Individuals using anticoagulants such as heparin [Hepalean] and warfarin [Coumadin] would be prudent to avoid garlic due to the increased risk of bleeding, due to garlic’s fibrinolytic and antiplatelet aggregation activities, observed in both patients with coronary artery disease and healthy subjects. Garlic has been associated with postoperative bleeding and spontaneous epidural hematoma6. It is likely that, as with ginkgo, any effect of garlic in this context is purely additive and not a true herb-drug interaction: no case reports of such an interaction have so far been identified. In addition, unsubstantiated anecdotal claims of elevated international normalized ratio (INR) and prothrombin times (PT) in patients stabilized on warfarin7 are not consistent with garlic’s anticoagulant effects: antiplatelet aggregation mechanisms would be expected to alter bleeding times, not INR and PT. 8
It is suggested that adjustment of insulin dose maybe required if garlic is consumed, on account of the latter’s hypoglycemic effect. 9
Information on the relationship between substances and disease is provided for general information, in order to convey a balanced review of the scientific literature. In many cases the relationship between a substance and a disease is tentative and additional research is needed to confirm such a relationship.
Other Common Names: Allium, ail (French), ajo (Spanish), camphor of the poor, nectar of the gods, poor man’s treacle, rustic treacle, stinking rose.
A. sativum is an erect perennial, the bulb producing several long, flat, firm, narrow leaves. The bulbs and bulblets (cloves), enclosed in membranous skins, are strongly scented especially when cut or crushed. Thought to have originated in west-central Asia, garlic is today cultivated worldwide. 1, 2
The medicinal parts are the whole fresh garlic bulbs or dried bulbs that consist of the main bulb with several secondary bulbs or cloves. Powdered garlic is made from the dehydrated bulbs, while garlic oil is obtained by steam distillation of the crushed fresh bulbs.
Commercial garlic preparations vary greatly in the way they are prepared and the concentration of the active ingredient, alliin, converted by the enzyme alliinase to allicin. Enteric coating of dried garlic is essential to efficient allicin production, by protecting alliinase from degradation by stomach acid. .
Oil-based garlic preparations may not be as effective as whole garlic cloves, because allicin is degraded to other less active sulfur compounds. .
The effectiveness of ‘deodorized’ garlic formulations is questioned by many sources, asserting that the healing benefits are linked to odoriferous constituents of garlic.
Variation of sulfur compound composition with different treatments of garlic cloves: Nearly all (95%) of the sulfur in intact garlic cloves is composed of two classes of compounds in comparable abundance, namely, S-alkylcysteine sulphoxides and γ-glutamyl-S-alkylcysteines; the most abundant sulfur compound is alliin (10 mg/g fresh weight or 30 mg/g dry weight) which accounts for about 80% of the total cysteine sulfoxides. When crushed no change occurs with the γ-glytamyl-S-alkylcysteines, but alliin and methiin are converted by the enzyme alliinase to sulfenic acids which undergo spontaneous condensation to form thiosulfinates such ass allicin (regarded as the most active constituent of whole fresh and dried cloves) and allyl methanethiosulfinate; cycloalliin is unchanged. Under long-term aqueous incubation, γ-glutamyl-S-alkenyl (allyl and 1-propenyl) cysteines are converted, through the agency of the enzyme transpeptidase, to S-allylcysteine and S-1-propenylcysteine. Steam distillation generates from the acyclic alliins, diallyl disulfide and trisulfide plus allylmethyl disulfide and trisulfide. Incubation in oil or organic solvents converts cycloalliin to vinyl dithiins and ajoenes. 3
No health hazards or side effects are known in conjunction with the proper administration of the designated therapeutic dosages. Large oral intakes can lead to gastrointestinal symptoms, changes to intestinal flora and allergic reactions.
Garlic is contraindicated in those slated for elective surgery and people with known allergy garlic, onion or tulip. 2
Although the German Commission E monograph mentions no contraindication for pregnancy or lactation for doses of 4 g/day of garlic cloves or equivalent preparations, other sources recommend against use of garlic during lactaion.5
Cardiovascular Disease (CVD): Data suggest that garlic might be useful in managing hypercholesterolemia and platelet function, both risk factors for cardiovascular disease. 10 Clinical studies have shown a modest but significant decrease in serum total and low density lipoprotein (LDL) cholesterol concentrations. However, high density lipoprotein (HDL) cholesterol and triglycerides are not affected by garlic. A variety of organosulfur compounds, primarily derived from the compound alliin (mainly allicin, possibly S-allylcysteine and other compounds), are believed to exert the cholesterol lowering effects of garlic. It is also thought that there may be a variety of mechanisms involved in the cholesterol –lowering activity of garlic. Two meta-analyses, addressing 5 of 28, 11 and 16 of 2512 trials reviewed, respectively, assessed the effect of garlic preparations on serum lipid levels. In the earlier publication, Warshafsky et al.11, the 5 randomized and placebo-controlled trials, garlic-treated patients showed, a consistently greater decrease in total cholesterol (TC) than those on placebo: daily consumption of the equivalent of ˝-1 clove of garlic produced an average decrease of 9%; 75% of patients had TC levels greater than 200 mg/dl. In the second publication by Silagy and Neil12 , involving data from 952 subjects; 12 of the 16 trials examined were double-blind placebo-controlled and 2 were of crossover design, while 2 were single-blind and 2 uncontrolled. The total daily dose of garlic ranged from 600-900 mg dried powder and 10 g of raw garlic to 18 mg of garlic oil; doses of aged garlic extract (AGE) were not specified. The conclusion of this meta-analysis was that all dosage forms of garlic reduced cholesterol levels by an average of 12% after one month of therapy, which reduction lasted for 6 months. Dried garlic powder also significantly lowered serum triglyceride levels (13%) as compared to placebo. However, a more recent meta-analysis of 13 trials, noted that 4 reported no significant differences in TC levels, compared to placebo13 , indicating a need for larger-scale longer-term studies with carefully characterized garlic preparations.
