Garlic

Purported Benefits, Side Effects & More
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Garlic

Purported Benefits, Side Effects & More
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Garlic

Common Names

  • Nectar of the gods
  • Camphor of the poor
  • Da-suan
  • La-suan
  • Stinking rose

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For Patients & Caregivers

Tell your healthcare providers about any dietary supplements you’re taking, such as herbs, vitamins, minerals, and natural or home remedies. This will help them manage your care and keep you safe.

What is it?

Evidence on whether garlic can help lower cholesterol levels or blood pressure are mixed.

Derived from the bulb or clove of the plant, garlic is used as a spice worldwide. It is also used traditionally for a variety of conditions such as maintaining circulatory function, to treat infections, and to boost the immune system.

Studies on whether garlic can reduce cholesterol or lower blood pressure are mixed, but other data suggest it may help reduce some cardiovascular risk factors.

Processing may reduce active compounds found in garlic products. For example, garlic powder and garlic essential oil do not contain allicin or ajoene, compounds believed to have cholesterol-reducing and blood-thinning properties.

Patients taking warfarin or other blood thinners should ask their doctor before taking garlic supplements.

What are the potential uses and benefits?
  • To treat cardiovascular disease

    Studies have shown mixed results, but garlic may help reduce some risk factors for cardiovascular disease. Patients should ask their doctor before taking garlic supplements.
  • To prevent and treat cancer

    Evidence on whether garlic can reduce risk of gastric and colorectal cancers is mixed.
  • To treat infections

    Data suggest garlic can stimulate the immune system, but it is unclear whether it can treat infections.
What are the side effects?
  • Headache
  • Fatigue
  • Offensive odor, bad breath
  • Stomach upset
  • Diarrhea
  • Changes in natural intestinal bacteria
  • Sweating
  • Low blood sugar
  • Altered platelet function
  • Increased bleeding risk

Case reports (Oral)

  • Bleeding, reduced blood clotting: Following excessive use of garlic supplements.
  • Redetected levels of HIV: In 2 cases due to interactions between garlic and darunavir. The effectiveness of the medication returned some time after garlic intake was stopped.
  • Chemical burns in the mouth lining: Following consumption of crushed garlic.
  • Allergic reactions: Two cases, following ingestion of garlic.
  • Liver toxicity: After a liver transplant patient was started on high-dose garlic supplementation to treat low levels of oxygen in the blood.

Case reports (Topical)

  • Chemical burns: Multiple cases from the topical use of garlic in attempts to treat warts, rash/itching, tooth pain, acne, and sore throat.
  • Skin rash: Due to “garlic necklace” on infant’s neck to treat nasal congestion
What else do I need to know?

Patient Warnings:

  • Garlic supplements should be discontinued 1–2 weeks before surgery because of the potential for increased bleeding.
  • Topical use of garlic preparations should be avoided. Chemical burns have occurred in attempts to treat a variety of conditions.

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 are taking protease inhibitors (darunavir/Prezista®, saquinavir/Fortovase®, Invirase®): In 2 cases of HIV patients who regularly consumed garlic, darunavir became ineffective. Other studies found garlic reduced blood levels of saquinavir and may reduce its effectiveness.
  • You are taking insulin: Dose adjustments may be required from potential blood sugar-lowering effects with garlic supplements. Take with caution and consult your doctor.
  • You are taking drugs that are substrates of CYP450 2C9, 2C19, or 3A4: Garlic may increase the risk of side effects of these drugs.
  • You are taking drugs that are substrates of P-glycoprotein: Garlic may reduce the activity of such drugs.
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For Healthcare Professionals

Brand Name
Garlique®, Kwai®, Kyolic®, One-a-day Garlic®
Scientific Name
Allium sativum
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. Processing can have a substantial effect on the chemical content of garlic because the volatile oil components are heat-sensitive and certain enzymes are acid-labile. The best measure of total activity of garlic is its ability to produce allicin, which in turn, results in the formation of other active constituents (1).

