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Garlic and Cancer Prevention: Questions and Answers 

The information included in this fact sheet is not to be used as the basis for a health claim. 

1. What is garlic?

Garlic is a vegetable (Allium sativum) that belongs to the Allium class of bulb-shaped 

plants, which also includes onions, chives, leeks, and scallions. Garlic is used for 

flavoring in cooking and is unique because of its high sulfur content. In addition to 

sulfur, garlic also contains arginine, oligosaccharides, flavonoids, and selenium, all of 

which may be beneficial to health (1). 

The characteristic odor and flavor of garlic comes from sulfur compounds formed from 

allicin, the major precursor of garlic’s bioactive compounds, which are formed when 

garlic bulbs are chopped, crushed, or damaged (2). Bioactive compounds are defined as 

substances in foods or dietary supplements, other than those needed to meet basic 

nutritional needs, that are responsible for changes in health status. 

2. What are the types of garlic preparations? 

Garlic supplements can be classified into four groups: Garlic essential oil, garlic oil 

macerate, garlic powder, and garlic extract. 

Key Points 

• Preliminary studies suggest that garlic consumption may reduce the risk of 

developing several types of cancer, especially cancers of the 

gastrointestinal tract (see Questions 3 and 4). Most of the studies 

evaluated different types of garlic preparations and used them in varying 

amounts (see Question 5). 

• If garlic consumption does reduce the risk of developing cancer, the 

amount needed to lower risk remains unknown (see Question 7). 

• Although usual garlic consumption rarely causes problems, higher intakes 

can cause side effects, including gastrointestinal distress (see Question 8). 
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• Garlic essential oil is obtained by passing steam through garlic. Commercially 

available garlic oil capsules generally contain vegetable oil, but only have a small 

amount of garlic essential oil because of its strong odor. 

• Garlic oil macerate products are made from encapsulated mixtures of whole garlic 

cloves ground into vegetable oil. 

• Garlic powder is produced by slicing or crushing garlic cloves, then drying and 

grinding them into powder. Garlic powder is used as a flavoring agent for 

condiments and food and is thought to retain the same ingredients as raw garlic.

• Garlic extract is made from whole or sliced garlic cloves that are soaked in an 

alcohol solution (an extracting solution) for varying amounts of time. Powdered 

forms of the extract also are available (3, 4). 

3. Do findings from population studies offer evidence that garlic may prevent cancer? 

Several population studies show an association between increased intake of garlic and 

reduced risk of certain cancers, including cancers of the stomach, colon, esophagus, 

pancreas, and breast. Population studies are multidisciplinary studies of population 

groups that investigate the cause, incidence, or spread of a disease or examine the effect 

of health-related interventions, dietary and nutritional intakes, or environmental 

exposures. An analysis of data from seven population studies showed that the higher the 

amount of raw and cooked garlic consumed, the lower the risk of stomach and colorectal 

cancer (5). 

The European Prospective Investigation into Cancer and Nutrition (EPIC) is an ongoing 

multinational study involving men and women from 10 different countries. This study is 

investigating the effects of nutrition on cancer. In the study, higher intakes of onion and 

garlic were associated with a reduced risk of intestinal cancer (6). 

The Iowa Women’s Study is a large prospective study investigating whether diet, 

distribution of body fat, and other risk factors are related to cancer incidence in older 

women. Findings from the study showed a strong association between garlic 

consumption and colon cancer risk. Women who consumed the highest amounts of garlic 

had a 50 percent lower risk of cancer of the distal colon compared with women who had 

the lowest level of garlic consumption (7).

Several population studies conducted in China centered on garlic consumption and cancer 

risk. In one study, investigators found that frequent consumption of garlic and various 

types of onions and chives was associated with reduced risk of esophageal and stomach 

cancers, with greater risk reductions seen for higher levels of consumption (8). Similarly, 

in another study, the consumption of allium vegetables, especially garlic and onions, was 

linked to a reduced risk of stomach cancer (9). In a third study, greater intake of allium 

vegetables (more than 10 g per day vs. less than 2.2 g per day), particularly garlic and 
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scallions, was associated with an approximately 50 percent reduction in prostate cancer 

risk (10). 

