Garlic has been used as both food and medicine in many cultures for thousands of years, dating as far back as the time that the Egyptian pyramids were built. Later, gravediggers in early eighteenth-century France drank a concoction of crushed garlic in wine which they believed would protect them from getting the plague that killed many people in Europe. More recently, during both World Wars I and II, soldiers were given garlic to prevent gangrene and today people use garlic to help prevent atherosclerosis (plaque build up in the arteries causing blockage and possibly leading to heart attack or stroke), improve high blood pressure, and reduce colds, coughs, and bronchitis.

Medical research has been underway to assess whether these traditional uses of garlic have scientific validity. While the science is not definitive at this point, much of the research is showing real promise and many clinicians continue to report improvements in the areas of infection and heart-related risk factors for their individual patients. For example, test tube and animal studies suggest that garlic can kill many types of bacteria, some viruses and fungal infections, and even intestinal parasites. The belief is that properties of garlic may prove to help support immune function and prevent infection in people. Some experts believe that science may prove that garlic is particularly useful when taken together with medications (like antibiotics) prescribed for these infections.

Garlic also has antioxidant properties. Antioxidants help scavenge free radicals -- particles that can damage cell membranes, interact with genetic material, and possibly contribute to the aging process as well as the development of a number of conditions including heart disease and cancer. Free radicals occur naturally in the body, but environmental toxins (including ultraviolet light, radiation, cigarette smoking, and air pollution) can also increase the number of these damaging particles. Antioxidants can neutralize free radicals and may reduce or even help prevent some of the damage they cause over time.

The conditions for which garlic is showing the most promise include:

Atherosclerosis
Studies suggest that fresh garlic and garlic supplements may prevent blood clots and destroy plaque. Blood clots and plaque block blood flow and contribute to the development of atherosclerosis. Blockage of blood flow to the heart, brain, and legs, can lead to heart attack, stroke, or peripheral vascular disease (PVD). People with PVD experience pain in the legs when they walk and move. If garlic does reduce the build up of plaque, then stroke, heart attacks, and PVD may be less likely to occur in people who eat garlic or take garlic supplements.

Garlic may also be beneficial for risk factors for heart disease, including high blood pressure, high cholesterol, and diabetes. An animal study suggests that garlic may help lower homocysteine levels as well. Homocysteine, similar to cholesterol, may contribute to increasing amounts of blood clots and plaque in blood vessels.

High Cholesterol
Long hailed for its beneficial effects, a number of studies have found that garlic reduces elevated total cholesterol levels more effectively than placebo. However, the extent to which garlic lowers cholesterol in these studies has been small, and study limitations make it somewhat difficult to draw any firm conclusions. More research with better-designed studies is needed in order to fully assess the safety and effectiveness of garlic and to determine the most appropriate dose and form (see Available Forms).

In the meantime, work with a qualified healthcare practitioner, knowledgeable in herbal medicine, to determine if garlic is safe and appropriate for you to try. The specialist will assess what other medications you are taking to make sure that there are no potentially dangerous interactions, and will follow your cholesterol levels closely.

High Blood Pressure
Studies suggest that raw garlic may lower blood pressure. Similar to cholesterol, however, the drop in blood pressure caused by garlic is fairly small. For this reason, further research is necessary before it can be routinely recommended for people with high blood pressure.

Since garlic is considered relatively safe and has a number of other potentially healthful benefits for the heart, a professional herbalist may recommend the use of this herb. Again, work closely with a knowledgeable herbal specialist to determine if garlic is safe and appropriate for you. A healthcare provider will also monitor your blood pressure closely while you are taking this herbal supplement.

Diabetes
Garlic has been used as a traditional dietary supplement for diabetes in Asia, Europe, and the Middle East. Preliminary studies in rabbits, rats, and limited numbers of people have demonstrated that garlic has some ability to lower blood sugars. One well-designed study conducted in Thailand, however, found that garlic was no better than placebo in lowering blood sugar levels in people with type 2 diabetes. More research in this area is warranted. (See Possible Interactions regarding some concern about using garlic with certain medications for diabetes.)

