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Garlic is valued in many parts of the world for its pungent aroma and flavor. It is possible that garlic’s biological activity and popularity in Mediterranean cuisines contribute to the healthful effects of the “Mediterranean diet.”

Most investigations of garlic’s health benefits have considered its medicinal rather than culinary uses, however.

Medicinal use of garlic goes back to Greek and Egyptian antiquity. Hippocrates prescribed it for leprosy, toothache, and chest pain. Galen considered it a cure-all readily accessible to everyone.

One old tradition holds that garlic protected four convicts from the plague in Marseilles. They were released from prison in 1721 to bury the dead with the expectation that they would succumb quickly. Their survival was attributed to their habit of imbibing garlic juice mixed with vinegar and wine.

Garlic was used in the nineteenth century for tuberculosis and into World War II for disinfecting battlefield wounds.

It is frequently used in an attempt to ward off or treat the common cold.

The herb is available in many forms, including fresh bulbs, oil-based extracts, dried powder, and steam-distilled extracts.

To maximize the anti-cancer activity of fresh garlic in cooking, crush or mince it at least ten minutes before heating.

Active Ingredients

Sulfur compounds give garlic its characteristic pungent aroma and probably account for some of the flavor. They also appear to be responsible for most of the medicinal properties of this herb, although the trace minerals germanium and selenium may also play a role.

An inert compound, alliin, is converted to allicin once the clove is cut or crushed.

In Europe, standardized extracts of garlic are supposed to contain at least 0.45 percent allicin, a compound that breaks down into most of the active components, such as ajoene.

Chemical analysis of garlic products shows that concentrations of sulfur compounds vary enormously.


Garlic is widely used for its cardiovascular benefits, although the results of two American trials on its ability to lower cholesterol were disappointing.

An analysis of twenty-six other studies showed cholesterol reduced, on the average, by approximately 10 percent.

In some studies, dangerous LDL cholesterol dropped by 16 percent, while other research has shown increases in beneficial HDL with long-term use.

Although the cholesterol-lowering power of garlic appears modest, the herb is reported to reduce oxidation of LDL and seems to have other cardioprotective effects.

Several garlic-derived chemicals can help slow blood clotting by keeping blood platelets from clumping together. In addition, garlic helps to break up or prevent blood clots through fibrinolytic action.

Since many heart attacks and strokes are believed to be caused by spontaneous clots in blood vessels, these anticoagulant actions could be very helpful.

Garlic may also lower blood pressure, but it is less effective in this respect than are medicines. It is helpful, however, in keeping blood vessels to the heart flexible in older people.

Research in rats and dogs also indicates that fresh garlic and garlic extracts can correct certain irregular heart rhythms.

Test tube research has established that garlic extracts are active against a range of bacteria, including such nasties as Staphylococcus aureus and Streptococcus pneumoniae. It is only about 1 percent as active as penicillin, however.

Garlic extract can also fight Helicobacter pylori, a bacterium that causes stomach ulcers. Perhaps garlic should be added to the combination of drugs used to eradicate this bug and cure ulcers, but we’ll have to await clinical research to confirm this.

Garlic extracts are comparable to antifungal drugs against fungal infections of the skin and the ear.

One of the most intriguing possibilities for garlic is that regular ingestion may help prevent cancer. Studies in China comparing people in one region where garlic is commonly eaten (20 grams, or approximately seven cloves a day, on average) with those in another region where daily consumption is less than half a clove found the garlic eaters were much less likely to suffer stomach cancer.

Other studies have indicated that people who eat garlic more often seem less susceptible to stomach or colon cancer. Animal research confirms that garlic has the potential to improve resistance to tumors, and test tube research shows that garlic can interfere with some cancer-causing chemicals.

Garlic can reduce blood sugar levels and may improve insulin response.


For cardiovascular conditions: one clove daily, equivalent to 6 to 10 mg of alliin, or 3 to 5 mg of allicin. Treatment is maintained indefinitely.

For common cold prevention/treatment: one clove three times a day, until symptoms resolve.

For a standardized product tested in Germany, look for Kwai. Read product label for proper dosage.

Special Precautions

Because garlic can slow blood clotting, German authorities recommend that patients avoid this herb in the period just prior to and following surgery.

Those with chronic digestive problems should be cautious, because high doses of garlic can irritate the intestinal tract.

Pregnant women should exercise moderation; at high doses, garlic extracts can stimulate uterine contractions in animals.

People with low thyroid function should be aware that concentrated garlic products may keep the thyroid gland from utilizing iodine properly. This could aggravate an underactive thyroid condition.

Adverse Effects

In rats, high doses of garlic led to weight loss and damage to the stomach lining. Humans taking garlic oil at a dose equivalent to twenty cloves daily for three months did not report problems.

Most people appear to tolerate garlic well, but some individuals experience digestive distress.

People who handle garlic products occasionally develop a skin reaction on exposure (contact dermatitis).
Ingesting fresh garlic and most extracts results in a characteristic breath odor. This has been linked to the active sulfur-containing compounds. Parsley is recommended as a home remedy for garlic breath.

Possible Interactions

Although there are no studies of interactions, in theory garlic could increase the risk of bleeding in people taking anticoagulants such as Coumadin, aspirin, Plavix, or Ticlid.

There is also a possibility that this herb could interact with drugs such as DiaBeta or Glucotrol that lower blood sugar. Careful monitoring is suggested for anyone combining garlic products with such prescription drugs. Garlic appears to inhibit an enzyme called CYP 2E1. In most cases, this interference is welcome, since this enzyme can make carcinogens more dangerous. But CYP 2E1 is also involved in the metabolism of acetaminophen (Panadol, Tylenol, etc.) and a muscle relaxant called chlorzoxazone (Parafon Forte). These drugs could possibly linger longer in people who are taking or eating garlic.

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About the Author
Joe Graedon is a pharmacologist who has dedicated his career to making drug information understandable to consumers. His best-selling book, The People’s Pharmacy, was published in 1976 and led to a syndicated newspaper column, syndicated public radio show and web site. In 2006, Long Island University awarded him an honorary doctorate as “one of the country's leading drug experts for the consumer.”.
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