Bottom Line: Garlic was shown to lower blood cholesterol and triglyceride levels but not in everyone. Garlic in the diet is associated with a lower risk of stomach and colorectal cancer, but there is no evidence that it can treat cancer.
Raw garlic contains compounds called alliin and allicin. In laboratory studies, these compounds and their breakdown products have been found to kill bacteria directly, reduce the number of platelets in the blood and slow clotting, and reduce the level of lipids in the blood. Compounds called ajoenes are also responsible for garlic's ability to prevent blood clots. Garlic may also reduce blood pressure. Garlic intake may protect against certain cancers possibly by decreasing tumor cell growth or stimulating the immune system. Because garlic has blood thinning property, patients taking warfarin or other blood thinners should ask their doctor before taking garlic supplements.
To fight skin infections Laboratory studies show that raw garlic has antibacterial, antiviral, antiparasitic, and antifungal activity. There is no proof from clinical trials that this effect occurs in the human body.
To prevent and treat cancer A few clinical trials suggest that high garlic consumption may lower the risk of stomach and colorectal cancers. No evidence supports the use of garlic as a cancer treatment.
To treat heart disease Several clinical trials show that use of garlic supplements reduces cholesterol and triglyceride levels in the blood, which may reduce the risk of heart disease, but a handful of clinical trials contradict these findings. Clinical trials studying the effect of garlic on blood pressure and risk of heart disease have shown mixed results.
As an antioxidant No scientific evidence supports this use.
To treat atherosclerosis Several clinical trials show that use of garlic supplements reduces blood pressure as well as cholesterol and triglyceride blood levels, which are risk factors for developing atherosclerosis, but a handful of clinical trials contradict these findings. Clinical and laboratory studies support garlic's antiplatelet activity, which may help prevent blood clots in patients with atheroclerosis.
To lower high cholesterol Several clinical trials show that use of garlic supplements reduces cholesterol and triglyceride levels in the blood, but a handful of clinical trials contradict these findings.
To lower high blood pressure Clinical trials studying the effects of garlic on blood pressure have shown mixed results.
To treat circulatory disorders Clinical and laboratory studies support garlic's antiplatelet activity, which may help prevent blood clots in patients with circulatory disorders.
High cholesterol Forty-six patients with high cholesterol took part in a study of garlic powder supplements. For 12 weeks, patients received either enteric-coated (specially prepared to avoid digestion in the stomach) capsules containing 9.6 mg allicin or an identical placebo capsule. Those taking the active garlic supplement experienced a decrease in total cholesterol and LDL ("bad") cholesterol, while those taking the placebo had an increase in HDL ("good") cholesterol. However, patients taking the active garlic pill also reported lower intakes of fat, carbohydrates, and alcohol than the placebo group, which may also be responsible for the differences seen in their cholesterol readings. These results suggest that, when combined with a low-fat diet, such garlic supplements may be effective in patients with mild to moderate high cholesterol.
Researchers analyzed all randomized, controlled trials that looked at the effects of garlic supplements in patients with cholesterol levels greater than 200 mg/dl. Out of 13 clinical trials, results showed that garlic is slightly more effective than a placebo in reducing total cholesterol levels. The researchers concluded that the use of garlic for high cholesterol is therefore of questionable value.
Heart disease A group of researchers analyzed all randomized, controlled trials performed between the years 1966 and 2000 that examined the effects of garlic on cardiovascular health. Overall, they found that garlic preparations might lead to small, short-term (one to three months) reductions in total cholesterol, triglycerides, and LDL ("bad") cholesterol and reductions in platelet aggregation in the formation of blood clots. Trials showed no overall effect on HDL ("good") cholesterol or blood glucose levels, and mixed results concerning garlic's effect on blood pressure. High blood pressure Researchers reviewed seven clinical trials that studied the effect of Kwai®, a popular garlic powder supplement, on high blood pressure. Three trials showed decreases in systolic blood pressure (the first of the two numbers in a blood pressure reading), while four showed decreases in diastolic blood pressure (the second number) with use of Kwai®. These results suggest that Kwai® may be beneficial in patients with mild hypertension, but the researchers suggest that more trials be done before it can be recommended as a routine therapy. Cancer prevention A review of 18 clinical trials studying the effect of garlic consumption in preventing stomach, colon, head and neck, lung, breast, and prostate cancers was performed. The results suggest that high intake of raw and cooked garlic may be associated with a protective effect against stomach and colorectal cancers only. However, this preventative effect may reflect the fact that people who consume large amounts of garlic also generally consume higher levels of fruits and vegetables.
