ARTICLE SECTIONS

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Background

Numerous controlled trials have examined the effects of oral garlic on serum lipids. Long-term effects on lipids or cardiovascular morbidity and mortality remain unknown. Other preparations (such as enteric-coated or raw garlic) have not been well studied.

Small reductions in blood pressure (<10 millimeters of mercury), inhibition of platelet aggregation, and enhancement of fibrinolytic activity have been reported, and may exert effects on cardiovascular outcomes, although evidence is preliminary in these areas.

Numerous case-control/population-based studies suggest that regular consumption of garlic (particularly unprocessed garlic) may reduce the risk of developing several types of cancer, including gastric and colorectal malignancies. However, prospective controlled trials are lacking.

Multiple cases of bleeding have been associated with garlic use, and caution is warranted in patients at risk of bleeding or prior to some surgical/dental procedures. Garlic does not appear to significantly affect blood glucose levels.

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Synonyms

2-propenesulfenic acid, aged garlic extract, aglio, ail, ail commun, ajo, ajoene, akashneem, alisat, alk(en)yl thiosulfates, allicin, Allicor?, Allii sativi bulbus, alliinase, allium, allitridium, allyl mercaptan, alubosa elewe, Amaryllidaceae (family), ayo-ishi, ayu, banlasun, camphor of the poor, clove garlic, da-suan, dai toan, dasuan, dawang, diallyl, diallyl disulphide, diallyl sulfide, diallyl sulphide, diethyl disulfide, diethyl hexasulfide, diethyl monosulfide, diethyl pentasulfide, diethyl tetrasulfide, diethyl trisulfide, dipropyl disulphide, dipropyl sulphide, dra thiam, (E)-ajoene, foom, garlic clove, garlic corns, garlic extract, garlic oil, garlic paste, garlic powder extract, Gartenlauch, hom khaao, hom kia, hom thiam, hua thiam, Karinat?, kesumphin, kitunguu-sumu, knoblauch, kra thiam, Krathiam, krathiam cheen, krathiam khaao, Kwai?, Kyolic?, l'ail, lahsun, lai, la-juan, lasan, lashun, la-suan, lasun, lasuna, lauch, lay, layi, lehsun, lesun, Liliaceae (family), lobha, majo, methyl allyl, naharu, nectar of the gods, Ninniku, pa-se-waa, poor man's treacle, rason, rasonam, rasun, rust treacle, rustic treacles, S-alk(en)yl cysteine sulfoxide, S-allylcysteine (SAC), seer, skordo, sluon, stinking rose, sudulunu, tafanuwa, ta-suam, ta-suan, tellagada, Tellagaddalu, thiam, thioallyl derivative, thiosulfinates, toi thum, tum, umbi bawang putih, vallaippundu, Velluli, vellulli, verum, vinyl dithiin, vinyldithiin, (Z)-ajoene.