Studies are mixed on whether garlic preparations can lower cholesterol (2) (3) (4) (5) (6) (7) (31) or blood pressure (45) (53) (55), but meta-analyses suggest reductions in cardiovascular risk factors (50) (56) (57) (58) (59).

Studies on protective effects against various cancers are also mixed. In a large trial, long-term garlic supplementation was associated with reduced risk of gastric cancer mortality but not incidence (63), although conflicting data did not find evidence for either (43). Whether garlic protects against Helicobacter pylori infection is also unclear (64). Meta-analyses are also mixed on whether garlic intake reduces colorectal cancer risk (65) (66). Inverse associations have been identified between garlic consumption and risk of endometrial (13), prostate (12), and other cancers (9). Garlic supplementation may also be associated with reduced risk of hematologic malignancies (40).

Because garlic is known to decrease platelet aggregation and potentially elevate INR values, it should not be used with anticoagulants or in patients with platelet dysfunction (17).

Purported Uses and Benefits
  • Cardiovascular disease
  • High cholesterol
  • Hypertension
  • Infections
  • Cancer
Mechanism of Action

Intact cells of garlic contain alliin, an odorless sulfur-containing amino acid derivative. When the cells are crushed, alliin comes into contact with the enzyme alliinase located in neighboring cells and is converted to allicin. Allicin has antibacterial and antimicrobial activity, but is highly odoriferous and unstable. It also has antiplatelet and antihyperlipidemic activities. Most authorities agree that the best measure of total activity of garlic is the 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). Garlic may also reduce oxidative stress and LDL oxidation and have antithrombotic effects (22). It might reduce blood pressure by causing smooth muscle relaxation and vasodilation (23).

Garlic may stimulate both humoral and cellular immunity, causing T-cell proliferation, restoring suppressed antibody responses (24), and stimulating macrophage cytotoxicity on tumor cells. It may increase selenium absorption with possible protection against tumorigenesis (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).

Warnings
  • Discontinue use of garlic at least 7 days prior to surgery (17).
  • Chemical burns with topical applications of garlic have been reported in attempts to treat a variety of conditions (44) (48) (51) (52) (67) (68) (69).
Adverse Reactions

Headache, fatigue, altered platelet function with potential for bleeding, offensive odor, GI upset, diarrhea, sweating, changes in the intestinal flora, hypoglycemia (16).

Case reports (Oral)

  • Anaphylaxis  (38) (54)
  • Perioperative bleeding: Following excessive garlic intake (46) (70).
  • Prolonged bleeding, spinal epidural hematoma, platelet dysfunction: Following excessive use of garlic (29).
  • Renal hematoma following lithotripsy: In a 51-year-old man, due to excessive odorless garlic ingestion. Treated with antibiotics and fluids; remission occurred after 5 months (35).
  • Viral rebound in HIV patients: Two cases occurred due to significant interactions between garlic and darunavir that resulted in sub-therapeutic DRV plasma concentrations (71).
  • Hepatotoxicity: After a liver transplant patient was started on high-dose garlic supplementation for presenting hypoxemia and hepatopulmonary syndrome (72).
  • Chemical burn of oral mucosa: Following consumption of crushed garlic (36).

Case reports (Topical)