Due to its potential for increasing bleeding time, you may want to discontinue garlic supplements a week or two before undergoing surgery. Consult with your surgeon.
This product is regulated by the FDA as a dietary supplement. Unlike approved drugs, supplements are not required to be manufactured under specific standardized conditions. The product may not contain the labeled amount or may be contaminated. In addition, it may not have been tested for safety or effectiveness.
You are taking warfarin or other blood thinners (Garlic may increase the risk of bleeding or bruising).
You take cyclosporine (Garlic can reduce its effectiveness and potentially cause transplant rejection).
You take protease inhibitors such as saquinavir (Fortovase®, Invirase®) (Garlic can significantly reduce their levels in the blood and reduce their effectiveness).
You are taking insulin (Dose adjustments may be required because of the occasional blood sugar-lowering effect of garlic supplements. Take with caution and consult your doctor).
Altered platelet function with potential for bleeding
Offensive odor, bad breath
Stomach upset
Diarrhea
Changes in the natural bacteria found in the intestines
Sweating
Low blood sugar
Contact dermatitis (inflammation, redness of the skin) when used topically.
Following excessive use of garlic supplements, one patient experienced prolonged bleeding time, diminished platelet blood clotting activity, and spinal epidural hematoma (collection of blood in the spinal canal).
Processing can have a significant effect on the amount of active substances in garlic: garlic powder and garlic essential oil do not contain allicin or ajoene, the compounds believed to be responsible for its cholesterol-reducing and blood-thinning activity.
In many of the clinical trials in which garlic showed beneficial results in heart disease, dietary changes may have played a large role in patients' improvement.
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 in 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). Studies evaluating the antithrombotic effects repeatedly show modest reduction in platelet aggregation, but varying levels of fibrinolytic activity. Research indicates mixed effects with regard to reductions in blood glucose, blood pressure, or risk of cardiovascular disease (8).
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). In addition, 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). Frequently reported adverse events include bad breath, headache, fatigue, GI upset, diarrhea, sweating, and possible hypoglycemia (16). 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). Garlic appears to induce cytochrome P450 (CYP) 3A4 and may enhance metabolism of many medications (e.g., cyclosporin and saquinavir) (18).
Volatile Oils: Small amount of oil-soluble sulfur compounds, including alliin, small amount of allicin, ajoene, diallyl trisulphide; terpenes such as citral, geraniol, linalool
Other constituents: S-methyl-l-cysteine sulfoxide, protein, minerals, vitamins, lipids, amino acids, prostaglandins (A2 and F 1 alpha)
Enzymes: Alliinase, peroxidases, myrosinase
Crushed raw Garlic
Volatile Oils: Contains most allicin (approx. 3.7 mg/g), alliin, ajoene, diallyl trisulphide; terpenes such as citral, geraniol, linalool
Water soluble compounds: S-allyl cysteine, S-allyl mercaptocysteine, saponins
Other constituents: S-methyl-l-cysteine sulfoxide, protein, minerals, vitamins, lipids, amino acids, prostaglandins (A2 and F 1 alpha)
Enzymes: Alliinase, peroxidases, myrosinase
Garlic Powder
Volatile Oils: Does not contain allicin. Contains alliin (approx. 1%), small amount of oil-soluble sulfur compounds
Other constituents: S-methyl-l-cysteine sulfoxide, protein, minerals, vitamins, lipids, amino acids, prostaglandins (A2 and F 1 alpha)
Enzymes: Alliinase, peroxidases, myrosinase
Garlic Essential Oil
No allicin or water-soluble components, not well standardized
Volatile Oils: Contains 1% or less oil-soluble sulfur compounds (19)(20)
The intact cells of garlic contain an odorless, sulfur-containing amino acid derivative known as alliin. When the cells are crushed, alliin comes into contact with the enzyme alliinase located in neighboring cells and is converted to allicin. Allicin is a potent antibiotic, but it is highly odoriferous and unstable. The ajoenes are apparently responsible for the antithrombotic properties of garlic. Allicin is described as possessing antiplatelet, antibiotic, and antihyperlipidemic activity. Most authorities agree that 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). In patients with hyperlipidemia, garlic might lower cholesterol levels by acting as an HMG-CoA reductase inhibitor (21). For atherosclerosis, garlic is believed to reduce oxidative stress and low-density lipoprotein oxidation and have antithrombotic effects (22). For hypertension, it is thought to reduce blood pressure by causing smooth muscle relaxation and vasodilation by activating the production of endothelium-derived relaxation factor (23). Garlic might stimulate both humoral and cellular immunity, causing T-cell proliferation, restoring suppressed antibody responses (24), and stimulating macrophage cytotoxicity on tumor cells. Garlic might increase selenium absorption with possible protection against tumororigenesis (25). In addition, garlic may protect against certain cancers by halting cell cycle progression and inducing apoptosis of cancer cells as well as by decreasing angiogenesis and influencing carcinogen metabolism (26)(27).