Evidence also suggests that increased garlic consumption may reduce pancreatic cancer 

risk. A study conducted in the San Francisco Bay area found that pancreatic cancer risk 

was 54 percent lower in people who ate larger amounts of garlic compared with those 

who ate lower amounts (11). 

In addition, a study in France found that increased garlic consumption was associated 

with a statistically significant reduction in breast cancer risk. After considering total 

calorie intake and other established risk factors, breast cancer risk was reduced in those 

consuming greater amounts of fiber, garlic, and onions (12). 

4. Do findings from clinical trials offer evidence that garlic may prevent cancer? 

Few clinical trials (research studies with people) have been done to examine the potential 

anticancer effects of garlic. 

Three randomized clinical trials have evaluated the effect of garlic intake on gastric 

cancer risk. In one study, which involved over 5,000 Chinese men and women at high 

risk for stomach cancer, researchers compared the effects of taking a combination of 200 

mg synthetic allitridum (an extract of garlic used as a medicine in China for over 3,000 

years) daily and 100 micrograms selenium every other day with taking a placebo (an 

inactive substance or treatment that looks the same as, and is given the same way as, an 

active drug or treatment being tested) for 5 years. In the group that received allitridum 

and selenium, the risk for all tumors combined was reduced by 33 percent and the risk for 

stomach cancer was reduced by 52 percent in comparison with the group that received 

only the placebo (13). 

In contrast, findings from another randomized trial involving individuals with 

precancerous stomach lesions found that garlic supplementation (800 mg garlic extract 

plus 4 mg steam-distilled garlic oil daily) did not improve the prevalence (number of 

existing cases) of precancerous gastric lesions or reduce the incidence (number of new 

cases) of gastric cancer (14). 

A third randomized study in Japan compared the effects of daily high-dose (2.4 mL) and 

low-dose (0.16 mL) intake of aged-garlic extract after 6 and 12 months of use on 

individuals with colorectal adenomas (noncancerous tumors). At the end of 12 months, 

67 percent of the low-intake group developed new adenomas compared with 47 percent 

in the high-intake group (15). 

The results of a small, nonrandomized study indicate that the application of garlic 

extracts to some skin tumors may be beneficial. In the study, which involved 21 persons 

with basal cell carcinoma, the application of ajoene (a sulfurous chemical found in garlic) 

to the skin for 1 month markedly decreased the size of 17 tumors, increased tumor size in 

3 patients, and resulted in no change in 1 other patient (16). Changes in tumor size 
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ranged from an 88 percent reduction to a 69 percent increase, with an overall median 

reduction of 47 percent. 

5. What are the current issues and controversies surrounding the use of garlic in 

cancer prevention? 

Study limitations, including the accuracy of reporting the amounts and frequency of 

garlic consumed, and the inability to compare data from studies that used different garlic 

products and amounts make an overall conclusion about garlic and cancer prevention 

extremely difficult. Since many of the studies looking at garlic use and cancer prevention 

have used multi-ingredient products, it is unclear whether garlic alone or in combination 

with other nutritional components may have the greatest effect. 

Well-designed dietary studies in humans using predetermined amounts of garlic 

(intervention studies) are needed to determine potentially effective intakes. Studies 

directly comparing various garlic preparations are also needed. 

6. How might garlic act to prevent cancer? 

Protective effects from garlic may arise from its antibacterial properties (17) or from its 

ability to block the formation of cancer-causing substances (18), halt the activation of 

cancer-causing substances (19), enhance DNA repair (20), reduce cell proliferation, or 

induce cell death (10). 

