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Complementary & Alternative Medicine
Index of Herbal Medicines, Supplements and Therapies
The decision to use products containing or claiming to contain garlic should be carefully considered.
Garlic (Allium sativum)
Be aware that the U.S. Food and Drug Administration does not strictly regulate herbs and dietary supplements. There is no guarantee of strength, purity or safety of products containing or claiming to contain garlic. Decisions to use herbs or supplements should be carefully considered. Individuals using prescription drugs should discuss taking herbs or supplements with a pharmacist or health care professional before starting.
Scientists have studied garlic for the following health problems:
A number of studies in humans have examined the effects of garlic on cholesterol levels. Most of these studies have been brief and have included few people. Overall, this research suggests that garlic lowers cholesterol levels a small amount. It is not clear how long its effects may last or what the long-term effects on health may be. In the future, longer studies with more people may provide stronger evidence. It should be noted that research using prescription drugs to lower blood cholesterol levels has shown better results than research using garlic.
High blood pressure
There are a few studies showing that garlic lowers blood pressure. However, these studies have been small, low quality and not fully convincing. Better studies need to be done before garlic can be recommended to treat high blood pressure.
Garlic may have positive effects on health problems that may lead to cardiovascular diseases, such as heart attack or stroke. For example, garlic may lower blood pressure, mildly "thin" the blood (anticoagulate) and reduce cholesterol levels. Laboratory and animal studies report that garlic may prevent atherosclerosis (cholesterol buildup in the arteries). However, there are no high-quality studies showing that garlic promotes cardiovascular health.
Several studies suggest that garlic may reduce the risk of developing cancer of the stomach or colon. However, these are only early results, and there are no definitive answers at this time. Studies are being done (many of them in China) to further investigate the use of garlic for cancer. Other cancers under examination include breast, head and neck, lung, prostate and urinary tract cancers.
Infections (bacterial, viral, fungal, other)
In laboratory experiments, garlic has been shown to be effective against bacteria, mycobacteria, viruses and fungi. Small studies have been conducted using garlic for acute viral respiratory infections in children. Other studies have shown that garlic may have some effect on athlete's foot and cryptococcal meningitis. However, few high-quality studies have been done in humans. Therefore, there is not enough information to recommend garlic to treat or prevent infections at this time.
Antiplatelet effects (blood thinning)
The effects of garlic on platelet aggregation have been assessed in several trials in humans. Although these studies have, overall, been of low quality, garlic does appear to possess some platelet-inhibiting properties. Dosing, safety, comparison to other agents, duration of effects, and clinical outcomes are not known, and the potential benefits of using garlic for this purpose are not clear. Because garlic has been associated with several cases of bleeding, therapy should be applied with caution (particularly in patients using other agents that may precipitate bleeding).
Peripheral vascular disease (blocked arteries in the legs), claudication, circulation
Garlic trials suggest modest short-term reductions in total cholesterol and low-density lipoprotein levels with oral garlic supplements. Long-term effects on lipids and atherosclerosis are not clear. There is limited evidence regarding the effects of garlic in patients with peripheral vascular disease or claudication. A small number of studies have explored this issue and reported favorable results, including increased walking distances. However, these studies have, overall, been poorly designed. There is currently insufficient evidence demonstrating effects of garlic on peripheral vascular disease, and further study is needed in this area.
In early study, self-reports of tick bites were significantly less in people receiving garlic over a sugar pill.
Upper respiratory tract infection
Garlic has a long history of use in the treatment of various infectious agents, including bacteria, fungi, and viruses. Garlic has shown some effectiveness in laboratory experiments. However, there is limited available evidence in humans.
Early study results in animals and humans are mixed. Further research is needed before a conclusion can be drawn.
Several case studies in humans have examined the effects of garlic on Helicobacter pylori infection, which can cause stomach ulcers, and found a lack of benefit.
Garlic has been suggested for many other uses, based on tradition or on scientific theories. However, these uses have not been thoroughly studied in humans, and there is limited scientific evidence about safety or effectiveness. Some of these suggested uses are for conditions that are potentially very serious and even life-threatening. You should consult a health care professional before taking garlic for any unproven use.
|Abnormal heart rhythms|
Age-related memory problems
Ascaridiasis (worms in the gut or liver)
Benign breast disease
Bile secretion problems
Dysmenorrhea (painful menstruation)
Heart rhythm disorders
| Hormonal effects|
Immune system enhancement
Induction of vomiting
Inflammatory bowel disease
Kidney damage from antibiotics
Parasites and worms
Peptic ulcer disease
Ringworm (Tinea corpori, Tinea cruris)
Snake venom protection
Stomach acid reduction
Stomach lining protection
Urinary tract infections
People with allergies to plants in the Liliaceae family should avoid garlic. There have been multiple cases of asthmatic reactions and allergic skin rashes related to garlic therapy. A severe allergic reaction to garlic was reported in a 23-year-old woman after taking young garlic.
