About Herbs, Botanicals & Other Products

Scientific Name
Allium sativum
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Common Name

Nectar of the gods, camphor of the poor, da-suan, la-suan, stinking rose

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Brand Name

Garlique®, Kwai®, Kyolic®, One-a-day Garlic®

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Clinical Summary

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 analysis of a systematic review shows 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. Research indicates 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).

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). Garlic supplemenation 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).

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Purported Uses
  • Atherosclerosis
  • Cancer prevention
  • Cancer treatment
  • Cardiovascular disease
  • Circulatory disorders
  • High cholesterol
  • Hypertension
  • Microbial infection
  • Skin infections
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Constituents

Aged Garlic Extract

  • Water-soluble compounds: S-allyl cysteine, S-allyl mercaptocysteine, saponins
  • 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)
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Mechanism of Action

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).

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Pharmacokinetics

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).

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Warnings

Discontinue use of garlic at least 7 days prior to surgery.
(17)

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Adverse Reactions
  • 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).
  • Case Report: A 51-year-old man developed renal haematoma after extracorporeal shock-wave lithotripsy (SWL) that occurred due to excessive odourless garlic ingestion. He was treated with antibiotics and fluids. His haematoma went into remission after five months (35).
  • Case Report: Chemical burn of oral mucosa has been reported following consumption of crushed garlic (36).
  • Case Report: Two episodes of severe, near-fatal anaphylaxis were reported in a 52-year-old man following ingestion of garlic (38).
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Herb-Drug Interactions
  • Insulin: Dose of insulin may require adjustment due to hypoglycemic effects of garlic (39).
  • Warfarin: Anticoagulant activity may be enhanced due to increased fibrinolytic activity and diminished human platelet aggregation (17).
  • 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).
  • P-Glycoprotein substrates: Garlic induces P-glycoprotein and can interfere with the metabolism of certain drugs (34).
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Herb Lab Interactions
  • Insulin
  • Increased PT and INR
  • Decreased cholesterol
  • Change in blood pressure
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Literature Summary and Critique

Galeone C, Pelucchi C, Levi F, et al. Onion and garlic use and human cancer. Am J Clin Nutr 2006;84: 1027-32.
This is an analyis of a large data set from Swiss and Italian case-control studies conducted between 1991 and 2004 to determine the association between frequency of onion and garlic use and cancer. Researchers found a protective role of a moderate frequency of onion consumption against colorectal, laryngeal, and ovarian cancers. The association was also significant for cancers of oral cavity and esophagus, but not for prostate, breast, or renal cell cancers. Data also showed a moderate frequency of garlic intake was inversely associated with colorectal and renal cell cancers, and a high frequency was significantly and inversely related to all cancers, except for breast and prostate.
Onion and Garlic may confer protection against certain common cancers.

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.

Stevinson C, et al. Garlic for treating hypercholesterolemia: a meta-analysis of randomized clinical trials. Ann Intern Med 2000;133:420-9.
The available data suggests that garlic is superior to placebo in reducing total cholesterol levels. However, the size of the effect is modest, and the robustness of the effect is debatable. The use of garlic for hypercholesterolemia is therefore of questionable value.

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.