A study was conducted on 46 hypercholesterolemic patients given dietary counseling on a low fat diet and enteric-coated garlic powder tablets with 9.6 mg allicin-releasing potential or placebo for 12 weeks. The garlic supplemented group had a small but significant reduction in total serum cholesterol concentrations by 4.2% and LDL-cholesterol by 6.6%, while the placebo group had a non-significant effect. 14
In a 12-week study proprietary standardized garlic tablets (Kwai®) was compared with benzafibrate, the most commonly prescribed blood lipid-lowering pharmaceutical in Germany. 15 A multi-center double-blind trial was conducted with 98 patients with primary hyperlipo-proteinemia, and having TC and/or triglyceride values greater than 250 mg.dl. Daily dosing, after a 6-week placebo run-in with 900 mg of garlic powder, standardized to 1.3% alliin and 600 mg of bezafibrate registered comparable, statistically significant decreases in TC, LDL cholesterol and triglyceride levels. HDL cholesterol values also increased significantly, with no appreciable differences between the two treatments.
Two subsequent trials with Kwai® did not confirm the positive effects on lipid lowering registered in the original study. 16 , 17 The first, a 12-week randomized, double-blind, placebo-controlled trial with 28 patients found no significant differences between treatment and placebo, as did the second, a similarly designed trial with 30 hypercholesterolemic children. Both studies utilized a dosing regimen of 300 mg of Kwai® three times daily.
It has been suggested that the failure of these two trials was due to changed bioavailability due to reformulation resulting in only one-third of release of allicin as compared to the older trials, as estimated under simulated gastrointestinal conditions.
A randomized double-blind placebo-controlled
crossover study assessed the effect of aged
garlic extract (AGE) on blood lipid levels in 41
men (ages 32-68) with moderate
hypercholesterolemia. 19 After a 4-week dietary
run-in, patients were administered 7.2 g of AGE
daily, in divided doses, or placebo for 6 months,
followed by a 4-mont crossover period. TC was
reduced by 6.1% 94 months) and 7.0% (6
months) in garlic-treated patients, with
inappreciable reduction under placebo. Systolic
blood pressure was reduced by an average of
5.5% in treated patients, negligibly with
An epidemiological study assessed the
protective effect of garlic consumption on
aortic elasticity in 101 elderly patients (aged
50-80 years) taking 300 mg/day of a
standardized garlic preparation for 2 or more
years. 20 The treatment group was matched with
101 control subjects. Pulse wave velocity
(PWV) and pressure-standardized elastic
vascular resistance (EVR) were used to
estimate the elastic properties of the aorta.
Both PWV and EVR were decreased in the
garlic-treated group, as compared with
controls. Platelet aggregation: Three randomized, double-blind, placebo-controlled crossover studies found that 900 mg/day of a standardized garlic powder significantly increased total euglobulin fibrinolytic and tissue plasminogen activator activity as compared to placebo. In addition, ex vivo platelet aggregation induced by adenosine diphosphate or collagen was significantly inhibited at 2 and 4 hours after garlic ingestion, and remained lower for 7-14 days of treatment.21 Consumption of 800 mg/day of a standardized garlic powder product for 4 weeks significantly decreased both the increased ratio of circulating platelet aggregation (56.3%), as compared to placebo. 22 Anticarcinogenic: Many epidemiological studies have suggested that certain natural foods, like garlic, could have anticarciongenic effects and may prevent development of tumors. Garlic may have a variety of pharmacological effects including tumor cell growth inhibition and chemopreventive activity. Epidemilogical evidence for a cancer-preventive effect of garlic comes from 9 epidemiological studies, 7 of which support the suggestion. [~23~]
A meta-analysis of epidemiologic literature was conducted on the association between garlic consumption and the risk of colon, stomach, head, neck, lung, breast and prostate cancers. For colorectal and stomach cancer reference categories ranged from 3.5 g per week to >28.8 g per week of garlic consumption. The authors concluded that a high intake of both raw and cooked garlic (21g or ~ 6 cloves/week) may be associated with a reduced incidence of stomach and colorectal cancers; a firm connection to protection could not be made because of confounding intakes possible publication bias, varying estimations of dose and other factors.