7. How much garlic may be useful for cancer prevention? 

The National Cancer Institute, part of the National Institutes of Health, does not 

recommend any dietary supplement for the prevention of cancer, but recognizes garlic as 

one of several vegetables with potential anticancer properties. Because all garlic 

preparations are not the same, it is difficult to determine the exact amount of garlic that 

may be needed to reduce cancer risk. Furthermore, the active compounds present in 

garlic may lose their effectiveness with time, handling, and processing. The World 

Health Organization’s (WHO) guidelines for general health promotion for adults is a 

daily dose of 2 to 5 g of fresh garlic (approximately one clove), 0.4 to 1.2 g of dried 

garlic powder, 2 to 5 mg of garlic oil, 300 to 1,000 mg of garlic extract, or other 

formulations that are equal to 2 to 5 mg of allicin. 

8. What are the safety considerations? 

Although garlic has been used safely in cooking, excessive consumption can cause some 

side effects, in addition to strong breath and body odors (4, 21). Garlic occasionally 

causes allergies that can range from mild irritation to potentially life-threatening 

problems. Ingestion of fresh garlic bulbs, extracts, or oil on an empty stomach may 

occasionally cause heartburn, nausea, vomiting, and diarrhea. Some animal and human 

studies suggest that garlic can lower blood sugar levels and increase insulin. 
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Garlic has been shown to interfere with several prescription drugs, especially the HIV 

medication saquinavir (brand names Invirase® and Fortovase®). Garlic can lower the 

serum levels of saquinavir by as much as 50 percent (22). Garlic also acts as a natural 

blood thinner and, thus, should be avoided by pregnant women, people about to undergo 

surgery, and people taking blood thinners, such as warfarin (brand name Coumadin®). 

Garlic bulbs are sometimes contaminated with the bacterium Clostridium botulinum. C. 

botulinum can grow and produce botulinum toxin in garlic-in-oil products that are not 

refrigerated and do not contain antibacterial agents. 

In addition, chemical burns, contact dermatitis, and bronchial asthma can occur when 

garlic is applied to the skin. Garlic should also be avoided by people who are prone to 

stomach conditions, such as ulcers, as it can exacerbate the condition or cause new ones 

(4). 

Selected References 

1. Milner JA. Garlic: Its anticarcinogenic and antitumorigenic properties. Nutrition 

Reviews 1996; 54:S82–S86. 

2. Ross SA, Finley JW, Milner JA. Allyl sulfur compounds from garlic modulate aberrant 

crypt formation. Journal of Nutrition 2006; 136(3 Suppl):852S–854S. 

3. Amagase H, Petesch BL, Matsuura H, Kasuga S, Itakura Y. Intake of garlic and its 

bioactive components. Journal of Nutrition 2001; 131(3s):955S–962S. 

4. Amagase H. Clarifying the real bioactive constituents of garlic. Journal of Nutrition

2006; 136(3 Suppl):716S–725S. 

5. Fleischauer AT, Arab L. Garlic and cancer: A critical review of the epidemiologic 

literature. Journal of Nutrition 2001; 131(3s):1032S–1040S. 

6. Gonzalez CA, Pera G, Agudo A, et al. Fruit and vegetable intake and the risk of stomach 

and oesophagus adenocarcinoma in the European Prospective Investigation into Cancer 

and Nutrition (EPIC-EURGAST). International Journal of Cancer 2006; 118(10): 

2559–2566. 

7. Steinmetz KA, Kushi LH, Bostick RM, Folsom AR, Potter JD. Vegetables, fruit, and 

colon cancer in the Iowa Women’s Health Study. American Journal of Epidemiology

1994; 139(1):1–15. 

8. Gao CM, Takezaki T, Ding JH, Li MS, Tajima K. Protective effect of allium vegetables 

against both esophageal and stomach cancer: A simultaneous case-referent study of a 

high-epidemic area in Jiangsu Province, China. Japanese Journal of Cancer Research

1999; 90(6):614–621. 
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9. Setiawan VW, Yu GP, Lu QY, et al. Allium vegetables and stomach cancer risk in 

China. Asian Pacific Journal of Cancer Prevention 2005; 6(3):387–395. 