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References
  1. Tyler, V. Herbs of Choice, the Therapeutical Use of Phytomedicinals. Binghamton: Pharmaceutical Press; 1994.
  2. Ackermann RT, et al. Garlic shows promise for improving some cardiovascular risk factors. Arch Intern Med 2001;161:813-24.
  3. Berthold HK, et al. Effect of a garlic oil preparation on serum lipoproteins and cholesterol metabolism: a randomized controlled trial. JAMA 1998;279:1900-2.
  4. Gardner CD, et al. Effect of Raw Garlic vs Commercial Garlic Supplements on Plasma Lipid Concentrations in Adults With Moderate Hypercholesterolemia: A Randomized Clinical Trial. Arch Intern Med. 2007 Feb 26;167(4):346-53.
  5. Isaacsohn JL, et al. Garlic powder and plasma lipids and lipoproteins: a multicenter, randomized, placebo-controlled trial. Arch Inter Med 1998;158:1189-94.
  6. Kannar D, et al. Hypocholesterolemic effect of an enteric coated garlic supplement. J Am Coll Nutr 2001;20:225-31.
  7. Stevinson C, et al. Garlic for treating hypercholesterolemia: a meta-analysis of randomized clinical trials. Ann Intern Med 2000;133:420-9.
  8. Silagy CA, et al. A meta-analysis of the effect of garlic on blood pressure. J Hypertension 1994;12:463-8.
  9. Galeone C, Pelucchi C, Levi F, et al. Onion and garlic use and human cancer. Am J Clin Nutr 2006;84: 1027-32.
  10. Fleischauer AT, et al. Garlic consumption and cancer prevention: meta-analyses of colorectal and stomach cancers. Am J Clin Nutr 2000;72:1047-52.
  11. Ngo SN, Williams DB, Cobiac L, et al. Does garlic reduce risk of colorectal cancer? A systematic review.J Nutr. Oct 2007;137(10):2264-2269.
  12. Hsing AW, Chokkalingam AP, Gao YT, et al. Allium vegetables and risk of prostate cancer: a population-based study. J Natl Cancer Inst. Nov 6 2002;94(21):1648-1651.
  13. Galeone C, Pelucchi C, Dal Maso L, et al. Allium vegetables intake and endometrial cancer risk. Public Health Nutr. Nov 6 2008:1-4.
  14. Ishikawa H, Saeki T, Otani T, et al. Aged garlic extract prevents a decline of NK cell number and activity in patients with advanced cancer. J Nutr. Mar 2006;136(3 Suppl):816S-820S.
  15. Tanaka S, Haruma K, Yoshihara M, et al. Aged garlic extract has potential suppressive effect on colorectal adenomas in humans. J Nutr. Mar 2006;136(3 Suppl):821S-826S.
  16. Blumenthal M. Herbal Medicine, Expanded Commission E Monographs, 1st ed. Austin: American Botanical Council; 2000.
  17. Ang-lee M, et al. Herbal Medicines and perioperative care. JAMA 2001;286:208-16.
  18. Piscitelli SC, et al. The effect of garlic supplements on the pharmacokinetics of saquinavir. Clin Infect.Dis. 2002;34:234-8.
  19. Amagase H, et al. Intake of garlic and its bioactive components. J Nutr 2001;131:955S-62S.
  20. Newall CA, et al. Herbal Medicines: A Guide for Health Care Professionals. London: Pharmaceutical Press; 1996.
  21. Qureshi AA, et al. Suppression of avian hepatic lipid metabolism by solvent extracts of garlic: impact on serum lipids. J Nutr 1983;113:1746-55.
  22. Dirsch VM, et al. Effect of allicin and ajoene, two compounds of garlic, on inducible nitric oxide synthase. Atherosclerosis 1998;139:333-9.
  23. Pedraza-Chaverri J, et al. Garlic prevents hypertension induced by chronic inhibition of nitric oxide synthesis. Life Sci 1998;62:71-7.
  24. Hodge G, et al. Allium sativum (garlic) suppresses leukocyte inflammatory cytokine production in vitro: potential therapeutic use in the treatment of inflammatory bowel disease. Cytometry 2002;48:209-15.
  25. Hirsch K, et al. Effect of purified allicin, the major ingredient in freshly crushed garlic, on cancer cell proliferation. Nutr Cancer 2000;38:245-54.
  26. Powolny AA, Singh SV. Multitargeted prevention and therapy of cancer by diallyl trisulfide and related Allium vegetable-derived organosulfur compounds. Cancer Lett. Oct 8 2008;269(2):305-314.
  27. Herman-Antosiewicz A, Powolny AA, Singh SV. Molecular targets of cancer chemoprevention by garlic-derived organosulfides.Acta Pharmacol Sin. Sep 2007;28(9):1355-1364.
  28. Song K, Milner JA. The influence of heating on the anticancer properties of garlic. J Nutrition 2001;131:1054S-7S.
  29. Rose KD, et al. Spontaneous spina epidural hematoma with associated platelet dysfunction from excessive garlic ingestion: a case report. Neurosurgery 1990;26:880-2.
  30. Brinker F. Herb Contraindications and Drug Interactions, 2nd ed. Sandy (OR): Eclectic Med Publications; 1998.
  31. Hasani-Ranjbar S, Nayebi N, Moradi L, Mehri A, Larijani B, Abdollahi M. The efficacy and safety of herbal medicines used in the treatment of hyperlipidemia; a systematic review. Curr Pharm Des. 2010;16(26):2935-47.
  32. De BK, Dutta D, Pal SK, Gangopadhyay S, Das Baksi S, Pani A. The role of garlic in hepatopulmonary syndrome: a randomized controlled trial. Can J Gastroenterol. 2010 Mar;24(3):183-8.
  33. Lissiman E, Bhasale AL, Cohen M. Garlic for the common cold. Cochrane Database Syst Rev. 2009 Jul 8;(3):CD006206.
  34. Hajda J, Rentsch KM, Gubler C, Steinert H, Stieger B, Fattinger K. Garlic extract induces intestinal P-glycoprotein, but exhibits no effect on intestinal and hepatic CYP3A4 in humans. Eur J Pharm Sci. 2010 Oct 7. [Epub ahead of print]
  35. Gravas S, Tzortzis V, Rountas C, Melekos MD. Extracorporeal shock-wave lithotripsy and garlic consumption: a lesson to learn. Urol Res. 2010 Feb;38(1):61-3.
  36. Bagga S, Thomas BS, Bhat M. Garlic burn as self-inflicted mucosal injury—a case report and review of the literature. Quintessence Int. 2008 Jun;39(6):491-4.
  37. Hasani-Ranjbar S, Nayebi N, Moradi L, et al. The efficacy and safety of herbal medicines used in the treatment of hyperlipidemia; a systematic review. Curr Pharm Des. 2010;16(26):2935-47.
  38. Vovolis V, Kalogiros L, Ivanova D, Koutsostathis N. Garlic-induced severe anaphylaxis in a nonatopic patient. J Investig Allergol Clin Immunol. 2010;20(4):356.
  39. Madkor HR, Mansour SW, Ramadan G. Modulatory effects of garlic, ginger, turmeric and their mixture on hyperglycaemia, dyslipidaemia and oxidative stress in streptozotocin-nicotinamide diabetic rats. Br J Nutr. 2011 Apr;105(8):1210-7.
  40. Walter RB, Brasky TM, Milano F, White E. Vitamin, mineral, and specialty supplements and risk of hematologic malignancies in the prospective VITamins And Lifestyle (VITAL) study.Cancer Epidemiol Biomarkers Prev. 2011 Jul 29. [Epub ahead of print]
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How It Works