A study investigating the influence of heating on the anticancer properties of garlic revealed that garlic and its lipid or water –soluble components have many pharmacologic properties. The study showed that 60 seconds of microwave heating or 45 minutes of oven heating could block garlic’s ability to inhibit mammary carcinogen metabolite binding to rat mammary epithelial cell DNA. The studies demonstrated that heating destroyed garlic’s active allyl sulfur compound, which may suppress the activity of the enzyme allinase. The generation of biologically active allyl sulfur compounds is dependent not on total allinase activity, but the time for which it is allowed to function. 1 Immune Function : Arachidonic acid acts as a precursor of eicosanoids, which are involved in many regulatory processes. Modification of arachidonic acid metabolism has been proposed to be a consequence of garlic’s action. Inhibition of eicosanoid synthesis affects eicosanoid-dependent regulatory processes, including those on immune response and anti-tumor immunity.
A study done to investigate arachidonic acid metabolism and immune response gave rats garlic oil, diallyl disulfide (200 mg/kg of body weight) or corn oil three times a week for 7 days. The study demonstrated that garlic oil acts as an immunomodulator by promoting T-lymphocyte function. Garlic oil also was found to inhibit hepatic 6-desaturase activity and decrease the arachidonic acid concentrations in hepatic membrane phospholipids, both of which showed immunomodulatory potential. 2
A study compared aged garlic extract, containing 1.6-2.4 mg/g (calculated as dry weight) of S-allylcysteine and solid materials, to 100 mg of Krestin (an anti-cancer medication) on mouse spleens. Aged garlic extract stimulated the proliferation of mouse spleen cells and the release of cytokines, such as IL-2, TNF-alpha, and IFN-gamma, increased NK activities and it enhanced phagocytosis of peritoneal macrophages. Aged garlic extract also stimulated reactivity of lymphocytes in response to cytokines and mitogens. Aged garlic extract was found to be as effective as Krestin in significantly inhibiting the growth of Sarcoma-180 and LL/2 lung carcinoma cells transplanted into mice. 3 Peripheral Arterial Occlusive Disease (PAOD): Basic therapy for peripheral arterial occlusive disease stage II is walking. Risk factors such as smoking should be eliminated and vasoactive treatments such as naftidrofuryl and pentoxifylline can be used to increase walking distance. Garlic is being considered as a new treatment for PAOD, because it may improve microcirculatory blood flow.
In a 12-week study, 800 mg of garlic powder was given to patients with peripheral arterial disease stage II. In the 5th week of treatment, the garlic group’s walking distance was increased by 28.5% (46 meters) and was increased by 18.1% (31 meters) in the placebo group. In the garlic group, the diastolic blood pressure, spontaneous thrombocyte aggregation, plasma viscosity and cholesterol concentration also decreased significantly. Overall, garlic may be useful in the long-term treatment of incipitent intermittent claudication. 4 Antimutagenic: Allicin, an important substance for the medicinal properties of garlic is rapidly converted to diallyl disulfide and other sulfur compounds because of instability. Ajoene is one of the derivatives of allicin and has been found to be a major sulfur-containing compound in oil-macerated garlic extract. Ajoene may have antimutagenic effects toward Salmonella typhimurium strains.
The Ames test was used to examine the antimutagenic effects of ajoene in rat liver preparations, in vitro. Ajoene inhibited mutagenisis induced by both benzo  pyrene (B  P) and 4-nitro-1,2-phenylenediamine (NPD) in a dose-dependent manner. Ajoene inhibition of mutagenesis was more effective for transition-type mutations than for the frame shift type. 5 Garlic Preparations: Garlic contains unique organosulfur compounds, which provide its characteristic flavor, odor and its potent biological activity. Different garlic products that are marketed may be divided into allicin-potential and nonallicin-potential products. Allicin products are made from raw garlic and the nonallicin products are made from processed garlic, and these may differ significantly in the substances they contain. There are many questions regarding whether all garlic products are created equal and which forms are best for improving health.
A study used three animal models and compared the pharmacologic activities of four garlic preparations, raw garlic juice (RGJ), heated garlic juice (HGJ), dehydrated garlic powder (DGP), and aged garlic extract (AGE). All four garlic preparations significantly enhanced natural killer and killer cell activities of the spleen cells of tumor-bearing mice, but only the AGE and HGJ inhibited the growth of inoculated tumor cells. The results suggested that each garlic preparation has different pharmacologic properties, but AGE may be the most useful garlic preparation. 6
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