  • Chemical burns: From topical garlic preparations, used in attempts to treat warts (44) (67), rash/itching (48), tooth pain (51) (68), acne (69), and sore throat (52).
  • Infant contact dermatitis: Due to “garlic necklace” on infant’s neck to treat nasal congestion (73).
Herb-Drug Interactions
  • Warfarin: Anticoagulant activity may be enhanced due to increased fibrinolytic activity and diminished human platelet aggregation (17) (49).
  • Protease inhibitors (darunavir, saquinavir): In 2 cases of HIV-infected patients, significant interactions between garlic and darunavir resulted in sub-therapeutic DRV plasma concentrations and viral rebound (71). In one case the patient ingested 15 garlic cloves per week, while in the other the patient could not quantify the amount, but said it was significant. In both patients, drug effectiveness eventually returned with cessation of garlic. In healthy volunteers, garlic supplements with the rough equivalent of two 4-g cloves daily significantly decreased mean serum concentration levels, peak levels by 54%, and trough levels by 49%, which can cause therapeutic failure (18).
  • Insulin: Dose of insulin may require adjustment due to hypoglycemic effects of garlic exhibited in animal studies (39).
  • CYP450 substrates: In vitro studies suggest that garlic products may inhibit 2C9 and 2C19, and interfere with drugs metabolized by these enzymes (41) (42). Studies on its effects on 3A4 are mixed (34) (41) (42).
  • P-Glycoprotein substrates: In healthy volunteers, garlic extract induced P-glycoprotein and can interfere with the metabolism of certain drugs (34).
Herb Lab Interactions
  • Insulin
  • Increased PT and INR
  • Decreased cholesterol
  • Change in blood pressure
Dosage (OneMSK Only)
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. Guercio V, Galeone C, Turati F, et al. Gastric cancer and allium vegetable intake: a critical review of the experimental and epidemiologic evidence. Nutr Cancer. 2014;66(5):757-73.
  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.
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  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.
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  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. 2012 Mar 14;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; 41(5):729-35.
  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 Oct;20(10):2298-308.
  41. Ho BE, Shen DD, McCune JS, et al. Effects of Garlic on Cytochromes P450 2C9- and 3A4-Mediated Drug Metabolism in Human Hepatocytes. Sci Pharm. 2010 Sep 30;78(3):473-81.
  42. Foster BC, Foster MS, Vandenhoek S, et al. An in vitro evaluation of human cytochrome P450 3A4 and P-glycoprotein inhibition by garlic. J Pharm Pharm Sci. 2001 May-Aug;4(2):176-84.
  43. Ma JL, Zhang L, Brown LM, et al. Fifteen-year effects of Helicobacter pylori, garlic, and vitamin treatments on gastric cancer incidence and mortality.J Natl Cancer Inst. 2012 Mar 21;104(6):488-92.
  44. Filobbos G, Chapman T, Gesakis K. Iatrogenic burns from garlic. J Burn Care Res. 2012 Jan-Feb;33(1):e21.
  45. Stabler SN, Tejani AM, Huynh F, Fowkes C. Garlic for the prevention of cardiovascular morbidity and mortality in hypertensive patients. Cochrane Database Syst Rev. 2012 Aug 15;8:CD007653.
  46. Erian M, McLaren G. Be wary of “natural” therapy in gynecological surgery. International Journal of Women’s Health. 2013:5 345–349.
  47. Watson CJ, Grando D, Fairley CK, et al. The effects of oral garlic on vaginal candida colony counts: a randomised placebo controlled double-blind trial. BJOG. 2014 Mar;121(4):498-506.
  48. Xu S, Heller M, Wu PA, Nambudiri VE. Chemical burn caused by topical application of garlic under occlusion. Dermatol Online J. 2014 Jan 15;20(1):21261.
  49. Ge B, Zhang Z, Zuo Z. Updates on the clinical evidenced herb-warfarin interactions. Evid Based Complement Alternat Med. 2014;2014:957362.
  50. Kwak JS, Kim JY, Paek JE, et al. Garlic powder intake and cardiovascular risk factors: a meta-analysis of randomized controlled clinical trials. Nutr Res Pract. 2014 Dec;8(6):644-54.
  51. Sisson D, Balmer C. A chemical burn from a garlic poultice applied to the face to treat toothache: a case report. Prim Dent J. 2014 Feb;3(1):28-9.
