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

Garlic (Allium sativum)

Abstract and key points

  • Garlic (Allium sativum) is a food item and a spice which has also been used for medicinal purposes.
  • Garlic is said to reduce the risk of certain cancers.
  • The evidence for this assumption is contradictory and not strong.
  • Serious adverse-effects are not on record but interactions with cancer drugs might be an issue.

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

Garlic (Allium sativum) is sometimes recommended for cancer-prevention. The epidemiological evidence is contradictory but some studies seem to indicate that, for some types of cancer, regular garlic intake might reduce the risk. Only very few clinical trials exist and their results are inconclusive. Serious adverse events are not to be expected but herb-drug interactions might occur.

Read about the regulation, supervision and reimbursement of herbal medicine at NAFKAMs website CAM Regulation.

What is it?

Description

Garlic is a bulbous plant of the Allium (onion) family commonly used for culinary purposes. For medicinal purposes fresh or dried parts of the bulb or the oil from the bulb are usually used.

Scientific / common name

Allium sativum L (Alliaceae/Liliaceae) is commonly referred to as garlic. Ajo is also sometimes used.

Ingredients

Alliin, diallyldisulfate, ajoen and others 1. Allicin, considered to be one of the main active ingredients, and other sulfur-containing compounds are formed from alliin enzymatically when garlic is crushed or chopped 1,2. It is considered that 1 mg alliin is equivalent to 0.45 mg allicin 2,3. Commercial garlic preparations are often standardised on the content of sulfur-containing constituents, particularly to alliin, or on the allicin yield 2.

Application and dosage

It is usually taken orally in the following dosages.
Dried bulb: 2- 4g three times daily for upper respiratory tract infections 4-6; 0.5-1.0g daily for the prophylaxis of atherosclerosis 7,8.
Fresh garlic: 4g daily 1-3.
Oil: 0.03-0.12 ml three times daily 2,9.
Juice of garlic (BPC 1949) 2-4 ml 2,10.
Extract: 600-900mg of standardised extract (1.3% alliin content) daily in divided doses 1.

History/providers

Garlic has been used medicinally in several ancient cultures. Prescribers today are mainly herbalists, naturopaths and doctors. Many consumers self-prescribe garlic supplements. Garlic is a common food and spice. It is offered as “over the counter “ (OTC) preparations (food supplements) in form of single-ingredient or multi-ingredients preparations by many providers (see list in 11).

Claims of efficacy

The Monograph of ESCOP (2003) recommends Allii sativi bulbus for the prophylaxis of atherosclerosis, for the treatment of elevated blood lipid levels insufficiently influenced by diet, for the improvement of the circulation in peripheral arterial vascular disease and for upper respiratory tract infections and catarrhal conditions 7,8.

The WHO-Monograph states uses supported by clinical data as adjuvant to the dietetic management in the treatment of hyperlipidaemia, and in the prevention of atherosclerosis, age dependent vascular changes and mild hypertension 4,8.

Claims of efficacy in the prevention of cancer are mainly based on in vitro and animal studies 2.

Mechanism of action

Garlic's mechanisms of action have been studied extensively. It has antibacterial, antiviral, antifungal, antihypertensive, blood glucose lowering, antithrombotic, antimutagentic and antiplatelet activities 1.

Several pathways have been identified for each of which the evidence is substantial:

  • Modulation of carcinogen metabolism 12
  • Inhibition of cell cycle progression 13
  • Induction of apoptosis 14-19
  • Histone modification 12
  • Inhibition of angiogenesis 12
  • Protection against DNA-damage 20
  • Inhibition of cell proliferation 21-24
  • Modulation of gene expression 21,25
  • Inhibition of tumor cell motility 26
  • Antioxidation 27
  • Modification of drug metabolising enzymes especially the family of the hepatic cytochrome P450 (CYP) enzymes2
  • Immunomodulatory activity 2,28.

Many of the findings from animal studies support these postulated mechanisms 17,22-24,27-30.

Alleged indications

It is claimed that garlic might reduce the cancer risk.

Prevalence of use

Garlic is widely used as a food and spice. Garlic supplements have become very popular in developed countries. Exact prevalence figures are not available.

Legal issues

Garlic preparations are sold in most countries as food supplements.

Cost and expenditure

A one week supply of garlic supplements would cost between €5 and €10.

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.

Is it safe?

Adverse effects

Garlic (Allium sativum) is generally considered to be non-toxic 1,2,47,48. Adverse effects that have been documented in humans include a burning sensation in the mouth and gastrointestinal tract, nausea, diarrhea, vomiting 2,47 and body odour. The allergenic potential of garlic is well recognized, and the allergens have been identified as diallyl disulfide, allylpropyl sulfide and allicin 2,5. Preclinical data on chronic toxicity are conflicting 2,49. High doses are stated to cause anaemia due to both decreased haemoglobin synthesis and haemolysis 2,50. Genotoxicity studies using the micronucleus test have reported both positive and negative findings 2. No evidence of mutagenicity has been reported when assessed using the Ames and Ree assay 2.

Contraindications

Allergy to Allium species, peptic ulcer, there may be an increased risk of bleeding with the use of garlic supplements in patients undergoing surgery 2.

Interactions

Garlic intake might increase the effects of anticoagulants. A potential interaction between garlic and warfarin has been documented 2,51.Garlic extracts are unlikely to alter the metabolism of drugs primarily dependent on the CYP2D6 or CYP3A4 pathway 52,53. However, in patients carrying a certain CYP3A5 allele, garlic could affect the clearance of docetaxel, leading to higher toxicity 52. Findings from in-vitro studies further suggest, that garlic suppresses the expression and activity of the CYP P450 subtype 2C9 (pharmacokinetics possibly affected: e.g. cyclophosphamid, diclofenac, haloperidol, ibuprofen, naproxen, paclitaxel, piroxi¬cam, and tamoxifen) 54. Slight changes were also found in CYP1A2 levels (increase) and CYP2E1 activity (decrease) (pharmacokinetics possibly affected: e.g. ondansetron, etoposide) 55.

An increase has been noted in the expression of duodenal P-glycoprotein after the ingestion of garlic extracts 56. This interaction is thought to be the most probable mechanism for the known impact that garlic supplements exert on the first-pass metabolism of HIV protease inhibitors 57-59.

Finally, alterations in the activity of the phase II biotransformation enzymes UDP glucuronosyltransferase and glutathion-S-transferase have been observed after the ingestion of garlic extracts 55.

Quality issues

Garlic preparations should adhere to GAP, GMP and in Europe to the European guidelines “Quality of Herbal Medicinal products”.

Warning

Insufficient information exists for garlic use beyond amounts consumed as part of the daily diet during pregnancy and lactation. Garlic is reputed to act as an abortifacient and to affect the menstrual cycle, and is also reported to be utero-active 2. There are only few clinical studies during pregnancy and lactation. However, there are not experimental or clinical reports on adverse effects during pregnancy or lactation 2. In view of this, doses of garlic greatly exceeding amounts used in foods should not be taken during pregnancy and lactation 2.

Other problems

Overdose may cause nausea and vomiting.

Citation

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

Document history

Fully revised and updated in August 2013 by Edzard Ernst.

Summary first published in September 2011, authored by Edzard Ernst.

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