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Evidence

Uses based on scientific evidence	Grade*
High cholesterol Multiple studies in humans have reported small reductions in total blood cholesterol and low-density lipoproteins ("bad cholesterol") over short periods of time (4 to 12 weeks). It is not clear if there are benefits after this amount of time. Effects on high-density lipoproteins ("good cholesterol") are not clear. This remains an area of controversy. Well-designed and longer studies are needed in this area.	B
Alopecia areata (hair loss) Application of garlic gel on the skin may be beneficial in the treatment of alopecia areata (hair loss). Additional study is needed.	C
Anti-fungal (applied to the skin) Several studies describe the application of garlic to the skin to treat fungal infections, including yeast infections. Take caution as garlic can cause severe burns and rash when applied to the skin of sensitive individuals.	C
Anti-platelet effects (blood thinning) The effects of garlic on platelet aggregation have been assessed in several human trials. 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).	C
Atherosclerosis ("hardening" of the arteries) Preliminary research in humans suggests that deposits of cholesterol in blood vessels may not grow as quickly in people who take garlic. It is not clear if this is due to the ability of garlic to lower cholesterol levels or to other effects of garlic.	C
Benign breast diseases Taking garlic supplements by mouth may improve some symptoms of benign breast disease. Additional study is needed.	C
Cancer Preliminary human studies suggest that regular consumption of garlic (particularly unprocessed garlic) may reduce the risk of developing several types of cancer including gastric and colorectal malignancies. Some studies use multi-ingredient products so it is difficult to determine if garlic alone may play a beneficial role. Further well-designed human clinical trials are needed to conclude whether eating garlic or taking garlic supplements may prevent or treat cancer.	C
Cryptococcal meningitis Preliminary study documented potential benefits of oral plus intravenous garlic in the management of cryptococcal meningitis. Further research is needed before recommending for or against the use of garlic in the treatment of this potentially serious condition, for which other treatments are available.	C
Familial hypercholesterolemia Familial hypercholesterolemia is a genetic disorder in which very high cholesterol levels run in families. Research in children with an inherited form of high cholesterol suggests that garlic does not have a large effect on lowering cholesterol levels in these patients.	C
Heart attack prevention in patients with known heart disease It is not clear if garlic prevents future heart attacks in people who have already had a heart attack. The effects of garlic on cholesterol levels may be beneficial in such patients.	C
High blood pressure Numerous human studies report that garlic can lower blood pressure by a small amount, but larger, well-designed studies are needed to confirm this possible effect.	C
Infections (bacterial, viral, fungal, other) In initial studies, 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. More high-quality human studies are necessary to recommend garlic to treat or prevent infections.	C
Oral candidiasis There is not enough evidence to suggest garlic is effective in the treatment of oral candidiasis.	C
Mosquito repellant There is currently not enough evidence to suggest that garlic helps repel mosquitoes.	C
Peripheral vascular disease (blocked arteries in the legs) Some human studies suggest that garlic may improve circulation in the legs by a small amount, but this issue remains unclear. Better-designed studies are needed.	C
Pre-eclampsia (prevention) There is not enough evidence to recommend increased garlic intake for preventing pre-eclampsia and its complications.	C
Sickle cell anemia Initial evidence suggests the antioxidant activity of garlic may benefit sickle cell anemia. Further study is necessary.	C
Systemic sclerosis There is insufficient evidence to recommend the use of garlic in systemic sclerosis.	C
Tick repellant In early study, self-reports of tick bites were significantly less in people receiving garlic over a placebo "sugar" pill. Further well-designed study is needed to confirm these results.	C
Upper respiratory tract infection Preliminary reports suggest that garlic may reduce the severity of upper respiratory tract infections. However, this has not been demonstrated in well-designed human studies.	C
Diabetes Animal studies suggest that garlic may lower blood sugar and increase the release of insulin, but studies in humans do not confirm this effect.	D
Stomach ulcers caused by Helicobacter pylori bacteria Early studies in humans show no effect of garlic on gastric or duodenal ulcers.	D

Key to grades
A Strong scientific evidence for this use
B Good scientific evidence for this use
C Unclear scientific evidence for this use
D Fair scientific evidence against this use (it may not work)
F Strong scientific evidence against this use (it likely does not work)

Grading rationale

Uses based on tradition or theory

Abortion, age-related memory problems, AIDS, allergies, antioxidant, antitoxin, anxiety, aphrodisiac, arthritis, asthma, bile secretion problems, bladder cancer, bloody urine, bone diseases, bronchitis, cholera, colds, cough, dental pain, digestive aid, diphtheria, diuretic (water pill), doxorubicin cardiotoxicity, dysentery, dysmenorrhea (painful menstruation), earache, emetic (induction of vomiting), emmenagogue (induction of menstruation), fatigue, fever, gallstones, headache, heart rhythm disorders, hemorrhoids, hepatopulmonary syndrome, HIV, hormonal effects, immune system stimulation, inflammation, inflammatory bowel disease, influenza, kidney problems, kidney damage from antibiotics, leukemia, liver health, liver tumors, malaria, muscle spasms, nephrotic syndrome, obesity, parasites and worms, perspiration, pneumonia, premenstrual syndrome (PMS), psoriasis, Raynaud's disease, sedative, sinus decongestant, snake venom protection, spermicide, stomach ache, stomach acid reduction, stress (anxiety), stroke, toothache, tuberculosis, urinary tract infections, typhus, urinary tract infections, vaginal irritation, vaginal trichomoniasis, warts, well-being, whooping cough.