Common Cold
A well-designed study of nearly 150 people supports the value of garlic for preventing and treating the common cold. In this study, people received either garlic supplements or placebo for 12 weeks during "cold season" (between the months of November and February). Those who received the garlic had significantly fewer colds than those who received placebo. Plus, when faced with a cold, the symptoms lasted a much shorter time in those receiving garlic compared to those receiving placebo.

Cancer
Test tube and animal studies suggest that garlic may have some anti-cancer activity. Observational, population-based studies (which follow groups of people over time) suggest that people who have more raw or cooked garlic in their diet are less likely to have certain types of cancer, particularly colon and stomach cancers. Dietary garlic may also offer some protection against the development of breast, prostate, and laryngeal (throat) cancers. However, these types of cancer have not been as extensively studied as colon and stomach cancer.

While these results are intriguing, more research is needed to best understand whether dietary intake of garlic and other substances in the same family (such as onions, leeks, scallions, chives, and shallots) truly help to prevent cancer. In addition, studies looking at garlic supplements (as opposed to dietary garlic) and cancer have been limited. Thus far, however, use of garlic supplements does not appear to reduce the risk of developing prostate, colon, stomach, lung, or breast cancer.

Tuberculosis
Numerous test tube studies have demonstrated that garlic extract inhibits the growth of different species of bacteria, including Mycobacterium tuberculosis, the organism responsible for tuberculosis. However, very high concentrations of garlic extract were needed to slow down the growth of M. tuberculosis in these studies, so some experts are concerned that these levels may be toxic to people. While further research in people is needed, one animal study found that garlic oil (which is a higher concentration than the extract) also inhibited M. tuberculosis and reduced the tuberculosis lesions in the lungs of these animals. Some scientists speculate that a combination of garlic extract or garlic oil with anti-tuberculosis drugs may eventually prove effective against the disease. Research to test this theory is needed.

Intestinal Parasites
Laboratory studies suggest that large quantities of fresh, raw garlic may have antiparasitic properties against the roundworm, Ascaris lumbricoides, which is the most common type of intestinal parasite. Garlic for this purpose, however, has not yet been tested in people.

Ear Pain from Otitis Media
Most children with an ear infection known as otitis media experience pain. Often, ear drops with pain killers are prescribed to relieve this discomfort. A recent study compared this standard treatment to a combination herbal extract (also used as ear drops) containing garlic as well as calendula, St. John's wort, and mullein flower. The herbal combination worked as well as the prescription ear drops. The number of children included in the study, however, was small. More research in this area would be helpful.

Garlic originally came from central Asia, and is now cultivated throughout the world. Garlic is a perennial that can grow two feet high or more. The most important part of this plant for medicinal purposes is the compound bulb. Each bulb is made up of 4 to 20 cloves, and each clove weighs about 1 gram. The parts of the plant used medicinally include fresh bulbs, dried bulbs, and oil extracted from the garlic.

There are several important components of garlic that have been identified, and many more that have not. Alliin is an odorless sulfur-containing chemical derived from the amino acid cysteine. When garlic bulbs are crushed, alliin is converted into another compound called allicin. Allicin appears to be at least one of the primary active compounds that gives garlic its characteristic odor and many of its healing benefits.

Allicin appears to have infection-fighting action as well as potential cardiovascular effects including, possibly, the ability to lower blood pressure and cholesterol. In addition, test tubes have shown that allicin has anti-cancer activities.

Allicin is further broken down to a compound called ajoene, which may be the substance that inhibits blockage in blood vessels from clots and atherosclerosis.

Garlic products are made from whole fresh garlic, fresh or dried garlic cloves, garlic powder made from the dried cloves, freeze-dried garlic, or oil garlic extracts.