Bleeding has been reported with garlic use; therefore, it is very important that you speak with a health care professional before taking garlic in amounts greater than the amounts normally found in foods. You may need to stop taking garlic supplements before some surgeries; discuss this with a health care professional.
Multiple cases of allergic reactions and skin rashes (dermatitis) have been reported. Asthma has been noted in garlic harvesters and has been attributed to garlic allergies in many cases. Swollen sinuses (rhinitis) have also occurred in people taking garlic. Some reactions may be caused by infestation of garlic with mites. Impurities in garlic products may also cause side effects. Skin contact with garlic has led to hand eczema in adults and severe burns in infants, children and adults. Bad breath, body odor, stomach ache, gastrointestinal irritation and diarrhea may also occur while taking garlic. Other reported side effects include dizziness, increased sweating, headache, itching, fever, chills, and runny nose. Garlic and pycnogenol have been shown to increase human growth hormone secretion in laboratory experiments. Thyroid levels, blood sugar, cholesterol and blood pressure may be lowered.
Pregnancy And Breast-Feeding
Garlic is likely safe to use during pregnancy and breast-feeding in the amounts commonly found in food. Pregnant women should avoid large amounts of garlic because of the increased risk of bleeding. In addition, an animal study and a laboratory study reported that garlic stimulates the uterus (although another study in rats showed no negative effects). Eating large amounts of garlic while breast-feeding may increase nursing time, milk odor and milk intake by the infant.
Be aware that many tinctures contain high levels of alcohol and should be avoided during pregnancy.
Interactions with drugs, supplements and other herbs have not been thoroughly studied. The interactions listed below have been reported in scientific publications. If you are taking prescription drugs, speak with a health care professional or pharmacist before using herbs or dietary supplements.
Interactions With Drugs
Garlic may increase the risk of bleeding when taken with anticoagulant drugs, such as warfarin (Coumadin), or antiplatelet drugs, such as clopidogrel (Plavix). Your blood may need to be monitored carefully if you take these together. There are reports of patients taking warfarin who experienced an increased tendency to bleed (measured by laboratory tests) after starting garlic supplements. Nonsteroidal anti-inflammatory drugs, such as ibuprofen (Advil), should also be used cautiously with garlic. Garlic use should be stopped before some surgeries; discuss this with a health care professional.
Patients taking saquinavir (Fortovase) for HIV or AIDS should consult a health care professional or pharmacist because garlic may significantly lower saquinavir levels and effects. Levels of other antivirals may also be affected.
Garlic may lower blood pressure. Patients taking blood pressure drugs should be monitored closely by a health care professional while using garlic. Dosing adjustments may be necessary.
Garlic may alter blood sugar levels. Patients taking oral drugs for diabetes or using insulin should be monitored closely by a health care professional while using garlic. Dosing adjustments may be necessary.
Garlic may lower cholesterol. Taking garlic with cholesterol-lowering drugs may further decrease cholesterol levels.
Garlic may alter levels of certain drugs metabolized by the liver's CYP450 enzyme system. Check with your doctor and pharmacist to check for potential interactions.
Be aware that many tinctures contain high levels of alcohol and may cause nausea or vomiting when taken with metronidazole (Flagyl) or disulfiram (Antabuse).
Garlic may alter levels of various anticancer drugs. Check with your oncologist and pharmacist before taking garlic.
Interactions With Herbs And Dietary Supplements
Garlic may increase the risk of bleeding caused by anticoagulant or antiplatelet herbs or supplements such as ginkgo
or horse chestnut seed extract.
One study showed that coleus forskolin
taken with garlic can increase antiplatelet effects, possibly increasing the risk of bleeding. You should speak with a health care professional before taking garlic with other herbs or supplements. Always read product labels carefully.
EPA (eicosapentaenoic acid), which is found in fish oils, may increase the risk of bleeding when taken with garlic. EPA may also lower cholesterol. One study showed that taking EPA with garlic could lower cholesterol even further.
Garlic may lower blood pressure. People considering garlic that are taking other herbs or supplements that may lower blood pressure, such as black cohosh
) or hawthorn
) should discuss therapy options with a health care professional.