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Dosing

Adults (18 years and older)

Human studies report the use of 4-12.3 milligrams of garlic oil by mouth daily. 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.

600 to 900 milligrams daily of non-coated, dehydrated garlic powder in three divided doses, standardized to 1.3% allicin content, has been used in human studies. The European Scientific Cooperative on Phytotherapy (ESCOP) recommends 3 to 5 milligrams allicin daily (1 clove or 0.5 to 1.0 gram dried powder) for the prevention of atherosclerosis. The World Health Organization (WHO) recommends 2 to 5 grams fresh garlic, 0.4 to 1.2 grams of dried powder, 2 to 5 milligrams oil, 300 to 1,000 milligrams of extract, or other formulations that are equal to 2 to 5 milligrams of allicin daily.

The European Scientific Cooperative on Phytotherapy (ESCOP) recommends 2 to 4 grams of dried bulb or 2 to 4 milliliters of tincture (1:5 dilution in 45% ethanol), by mouth three times a day for upper respiratory tract infections.

Children (younger than 18 years)

Safety or effectiveness of garlic supplements has not been proven in children. Garlic in amounts found in food is likely safe in most children.

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Safety

Allergies

People with a known allergy to garlic, any of its ingredients, or to other members of the Liliaceae (lily) family, including hyacinth, tulip, onion, leek, and chives, should avoid garlic. Allergic reactions have been reported with garlic taken by mouth, inhaled, or applied to the skin. Some of these reactions are severe including throat swelling and difficulty breathing (anaphylaxis). It has been suggested that some cases of asthma from inhaling garlic may be due to mites on the garlic. Fresh garlic applied to the skin may be more likely to cause rashes than garlic extract.

Side Effects and Warnings

Bad breath, body odor, and allergic reactions are the most common reported side effects of garlic. Fresh garlic has caused rash or skin burns, both in people taking garlic therapy and in food preparers handling garlic. Most reactions improve after stopping garlic therapy. Other reported side effects include dizziness, increased sweating, headache, itching, fever, chills, asthma flares, and runny nose.

Bleeding is a potentially serious side effect of garlic use, including bleeding after surgery and spontaneous bleeding. Several cases of bleeding are reported, which may be due to effects of garlic on blood platelets, or to increased breakdown of blood clots (fibrinolysis). There is debate about the effects of garlic in people treated with warfarin (Coumadin?), but studies suggest that garlic does not alter the International Normalized Ratio (INR) values that are used to measure the effect of warfarin on blood thinning. Garlic should be stopped prior to some surgical or dental procedures due to an increased risk of bleeding. Caution is urged for people who have bleeding disorders or who take blood thinning medications (anticoagulants, aspirin/anti-platelet agents, non-steroidal anti-inflammatory drugs such as ibuprofen or naproxen) or herbs/supplements that may increase the risk of bleeding. Dosing adjustments may be necessary.

Garlic or its ingredients may lower blood sugar levels and increase the release of insulin. However, studies in humans do not show changes in blood sugar control in people with or without diabetes. Nonetheless, caution is advised in people with diabetes or hypoglycemia, and in those taking drugs, herbs, or supplements that affect blood sugar. Blood sugar levels may need to be monitored by a healthcare professional, and medication adjustments may be necessary. Informal reports describe low iodine absorption in the thyroid and low levels of thyroid hormone (hypothyroidism) with garlic supplementation. A few reports suggest that garlic and garlic-like plants may be linked to nodules or tumors of the thyroid. Reduced sperm counts have been reported in rats.

Dehydrated garlic preparations or raw garlic taken by mouth may cause burning of the mouth, abdominal pain or fullness, poor appetite, gas, belching, nausea, vomiting, irritation of the stomach lining, changes in the bacteria in the gut, heartburn, diarrhea, or constipation. One report describes bowel obstruction in a man who ate a whole garlic bulb. Garlic should be used cautiously by people with stomach ulcers or who are prone to stomach irritation.