  52. Karabacak E, Aydın E, Kutlu A, Dogan B. An unusual garlic burn occurring on an unexpected area. BMJ Case Rep. 2014 Apr 7;2014.
  53. Ried K. Garlic Lowers Blood Pressure in Hypertensive Individuals, Regulates Serum Cholesterol, and Stimulates Immunity: An Updated Meta-analysis and Review. J Nutr. 2016 Feb;146(2):389S-96S.
  54. Treudler R, Reuter A, Engin AM, Simon JC. A Case of Anaphylaxis After Garlic Ingestion: Is Alliinase the Only Culprit Allergen? J Investig Allergol Clin Immunol. 2015;25(5):374-5.
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  56. Sun YE, Wang W, Qin J. Anti-hyperlipidemia of garlic by reducing the level of total cholesterol and low-density lipoprotein: A meta-analysis. Medicine (Baltimore). May 2018;97(18):e0255.
  57. Taghizadeh M, Hamedifard Z, Jafarnejad S. Effect of garlic supplementation on serum C-reactive protein level: A systematic review and meta-analysis of randomized controlled trials. Phytother Res. Feb 2019;33(2):243-252.
  58. Darooghegi Mofrad M, Milajerdi A, Koohdani F, et al. Garlic Supplementation Reduces Circulating C-reactive Protein, Tumor Necrosis Factor, and Interleukin-6 in Adults: A Systematic Review and Meta-analysis of Randomized Controlled Trials. J Nutr. Apr 1 2019;149(4):605-618.
  59. Shabani E, Sayemiri K, Mohammadpour M. The effect of garlic on lipid profile and glucose parameters in diabetic patients: A systematic review and meta-analysis. Prim Care Diabetes. Feb 2019;13(1):28-42.
  60. Atkin M, Laight D, Cummings MH. The effects of garlic extract upon endothelial function, vascular inflammation, oxidative stress and insulin resistance in adults with type 2 diabetes at high cardiovascular risk. A pilot double blind randomized placebo controlled trial. J Diabetes Complications. May-Jun 2016;30(4):723-727.
  61. Matsumoto S, Nakanishi R, Li D, et al. Aged Garlic Extract Reduces Low Attenuation Plaque in Coronary Arteries of Patients with Metabolic Syndrome in a Prospective Randomized Double-Blind Study. J Nutr. Feb 2016;146(2):427s-432s.
  62. Szulinska M, Kregielska-Narozna M, Swiatek J, et al. Garlic extract favorably modifies markers of endothelial function in obese patients -randomized double blind placebo-controlled nutritional intervention. Biomed Pharmacother. Jun 2018;102:792-797.
  63. Li WQ, Zhang JY, Ma JL, et al. Effects of Helicobacter pylori treatment and vitamin and garlic supplementation on gastric cancer incidence and mortality: follow-up of a randomized intervention trial. Bmj. Sep 11 2019;366:l5016.
  64. Li Z, Ying X, Shan F, et al. The association of garlic with Helicobacter pylori infection and gastric cancer risk: A systematic review and meta-analysis. Helicobacter. Oct 2018;23(5):e12532.
  65. Zhou X, Qian H, Zhang D, et al. Garlic intake and the risk of colorectal cancer: A meta-analysis. Medicine (Baltimore). Jan 2020;99(1):e18575.
  66. Chiavarini M, Minelli L, Fabiani R. Garlic consumption and colorectal cancer risk in man: a systematic review and meta-analysis. Public Health Nutr. Feb 2016;19(2):308-317.
  67. Schimmel J, Camarena-Michel A, Hoyte C. Pediatric Cases Involving Chemical Burns to Garlic Used to Treat Warts. Dermatitis. Jan/Feb 2019;30(1):80-82.
  68. Vargo RJ, Warner BM, Potluri A, et al. Garlic burn of the oral mucosa: A case report and review of self-treatment chemical burns. J Am Dent Assoc. Oct 2017;148(10):767-771.
  69. Hisham A, Mohamed Sukur S, Basiron N. A case of facial burn due to the misuse of garlic face mask for acne. Australas J Dermatol. Nov 2018;59(4):336-337.
  70. Woodbury A, Sniecinski R. Garlic-Induced Surgical Bleeding: How Much Is Too Much? A A Case Rep. Dec 15 2016;7(12):266-269.
  71. Cloarec N, Solas C, Ladaique A, et al. Sub-therapeutic darunavir concentration and garlic consumption; a Mediterranean drug-food interaction, about 2 cases. Eur J Clin Pharmacol. Oct 2017;73(10):1331-1333.
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  73. Esfahani A, Chamlin SL. Garlic Dermatitis on the Neck of an Infant Treated for Nasal Congestion. Pediatr Dermatol. Jul 2017;34(4):e212-e213.
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