, the active antiplatelet component in garlic, potentiated the effect of physiologically and pharmacologically active antiplatelet agents such as dipyridamole and indomethacin in vitro (Apitz-Castro et al. 1986).

When administered to patients with myocardial infarction, the essential oil of garlic increased fibrinolysis by 63% to 69%, thereby enhancing the activity of warfarin (Bordia et al. 1977). Two cases of a possible interaction between warfarin and garlic have been reported in patients stabilized on anticoagulant therapy (Stockley 1999). In one case, the patient's INR values more than doubled and there was an incident of hematuria eight weeks after ingestion of garlic (3 pearles/day). This situation resolved when the garlic was discontinued, but the INR rose again when the patient started taking two garlic tablets daily. In the other case, the patient's INR also increased more than two-fold while the patient was taking garlic (6 tablets/day). Because of its antiplatelet and antithrombotic activity, garlic should be used with caution in patients taking oral anticoagulants (Rose et al. 1990).

The U.S. FDA classifies garlic as a dietary supplement. Bulbous garlic products are sold as nonprescription drugs in France and Germany.

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