Drug Profile

Background

“Garlic is as good as ten mothers,” or at least so claims a documentary extolling the virtues of the plant.[1] From its role as a currency in ancient Egypt to warding off vampires in the Balkans, the plant has always been steeped in myth and mystery. Garlic, also known by its plant name Allium sativum, has been traced by historians to nomadic Siberian tribes 5,000 years ago who spread it to other areas.[2] But is it any good for your health?

Alliin and methiin are two chemicals in garlic of particular interest. When you crush or chop garlic, these chemicals interact with enzymes from which they are usually separated in the plant to ultimately form a class of compounds called thiosulfinates, which give garlic its characteristic flavor. (Culinary tip: the more you chop your garlic, the more the enzymes come into contact with alliin and methiin, and the more flavored your dinner.) Further exposure to oil and high temperatures produce polysulfides, which have the characteristic garlic odor. The thiosulfinates have been linked in laboratory studies to cholesterol lowering, while the polysulfides have been connected to cancer prevention.[2]

Over the millennia, garlic has been recommended as an antibacterial agent, a diuretic, and a laxative as well as for arthritis, asthma, athlete’s foot, baldness, cancer, cardiovascular disease, dog and snakebites, freckles, hemorrhoids, lice, plague, thinning of the blood, toothache, tuberculosis, and wound healing. It is only in the last couple of decades that researchers have actually subjected garlic to randomized, placebo-controlled trials. This review is restricted to such trials, because they are the only reliable method for establishing the efficacy of garlic in these settings.

Claimed Uses

Cholesterol Lowering

A major review of 37 studies of the effectiveness of garlic in lowering cholesterol levels in the blood was undertaken for the Agency for Healthcare Research and Quality and published in 2000.[2] Although shorter studies (three months) typically showed small reductions in cholesterol (smaller than is often obtained with prescription cholesterol-lowering drugs), combining all eight placebo-controlled studies with at least six months of data showed no effect of garlic in total cholesterol reduction.

Since that review, four randomized, placebo-controlled trials have been conducted. Two of these showed no cholesterol-lowering effect for garlic.[3],[4] In a third study, there was an apparent beneficial effect of garlic on total cholesterol in patients at high risk for preeclampsia (toxemia of pregnancy).[5] However, this study was not properly blinded (either the doctors or the patients knew who was getting garlic and who placebo) and the apparent difference was due to baseline differences between the two study groups, a difference that should not have existed if randomization had occurred properly. In the fourth study, beneficial effects upon many cholesterol-related blood measurements were reported after 12 weeks.[6] However, during the study the garlic-treated group reduced their intake of carbohydrates, fat, and alcohol more than did the placebo group, complicating study interpretation.

Studies of garlic and other strong-tasting substances are often prone to this problem, called unblinding by researchers. The problem is that both the garlic tablets and the patients consuming them have a distinctive smell and so patients can often tell (or be told by their partners) that they are in the treated group. This can lead to different dietary patterns between the two study groups, undermining the randomization. A second problem, not unique to studies of garlic, is that there are multiple formulations on the market. Because these are not standardized, it is hard to know if the results from one formulation can be applied to another. The Agency for Healthcare Research and Quality review accurately summarized other limitations in the data when it stated: “Interpreting the [cholesterol] results is best tempered by recognizing that trials often had unclear randomization processes, short durations and no [adjustment for patients who left the study].”[2]

Cancer Prevention

Claims for the effectiveness of garlic in preventing various cancers rest primarily on case-control studies: those that assess garlic intake (either in the diet or as a supplement) in people with cancer compared to those without cancer. However, such studies cannot, on their own, prove that any observed relationship between diet and cancer is causal. Patients with cancer may have diets that are different from patients without cancer in many different ways, not just with respect to garlic; they may even start taking garlic once the early symptoms of the cancer appear, producing a spurious result. Cancer patients are likely to recall their diets differently from patients without cancer. Indeed, all patients struggle to describe dietary patterns that stretch back decades.

Thus, garlic could only be recommended as a cancer preventive if it reduced the number of cases of cancer in a randomized controlled trial. In fact, for almost a decade, there has been such a study under way in China in which patients with precancerous lesions in their stomachs are randomized to various combinations of vitamins, micronutrients, and garlic or a placebo. The trial’s clinical results have not yet been published.[7]

In October 2000, the Agency for Healthcare Research and Quality published a review on the subject of garlic and cancer. Here are the authors’ conclusions:

Scant data, primarily from case-control studies, suggest, but do not prove, dietary garlic consumption is associated with decreased odds of laryngeal, gastric, colorectal, and endometrial cancer and adenomatous colorectal polyps. Single case-control studies suggest, but do not prove, dietary garlic consumption is not associated with breast or prostate cancer. No epidemiological study has assessed whether using particular types of garlic supplements is associated with reductions in cancer incidence. Preliminary 3-year evidence from a large cohort study suggests consumption of “any” garlic supplement does not reduce risk of breast, lung, colon, or gastric cancer.[2]

In sum, there is inadequate evidence to support garlic supplementation to reduce the risk of any kind of cancer.

High Blood Pressure

The Agency for Healthcare Research and Quality review[2] found that an effect of garlic in lowering blood pressure was usually absent, small even when present, and that no firm conclusions could be drawn due to multiple study design defects. Two subsequent randomized, controlled trials on garlic for high blood pressure showed no effect in reducing blood pressure.[3],[4],[5]

Diabetes

The Agency for Healthcare Research and Quality review[2] concluded that garlic had “no clinically significant effect” on blood sugar in patients with or without diabetes. We were unable to identify any randomized, controlled trials on garlic for hypertension published since that review.

Interactions with Other Drugs

The U.S. Food and Drug Administration has warned that garlic can increase the effectiveness of warfarin (COUMADIN), which could lead to bleeding.[8] While the warfarin label must warn of this interaction with garlic, garlic may be sold without warning of the interaction with warfarin. The 2003 edition of Evaluations of Drug Interactions lists saquinavir (FORTOVASE, INVIRASE) as having a “clinically significant” interaction with garlic.

Adverse Effects

Garlic may lead to bleeding abnormalities because it inhibits platelets from aggregating and hence clots from forming. There is a case of a man who developed bleeding around his spinal cord due to garlic ingestion[9] and another of bleeding after surgery.[10] Garlic can cause halitosis (bad breath) and body odor. It has been associated with intestinal gas, pain in the esophagus and abdomen, and obstruction of the small intestine, although these associations are not proven.[2]

Conclusion

There are not adequate data to support the use of garlic for cholesterol lowering, cancer prevention, hypertension, or diabetes. As an article in a leading medical journal said, “Garlic for flavour, not cardioprotection [protecting the heart].”[11]