Garlic has been used for medical purposes since ancient times. For example the Egyptians used garlic to treat wounds, infections and tumours, while Roman gladiators ate garlic before combat. But what is the scientific evidence?

Population studies have been generally positive as regards evidence that garlic may help prevent cancer. For instance the National Cancer Institute in the USA reports on seven population studies, including a population study in Europe (across ten countries), a large study of women in Iowa, three population studies in China and a study in the San Francisco Bay area – which suggested respectively that a higher intake of garlic was associated with a reduced risk of intestinal cancer; a reduced risk of colon cancer in older women; a reduced risk of esophegal and stomach cancers; a reduction in prostate cancer risk; and a reduced risk of pancreatic cancer. In two of the studies a higher intake of onions (from the same plant family as garlic) also appeared to have a beneficial effect.

Clinical trials have been fewer and produced more mixed results. The National Cancer Institute reports that difficulties assessing the findings include the fact that different garlic preparations have often been used, in different strengths and frequencies and sometimes in combination with other nutritional components, making it difficult to compare like with like. In addition allicin is unstable, and changes into a different chemical rather quickly. Having said this a majority of the small number of clinical trials reported appear to have had some positive effect. The NCI concludes, ‘Well- designed 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.’

The NCI observes that garlic is unique because of its high sulfur content – and that in addition to sulfur, garlic also contains arginine, oligosaccharides, flavonoids, and selenium, all of which may be beneficial to health. It hypothesizes that protective effects from garlic may arise from its antibacterial properties or from its ability to block the formation of cancer-causing substances, halt the activation of cancer-causing substances, enhance DNA repair, reduce cell proliferation, or induce cell death.

Types of Garlic in the studies have varied and included raw garlic; garlic essential oil (achieved by steaming); garlic oil macerate (encapsulated mixtures of whole garlic cloves ground into vegetable oil); garlic powder (dried and ground after slicing or crushing); and garlic extract (soaked in an alcohol solution). The medical benefits of garlic are believed to be reduced by cooking and by deoderized preparations which reduce garlic’s allicin producing ability.

As regards the other health benefits claimed for garlic, Medline Plus reports that the Natural Medicines Comprehensive Database rates garlic as being possibly effective for:

  • High blood pressure
  • Hardening of the arteries
  • Tick bites
  • Fungal infections of the skin (via a garlic gel)

but possibly ineffective for diabetes, H.pylori and high cholesterol (the latter seen for instance in a study by Christopher Gardner et al at Stanford University reported in 2007).

The Database reports there is insufficient evidence to rate the effectiveness of garlic for benign prostatic hyperplasia (BHP), the common cold and corns – although it reports that preliminary research in these areas has suggested possible health benefits. For example a 2001 trial involving 146 volunteers who took either a garlic supplement or a placebo for 12 weeks revealed that participants taking the garlic supplement had significantly fewer colds and recovered faster – although the authors concluded that more studies were needed to validate this finding.

A review article published by researchers from Liverpool John Moores University revealed several beneficial effects of garlic on cardiovascular system suggesting garlic may be helpful for treating people who already have high blood pressure. They concluded that more research was needed and concluded, ‘so enjoy garlic as part of your diet but don’t stop taking your blood pressure medication.’

A study conducted at Boston University School of Medicine suggested garlic had an antibiotic effect, killing14 types of bacteria in ear infections among children – reported on the Healthy Food, Healthy Life website. There is also some evidence that fresh (but not aged) garlic, can kill bacteria such as E. coli, antibiotic-resistant Staphylococcus aureus, and Salmonella enteritidis in the laboratory.

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, or 300 to 1,000 mg of garlic extract.

Health Warning.

Any substance which is powerful enough to have a clinical effect could potentially have an adverse effect in certain circumstances and garlic is no exception.

The National Cancer Institute reports that while normal garlic consumption rarely causes problems (other then characteristic garlic breath) higher intakes can have side effects. In particular it can occasionally cause allergies of varying degrees of seriousness; and eating fresh garlic bulbs, extracts or oil on an empty stomach can occasionally cause heartburn, nausea, vomiting and diarrhea.

The NCI also advises that garlic acts as a natural blood thinner , so should be avoided by pregnant women, people about to undergo surgery, and people taking blood thinners, such as warfarin (brand name Coumadin®). Other sources advise against taking garlic with fish oil and herbs that slow blood clotting, such as clove, ginger, gingko and turmeric

Medline Plus advises against taking garlic and medications used for HIV/AIDS.

Medline Plus also advises caution as regards taking garlic with birth control pills; cyclosporine; medications changed by the liver; certain heart medications called calcium channel blockers; cancer drugs; and fungus-fighting drugs.

On the evidence available so far it seems reasonable to conclude that:

  • Well designed dietary studies in humans, comparing like with like as regards the amount and type of garlic preparations, are needed to ascertain how much reliance can be placed on the laboratory, epidemiological and small scale clinical trails conducted so far.
  • The health warnings above should be heeded if you are in one of the categories at potential risk.
  • With these caveats and provided it is not taken in excess, garlic appears to have a number of potential health benefits. 

Published 19/05/2011, Review date August 2014