Multiple studies show a small reduction in blood cholesterol levels after garlic supplements are taken by mouth. Small reductions in blood pressure are also commonly reported. One case of heart attack is noted in a healthy man after taking a large amount of garlic by mouth.

Contamination of garlic products has been reported. In Vancouver, British Columbia, a commercial preparation of chopped garlic was linked to botulism. One report describes overdose of colchicine and even death after meadow saffron ( Colchicum autumnale ) was mistaken for wild garlic ( Allium ursinium ).

Garlic and pycnogenol have been shown to increase human growth hormone secretion in laboratory experiments.

Pregnancy and Breastfeeding

Garlic is likely safe during pregnancy in amounts usually eaten in food, based on historical use. However, garlic supplements or large amounts of garlic should be avoided during pregnancy due to a possible increased risk of bleeding. In addition, early animal studies suggest that garlic may cause contraction of the uterus. Many tinctures contain high levels of alcohol, and should be avoided during pregnancy.

Garlic is likely safe during breastfeeding in amounts usually eaten in food, based on historical use. However, some mothers who take garlic supplements report increased nursing time, milk odor, and reduced feeding by the infant. The safety of garlic supplements during breastfeeding is not known.

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Methodology

This patient information is based on a professional level monograph edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).

Monograph methodology

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Selected references

1. Ashraf R, Aamir K, Shaikh AR, et al. Effects of garlic on dyslipidemia in patients with type 2 diabetes mellitus. J Ayub Med Coll Abbottabad 2005 Jul-Sep;17(3):60-4.
2. Fleischauer AT, Arab L. Garlic and cancer: a critical review of the epidemiologic literature. J Nutr 2001;131(3s):1032S-1040S.
3. Gail MH, You WC. A factorial trial including garlic supplements assesses effect in reducing precancerous gastric lesions. J Nutr 2006 Mar;136(3 Suppl):813S-815S.
4. Gardner CD, Lawson LD, Block E, 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):325-6.
5. Hassan HT. Ajoene (natural garlic compound): a new anti-leukaemia agent for AML therapy. Leuk Res 2004;28(7):667-671.
6. Hajheydari Z, Jamshidi M, Akbari J, et al. Combination of topical garlic gel and betamethasone valerate cream in the treatment of localized alopecia areata: a double-blind randomized controlled study. Indian J Dermatol Venereol Leprol 2007 Jan-Feb;73(1):29-32.
7. Josling P. Preventing the common cold with a garlic supplement: a double-blind, placebo-controlled survey. Adv Ther 2001;18(4):189-193.
8. McNulty CA, Wilson MP, Havinga W, et al. A pilot study to determine the effectiveness of garlic oil capsules in the treatment of dyspeptic patients with Helicobacter pylori. Helicobacter 2001;6(3):249-253.
9. Meher S, Duley L. Garlic for preventing pre-eclampsia and its complications. Cochrane Database Syst Rev 2006 Jul 19;3:CD006065.
10. Sabitha P, Adhikari PM, Shenoy SM, et al. Efficacy of garlic paste in oral candidiasis. Trop Doct 2005;35(2):99-100.
11. Siegel G. Long-term effect of garlic in preventing arteriosclerosis - results of two controlled clinical trials. Eur Phytojournal 2001;Symposium posters(1):1.
12. Takasu J, Uykimpang R, Sunga MA, et al. Aged garlic extract is a potential therapy for sickle-cell anemia. J Nutr 2006 Mar;136(3 Suppl):803S-805S.
13. Tanaka S, Haruma K, Yoshihara M, et al. Aged garlic extract has potential suppressive effect on colorectal adenomas in humans. J Nutr 2006 Mar;136(3 Suppl):821S-826S.
14. You WC, Brown LM, Zhang L, et al. Randomized double-blind factorial trial of three treatments to reduce the prevalence of precancerous gastric lesions. J Natl Cancer Inst 2006 Jul 19;98(14):974-83.
15. Wojcikowski K, Myers S, Brooks L. Effects of garlic oil on platelet aggregation: a double-blind placebo-controlled crossover study. Platelets 2007 Feb;18(1):29-34.

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