Not all garlic contains the same amount of active ingredients. In fact, there is a fairly wide variation in the amount of allicin and other important ingredients in both fresh garlic and commercial products. The amount present depends on where the garlic is grown as well as how the product is prepared. Some experts believe that the wide variation in the quantity of active ingredients in garlic preparations explains why there is some variability in how well the substances lower cholesterol, improve blood pressure, and fight infection in different people.

Aged garlic products are made by fermenting garlic. Fermentation may reduce the amount of active ingredients in garlic. In addition, cooking garlic at very high temperatures may destroy its active components.

It is important to carefully read the label on all garlic products. It is best to use standardized garlic products to ensure that you are getting a specified concentration of allicin and other active substances. Also, follow the directions of a qualified healthcare practitioner with knowledge and experience in herbal medicine.

Pediatric

An appropriate medicinal dose for children has not been established. For this reason, use of garlic for health-related reasons in children should be directed by a qualified healthcare practitioner who has experience treating children with herbal remedies.

Adult

• Whole garlic clove: 2 to 4 grams per day of fresh, minced garlic clove (each clove is approximately 1 gram)
• Capsules or tablets of freeze-dried garlic standardized to 1.3% alliin or 0.6% allicin: 600 to 900 mg daily
• Infusion: 4 grams in 150 mL of water/day
• Fluid extract of 1:1 (g/mL) solution: 4 mL/day
• Tincture of 1:5 (g/mL) solution: 20 mL/day
• Oil: 0.03 to 0.12 mL three times a day

The use of herbs is a time-honored approach to strengthening the body and treating disease. Herbs, however, contain active substances that can trigger side effects and that can interact with other herbs, supplements, or medications. For these reasons, herbs should be taken with care, under the supervision of a practitioner knowledgeable in the field of botanical medicine.

Garlic is considered to have very low toxicity and is listed as Generally Recognized as Safe (GRAS) by the Food and Drug Administration (FDA) of the United States.

Side effects from garlic include upset stomach, bloating, bad breath, body odor, and a stinging sensation on the skin from handling too much fresh or dried garlic. Handling garlic may also cause the appearance of skin lesions. Other side effects that have been reported by those taking garlic supplements include headache, fatigue, loss of appetite, muscle aches, dizziness described as vertigo (namely, the room spinning), and allergies such as an asthmatic reaction or contact dermatitis (skin rash).

Garlic has blood-thinning properties so people with bleeding disorders, such as hemophilia or platelet disorders, should not use garlic supplements or medicinal doses of garlic. This is also important to know if you are going to have surgery or deliver a baby. Too much garlic can increase your risk for bleeding during or after those procedures.

Some experts recommend that pregnant and breastfeeding women avoid garlic. This may be due to the fact that a safe dose of medicinal garlic has not been established for infants and children.

If you are currently being treated with any of the following medications, you should not use garlic supplements without first talking to your healthcare provider.

Antiplatelet medications
Garlic may exaggerate the activity of medications that inhibit the action of platelets in the body. Examples of such medications include indomethacin, dipyridamole, and aspirin.

Blood-thinning medications
There have been reports of a possible interaction between garlic and warfarin that could increase the risk of bleeding in people taking this blood thinning medication. Therefore, when taking medications that may thin the blood, such as aspirin and warfarin, you should refrain from consuming large quantities of garlic, either fresh or commercially prepared.

Diabetes medications
When used with a class of medications for diabetes called sulfonylureas, garlic may lower blood sugar considerably. Medications from this class include chlorpropamide, glimepiride, and glyburide. When using garlic with these medications, blood sugars must be followed closely.

Protease inhibitors
Garlic may reduce blood levels of protease inhibitors, a medication used to treat people with the human immunodeficiency virus (HIV), including indinavir, ritinavir, and saquinavir.

Other
It is thought that garlic may behave similarly to a class of cholesterol lowering medications called statins (such as atorvastatin, pravastatin, and lovastatin) and to a class of blood pressure lowering medications called ACE inhibitors (including enalapril, captopril, and lisinopril). It is not known, therefore, whether it is safe to take this supplement in large quantities with these medications or not. This possible interaction has never been tested in scientific studies.

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