Written by Edzard Ernst and the CAM-Cancer Consortium.
Updated August 14, 2013

Garlic (Allium sativum)

Does it work?

This summary is currently (April 2016) being revised. The version published here was last updated in August 2013.

The literature on garlic (Allium sativum) as a means of reducing cancer risk is huge. Therefore, the following section is mainly based on authoritative reviews.

Reviews

Most of the evidence for garlic as a cancer preventative in humans rests, of course, on epidemiological data which unfortunately is contradictory. For instance, some investigations report that regular garlic consumption marginally lowers the risk of prostate cancer 31, gastric cancer 32,33, colorectal cancer in women 34, haematologic cancers 35 and multiple melanoma 36, while other studies find no such effect on colorectal cancer 37 prostate cancer 38,39, cervical cancer 40 and bladder cancer 41. There might be numerous explanations for these contradictory findings, e.g. the nature, quantity and quality of the garlic intake 27.

A 2009 review related to all types of cancer 42. The authors found no protective effect for gastric, breast, lung or endometrial cancer. Limited evidence was found for an inverse relationship between garlic consumption and the risk of colon, prostate, oesophageal, larynx, oral, ovary or renal cancers.

A meta-analysis published in 2000 of all cohort and case control studies suggested that the regular intake of garlic reduces the cancer risk: the relative risk for colon cancer was 0.69 (95% CI = 0.55-0.89) for stomach cancer it was 0.53 (95% CI – 0.31-0.92) 43. Because of heterogeneity between data sets, the authors' conclusion was cautious: "high intake of garlic may be associated with a protective effect against stomach and colorectal cancer" 43.

The most recent review and meta-analysis was focussed purely on gastric cancer 44. The authors analyzed 19 case-control and 2 cohort studies with a total of 543,220 subjects. They pooled the relative risks from individual studies using a random-effects model and performed dose-response, heterogeneity, and publication bias analyses. In the pooled analysis of all studies, consumption of large amounts of Allium vegetables (in a comparison of the highest and lowest consumption groups) reduced the risk for gastric cancer (odds ratio, 0.54; 95% confidence interval, 0.43-0.65). Specific analyses for onion, garlic, leek, Chinese chive, scallion, garlic stalk, and Welsh onion yielded similar results, except for onion leaf. The estimated summary odds ratio for an increment of 20 g/day of Allium vegetables consumed (approximately the average weight of 1 garlic bulb) was 0.91 (95% confidence interval, 0.88-0.94), based on case-control studies from the dose-response meta-analysis. The authors concluded that the “consumption of high levels of Allium vegetables reduced the risk for gastric cancer risk. Because of potential confounding factors and exposure misclassification, further studies are required to establish this association.”

Clinical trials

Relatively few clinical trials of garlic to prevent cancer are available 45,46,60-62. Their key data are summarized in table 1. Collectively these studies fail to show conclusively that garlic-intake reduces the cancer risk.

Citation Edzard Ernst, CAM-Cancer Consortium. Garlic (Allium sativum) [online document]. http://ws.cam-cancer.org/The-Summaries/Herbal-products/Garlic-Allium-sativum. August 14, 2013.

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