Preparation of garlic, such as heating, microwaving, or drying, can substantially reduce the allyl sulfur compounds (allicin and alliin). Crushed raw garlic is highest in these components. S-allyl cysteine (SAC) is well absorbed after oral administration and can be detected in the plasma, liver, and kidney. It is metabolized to N-acetyl-SAC and excreted in the urine. Allicin has not been detected in the bloodstream and is thought to undergo extensive first-pass hepatic metabolism. Allinase, required to convert alliin to allicin, is acid-labile (28).
Reported (Oral): Headache, fatigue, altered platelet function with potential for bleeding, offensive odor, GI upset, diarrhea, sweating, changes in the intestinal flora, hypoglycemia (16) Case Report (Oral): Prolonged bleeding time with spinal epidural hematoma and platelet dysfunction has occurred following excessive usage of garlic (29). Reported (Topical): Contact dermatitis
Insulin: Dose of insulin may require adjustment due to hypoglycemia effects of garlic. Warfarin: Anticoagulant activity may be enhanced due to increased fibrinolytic activity and diminished human platelet aggregation. Cytochrome P450 3A4: Garlic may cause induction of the 3A4 isoenzyme resulting in enhanced metabolism of certain drugs.
Cyclosporine: Effectiveness might be decreased by garlic's ability to induce metabolism and decrease levels of drugs like cyclosporine which are substrates of cytochrome P450 3A4. It can potentially cause transplant rejection (30).
Saquinavir (Fortovase, Invirase): Consuming garlic can significantly decrease serum concentration levels. Garlic can decrease peak levels by 54% and mean trough levels by 49%. These reductions in levels can cause therapeutic failure and increase development. It is suspected that garlic induces cytochrome P450 metabolism of saquinavir. Patients taking other protease inhibitors may be affected; however, only saquinavir interaction has been reported (18).
Ackermann RT, et al. Garlic shows promise for improving some cardiovascular risk factors. Arch Intern Med 2001;161:813-24. Compared with placebo, garlic preparations may lead to small reductions in the total cholesterol level at 1 month and at 3 months, but not at 6 months. Changes in low-density lipoprotein levels and triglyceride levels paralleled total cholesterol level results; no statistically significant changes in high-density lipoprotein levels were observed. Trials also reported significant reductions in platelet aggregation and mixed effects on blood pressure outcomes.
Fleischauer AT, et al. Garlic consumption and cancer prevention: meta-analyses of colorectal and stomach cancers. Am J Clin Nutr 2000;72:1047-52. In this meta-analysis of colorectal and stomach cancer, the reference categories ranged from no consumption to consumption of 3.5 g/wk, whereas the highest categories ranged from any consumption to more than 28.8 g. The results show that high intake of raw and cooked garlic may be associated with a protective effect against stomach and colorectal cancers. Heterogeneity of effect estimates, differences in dose estimation, publication bias, and possible hypotheses preclude sole reliance on summary effect estimates.