Garlic may alter blood sugar levels. Patients taking other herbs or supplements that may affect blood sugar levels, such as bitter melon
), should be monitored closely by a health care professional while taking garlic.
Garlic may lower cholesterol. Taking garlic with cholesterol-lowering agents may decrease cholesterol levels further.
Vitamin E may have positive effects on cardiovascular disease and atherosclerosis. Taking garlic with vitamin E may increase these effects.
Garlic may interact with herbals and dietary supplements that are metabolized by the liver's YP450 enzyme system. Speak with your doctor and pharmacist about potential interactions.
Garlic and pycnogenol have been shown to increase human growth hormone secretion in laboratory experiments.
The doses listed below are based on scientific research, publications or traditional use. Because most herbs and supplements have not been thoroughly studied or monitored, safety and effectiveness may not be proven. Brands may be made differently, with variable ingredients even within the same brand. Combination products often contain small amounts of each ingredient and may not be effective. Appropriate dosing should be discussed with a health care professional before starting therapy; always read the recommendations on a product's label. The dosing for unproven uses should be approached cautiously, because scientific information is limited in these areas.
Adults (Aged 18 Or Older)
Raw garlic cloves: A dose of one-half to two raw garlic cloves (two to six grams) up to four times per day has been taken by mouth.
Garlic pills: A dose of 600 to 900 milligrams per day, divided into three doses, has been taken by mouth. The European Scientific Cooperative on Phytotherapy (ESCOP) recommends three to five milligrams of allicin daily (one clove or 0.5 to one gram of dried powder) for prevention of atherosclerosis. The World Health Organization (WHO) recommends two to five grams of fresh garlic, 0.4 to 1.2 grams of dried powder, two to five milligrams of oil, 300 to 1000 milligrams of extract, or other formulations that are equal to two to five milligrams of allicin daily.
A dose of 0.4 to 1.2 grams per day, divided into three doses, has been taken by mouth.
Oil extract of garlic: A dose of one or two capsules or 0.3 to 0.12 milliliters three times per day has been taken by mouth. Some sources report that steam-distilled oils, oil from crushed garlic, and aged garlic in alcohol may be less effective for some uses, particularly as a blood thinner.
Garlic juice: A dose of two to four milliliters three times per day has been taken by mouth.
Garlic syrup: A dose of two to eight milliliters three times per day has been taken by mouth.
Tincture: A dose of two to four milliliters of a tincture (1:5; 45 percent alcohol) three times per day has been taken by mouth.
Children (Younger Than 18)
There is not enough scientific evidence to recommend garlic supplements for children at this time. The amounts of garlic found in foods are thought to be safe.
Although garlic has been suggested for many conditions, it has been most studied for high cholesterol, cardiovascular health, high blood pressure, cancer prevention and infections. Scientific research suggests that garlic may lower cholesterol levels a small amount. Garlic has not been proven for any other health condition.
The amount of garlic found in foods is considered safe. When used in larger amounts, you must be careful because of the risk of bleeding. Garlic may also lower blood pressure or blood sugar levels. If you are taking anticoagulants, nonsteroidal anti-inflammatory drugs, blood pressure drugs, insulin or other diabetic drugs or if you are pregnant, you should speak with a health care professional or pharmacist before taking garlic supplements. Consult a health care professional immediately if you experience side effects.
The information in this monograph was prepared by the professional staff at Natural Standard, based on thorough systematic review of scientific evidence. The material was reviewed by the Faculty of the Harvard Medical School with final editing approved by Natural Standard.
- Natural Standard: An organization that produces scientifically based reviews of complementary and alternative medicine (CAM) topics
- National Center for Complementary and Alternative Medicine (NCCAM): A division of the U.S. Department of Health & Human Services dedicated to research
Selected Scientific Studies: Garlic
Natural Standard reviewed more than 575 articles to prepare the professional monograph from which this version was created.
Some of the more recent studies are listed below:
- Ackermann R, Mulrow C. Duration of the hypocholesterolemic effect of garlic supplements. Arch Intern Med 2001;161(20):2505-2506.
- Ackermann RT, Mulrow CD, Ramirez G, et al. Garlic shows promise for improving some cardiovascular risk factors. Arch Intern Med 2001;161(6):813-824.
- Bassioukas K, Orton D, Cerio R. Occupational airborne allergic contact dermatitis from garlic with concurrent type I allergy. Contact Dermatitis 2004;Jan, 50(1):39-41.
