Garlic : Dosing and Safety

by MedlinePlus

(Page 2 of 3)

Dosing

Standardization

Standardization involves measuring the amount of certain chemicals in products to try to make different preparations similar to each other. It is not always known if the chemicals being measured are the "active" ingredients. Although allicin was once thought to be the major active ingredient in garlic, it now appears that additional compounds may contribute to the effects of garlic. The amounts of these compounds likely vary with the process used to manufacture the garlic product. Dried garlic powder is thought to have similar activity to fresh crushed garlic. However, garlic products prepared by other methods may not have as strong effects. For example, steam-distilled oils, oils from crushed garlic, and aged-garlic in alcohol may have less blood-thinning effects. The method of processing may be as important as the ingredients reported.

The standardized garlic powder product Kwai® has been used in numerous studies (manufactured by Lichtwer Pharma GmbH, Berlin, Germany). It is standardized to contain 1.3% allicin. Other research has used a standardized preparation containing 220 milligrams of garlic powder containing 2.4 milligrams of allicin. In the United States, pharmacy-grade garlic contains 0.3% (powdered) to 0.5% (fresh, dried) allicin, while in Europe, pharmacy-grade garlic must has at least 0.45% allicin.

Adults (18 years and older)

Tablets or capsules: 600 to 900 milligrams daily of non-coated, dehydrated garlic powder in three divided doses, standardized to 1.3% allicin content, has been used in human studies. The European Scientific Cooperative on Phytotherapy (ESCOP) recommends 3 to 5 milligrams allicin daily (1 clove or 0.5 to 1.0 gram dried powder) for prevention of atherosclerosis. The World Health Organization (WHO) recommends 2 to 5 grams fresh garlic, 0.4 to 1.2 grams of dried powder, 2 to 5 milligrams oil, 300 to 1000 milligrams of extract, or other formulations that are equal to 2 to 5 milligrams of allicin daily.

Oil: Human studies report the use of 4 to 12.3 milligrams of garlic oil by mouth daily. Steam-distilled oils, oil from crushed garlic, and aged-garlic in alcohol may be less effective for some uses, particularly as a blood thinner.

Tincture: The European Scientific Cooperative on Phytotherapy (ESCOP) recommends 2 to 4 grams of dried bulb or 2 to 4 milliliters of tincture (1:5 dilution in 45% ethanol), by mouth three times a day for upper respiratory tract infections.

Children (younger than 18 years)

Safety or effectiveness of garlic supplements has not been proven in children. One small study involving children with a hereditary form of high cholesterol showed no benefit from 900 milligrams of dehydrated garlic powder tablets (Kwai®) taken in three divided daily doses when compared to placebo (sugar pills). Garlic in amounts found in food is likely safe.

Safety

The U.S. Food and Drug Administration does not strictly regulate herbs and supplements. There is no guarantee of strength, purity or safety of products, and effects may vary. You should always read product labels. If you have a medical condition, or are taking other drugs, herbs, or supplements, you should speak with a qualified healthcare provider before starting a new therapy. Consult a healthcare provider immediately if you experience side effects.

Allergies

People with a known allergy to garlic, any of its ingredients, or to other members of the Liliaceae (lily) family, including hyacinth, tulip, onion, leek, and chives, should avoid garlic. Allergic reactions have been reported with garlic taken by mouth, inhaled, or applied to the skin. Some of these reactions are severe including throat swelling and difficulty breathing (anaphylaxis). It has been suggested that some cases of asthma from inhaling garlic may be due to mites on the garlic. Fresh garlic applied to the skin may be more likely to cause rashes than garlic extract.

Side Effects and Warnings

Bad breath, body odor, and allergic reactions are the most common reported side effects of garlic. Fresh garlic has caused rash or skin burns, both in people taking garlic therapy and in food preparers handling garlic. Most reactions improve after stopping garlic therapy. Garlic products should not be applied to the skin of infants or children due to multiple reports of skin burns, and should be used cautiously in adults. Other reported side effects include dizziness, increased sweating, headache, itching, fever, chills, asthma flares, and runny nose.

Bleeding is a potentially serious side effect of garlic use, including bleeding after surgery and spontaneous bleeding. Several cases of bleeding are reported, which may be due to effects of garlic on blood platelets, or to increased breakdown of blood clots (fibrinolysis). There is debate about the effects of garlic in people treated with warfarin (Coumadin®), but studies suggest that garlic does not alter the International Normalized Ratio (INR) values that are used to measure the effect of warfarin on blood thinning. Garlic should be stopped prior to some surgical or dental procedures due to an increased risk of bleeding. Caution is urged for people who have bleeding disorders or who take blood thinning medications (anticoagulants, aspirin/anti-platelet agents, non-steroidal anti-inflammatory drugs such as ibuprofen or naproxen) or herbs/supplements that may increase the risk of bleeding. Dosing adjustments may be necessary.

Animal studies suggest that garlic or its ingredients may lower blood sugar levels and increase the release of insulin. However, studies in humans do not show changes in blood sugar control in people with or without diabetes. Nonetheless, caution is advised in people with diabetes or hypoglycemia, and in those taking drugs, herbs, or supplements that affect blood sugar. Blood sugar levels may need to be monitored by a healthcare provider, and medication adjustments may be necessary. Informal reports describe low iodine absorption in the thyroid and low levels of thyroid hormone (hypothyroidism) with garlic supplementation. A few reports suggest that garlic and garlic-like plants may be linked to nodules or tumors of the thyroid. Reduced sperm counts have been reported in rats, but not in humans.

Dehydrated garlic preparations or raw garlic taken by mouth may cause burning of the mouth, bad breath, abdominal pain or fullness, poor appetite, gas, belching, nausea, vomiting, irritation of the stomach lining, changes in the bacteria in the gut, heartburn, diarrhea, or constipation. One report describes bowel obstruction in a man who ate a whole garlic bulb. Garlic should be used cautiously by people with stomach ulcers or who are prone to stomach irritation.

Multiple studies show a small reduction in blood cholesterol levels after garlic supplements are taken by mouth. Small reductions in blood pressure are also commonly reported. One case of heart attack is noted in a healthy man after taking a large amount of garlic by mouth.

Contamination of garlic products has been reported.

In Vancouver, British Columbia, a commercial preparation of chopped garlic was linked to botulism. One report describes overdose of colchicine and even death after meadow saffron ( Colchicum autumnale ) was mistaken for wild garlic ( Allium ursinium ).

Garlic and pycnogenol have been shown to increase human growth hormone secretion in laboratory experiments.

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