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.

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Purported Uses
  • 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.
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Research Evidence

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.

Cancer prevention
An analyis of a large data set from Swiss and Italian case-control studies done between 1991 and 2004 was conducted to determine the association between frequency of onion and garlic use and cancer. Researchers found a protective role of a moderate frequency of onion consumption against colorectal, laryngeal, and ovarian cancers. The association was also significant for cancers of oral cavity and esophagus, but not for prostate, breast, or renal cell cancers. Data also showed a moderate frequency of garlic intake was inversely associated with colorectal and renal cell cancers, and a high frequency was significantly and inversely related to all cancers, except for breast and prostate. Onion and Garlic may offer protection against certain common cancers.

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.

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Patient Warnings
  • 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.
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Do Not Take If
  • 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).
  • If you are taking drugs that are substrates of P-Glycoprotein (Garlic may increase the risk of side effects of these drugs).
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Side Effects
  • Headache
  • Fatigue
  • 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.
  • Case Report: 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).
  • Case Report: A 51-year-old man developed renal haematoma after extracorporeal shock-wave lithotripsy (SWL) that occurred due to excessive odourless garlic ingestion. He was treated with antibiotics and fluids. His haematoma went into remission after five months.
  • Case Report: Chemical burn of oral mucosa has been reported following consumption of crushed garlic.
  • Case Report: Two episodes of severe, near-fatal anaphylaxis were reported in a 52-year-old man following ingestion of garlic.
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Special Point
  • 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.
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Dosage (Inside MSKCC Only)
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Aliases
Allium Sativum
Kwai®
Kyolic®
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E-mail your questions and comments to aboutherbs@mskcc.org.
Last updated: December 7, 2011