- Borek C. Garlic supplements and saquinavir. Clin Infect Dis 2002;Aug 1, 35(3):343. Comment in: Clin Infect Dis 2002;Jan 15, 34(2):234-238.
- Carden SM, Good WV, Carden PA, Good RM. Garlic and the strabismus surgeon. Clin Experiment Ophthalmol 2002;Aug, 30(4):303-304.
- Dietz DM, Varcelotti JR, Stahlfeld KR. Garlic burns: a not-so-rare complication of a naturopathic remedy? Burns 2004;Sep, 30(6):612-613.
- Dikasso D, Lemma H, Urga K, et al. Investigation on the antibacterial properties of garlic (Allium sativum) on pneumonia causing bacteria. Ethiop Med J 2002;Jul, 40(3):241-249.
- Durak I, Aytac B, Atmaca Y, et al. Effects of garlic extract consumption on plasma and erythrocyte antioxidant parameters in atherosclerotic patients. Life Sci 2004;Sep 3, (16):1959-1966.
- Durak I, Davutcu M, Aytac, et al. Effects of garlic extract consumption on blood lipid and oxidant/antioxidant parameters in humans with high blood cholesterol. J Nutr Biochem 2004;Jun, 15(6):373-377.
- Ernst E. Can Allium vegetables prevent cancer? Phytomed 1997;4(1):79-83.
- Fleischauer AT, Arab L. Garlic and cancer: a critical review of the epidemiologic literature. J Nutr 2001;131(3s):1032S-1040S.
- Fleischauer AT, Poole C, Arab L. Garlic consumption and cancer prevention: meta-analyses of colorectal and stomach cancers. Am J Clin Nutr 2000;72(4):1047-1052.
- Gallicano K, Foster B, Choudhri S. Effect of short-term administration of garlic supplements on single-dose ritonavir pharmacokinetics in healthy volunteers. Br J Clin Pharmacol 2003;Feb, 55(2):199-202.
- Groppo FC, Ramacciato JC, Simoes RP, et al. Antimicrobial activity of garlic, tea tree oil, and chlorhexidine against oral microorganisms. Int Dent J 2002;Dec, 52(6):433-437.
- Hassan HT. Ajoene (natural garlic compound): a new anti-leukaemia agent for AML therapy. Leuk Res 2004;Jul, 28(7):667-671.
- Hsing AW, Chokkalingam AP, Gao YT, et al. Allium vegetables and risk of prostate cancer: a population-based study. J Natl Cancer Inst 2002;Nov 6, 94(21):1648-1651.
- Hughes TM, Varma S, Stone NM. Occupational contact dermatitis from a garlic and herb mixture. Contact Dermatitis 2002;Jul, 47(1):48.
- Li H, Li HQ, Wang Y, et al. An intervention study to prevent gastric cancer by micro-selenium and large dose of allitridum. Chin Med J (Engl) 2004;Aug, 117(8):1155-1160.
- Markowitz JS, Devane CL, Chavin KD, et al. Effects of garlic (Allium sativum L.) supplementation on cytochrome P450 2D6 and 3A4 activity in healthy volunteers. Clin Pharmacol Ther 2003;Aug, 74(2):170-177.
- Rai J. Adverse interactions between low-dose aspirin/warfarin and garlic/ginseng/Gingko biloba. Indian Heart J 2004;Mar-Apr, 56(2):176.
- Silagy CA, Neil HA. A meta-analysis of the effect of garlic on blood pressure. J Hypertension 1994;12(4):463-468.
- Stevinson C, Pittler MH, Ernst E. Garlic for treating hypercholesterolemia: a meta-analysis of randomized clinical trials. Ann Intern Med 2000;133(6):420-429.
- Tanami J, Veeramanomai S, Indrakosas N. The efficacy of cholesterol-lowering action and side effects of garlic enteric coated tablets in man. J Med Assoc Thai 2004;Oct, 87(10):1156-1161.
- Turner B, Molgaard C, Marckmann P. Effect of garlic (Allium sativum) powder tablets on serum lipids, blood pressure and arterial stiffness in normo-lipidaemic volunteers: a randomized double-blind, placebo-controlled trial. Br J Nutr 2004;Oct, 92(4):701-706.
- Warshafsky S, Kamer RS, Sivak SL. Effect of garlic on total serum cholesterol: a meta-analysis. Ann Intern Med 1993;119(7 Pt 1):599-605.
- Zochling J, March L, Lapsley H, et al. Use of complimentary medicines for osteoarthritis: a prospective study. Ann Rheum Dis 2004;May, 63(5):549-554.
Last updated July 12, 2005
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