10. Hsing AW, Chokkalingam AP, Gao YT, et al. Allium vegetables and risk of prostate 

cancer: A population-based study. Journal of the National Cancer Institute 2002; 

94(21):1648–1651. 

11. Chan JM, Wang F, Holly EA. Vegetable and fruit intake and pancreatic cancer in a 

population-based case-control study in the San Francisco bay area. Cancer Epidemiology 

Biomarkers & Prevention 2005; 14(9):2093–2097. 

12. Challier B, Perarnau JM, Viel JF. Garlic, onion and cereal fibre as protective factors for 

breast cancer: A French case-control study. European Journal of Epidemiology 1998; 

14(8):737–747. 

13. Li H, Li HQ, Wang Y, et al. An intervention study to prevent gastric cancer by microselenium and large dose of allitridum. Chinese Medical Journal (English) 2004; 

117(8):1155–1160. 

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. Journal of the 

National Cancer Institute 2006; 98(14):974–983. 

15. Tanaka S, Haruma K, Kunihiro M, et al. Effects of aged garlic extract (AGE) on 

colorectal adenomas: A double-blinded study. Hiroshima Journal of Medical Sciences

2004; 53(3–4):39–45. 

16. Tilli CM, Stavast-Kooy AJ, Vuerstaek JD, et al. The garlic-derived organosulfur 

component ajoene decreases basal cell carcinoma tumor size by inducing apoptosis. 

Archives of Dermatological Research 2003; 295(3):117–123. 

17. Ruddock PS, Liao M, Foster BC, et al. Garlic natural health products exhibit variable 

constituent levels and antimicrobial activity against Neisseria gonorrhoeae, 

Staphylococcus aureus and Enterococcus faecalis. Phytotherapy Research 2005; 

19(4):327–334. 

18. Shenoy NR, Choughuley AS. Inhibitory effect of diet related sulphydryl compounds on 

the formation of carcinogenic nitrosamines. Cancer Letters 1992; 65(3):227–232. 

19. Milner JA. Mechanisms by which garlic and allyl sulfur compounds suppress carcinogen 

bioactivation. Garlic and carcinogenesis. Advances in Experimental Medicine and 

Biology 2001; 492:69–81. 

20. L'vova GN, Zasukhina GD. Modification of repair DNA synthesis in mutagen-treated 

human fibroblasts during adaptive response and the antimutagenic effect of garlic extract. 

Genetika 2002; 38(3):306–309. 
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21. Boon H, Wong J. Botanical medicine and cancer: A review of the safety and efficacy. 

Expert Opinion on Pharmacotherapy 2004; 5(12):2485–2501. 

22. Piscitelli SC, Burstein AH, Welden N, Gallicano KD, Falloon J. The effect of garlic 

supplements on the pharmacokinetics of saquinavir. Clinical Infectious Diseases 2002; 

34(2):234–238. 

# # # 

Related NCI materials and Web pages: 

• National Cancer Institute Fact Sheet 6.7, Cancer: Questions and Answers

(http://www.cancer.gov/cancertopics/factsheet/Sites-Types/general) 

• NCI’s Complementary and Alternative Medicine Home Page 

(http://www.cancer.gov/cancertopics/treatment/cam) 

• What You Need To Know AboutTM Cancer

(http://www.cancer.gov/cancertopics/wyntk/overview)

For more help, contact: 

NCI’s Cancer Information Service 

Telephone (toll-free): 1–800–4–CANCER (1–800–422–6237) 

TTY (toll-free): 1–800–332–8615

LiveHelp® online chat: https://cissecure.nci.nih.gov/livehelp/welcome.asp

This fact sheet was reviewed on 1/22/08