Herbs & Supplements:
Garlic
Allium sativum
Principal Proposed Uses
•
Atherosclerosis, Lowering Cholesterol, Reducing Blood Pressure, "Thinning the Blood", Cold Prevention, Heart Attack Prevention
Other Proposed Uses
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Insect Repellent, Cancer Prevention, Topical Antibiotic and Antifungal, Athlete's Foot, Immune Stimulant, Candida, Vaginal Infections, Asthma, Diabetes
Probably Ineffective Uses
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Oral Antibiotic, Ear Infections
The story of garlic's role in human history could fill a book, as indeed it has, many times. Its species name, sativum, means cultivated, indicating that garlic does not grow in the wild. So fond have humans been of this herb that garlic can be found almost everywhere in the world, from Polynesia to Siberia. Interestingly, as far back as the first century A.D., Dioscorides wrote of garlic's ability to "clear the arteries."
From Roman antiquity through World War I, garlic poultices were used to prevent wound infections. The famous microbiologist, Louis Pasteur, performed some of the original work showing that garlic could kill bacteria. In 1916, the British government issued a general plea for the public to supply it with garlic in order to meet wartime needs. Garlic was called "Russian penicillin" during World War II because, after running out of antibiotics, the Russian government turned to this ancient treatment for its soldiers.
After World War II, Sandoz Pharmaceuticals manufactured a garlic compound for intestinal spasms, and the Van Patten Company produced another for lowering blood pressure.
What Is Garlic Used for Today?
In Europe, garlic has come to be seen as an all-around treatment for preventing atherosclerosis, the cause of heart disease and strokes. Garlic may fight atherosclerosis in many ways, such as protecting against free radicals, countering the tendency of the blood to clot, and possibly reducing blood pressure and cholesterol levels.1–28 However, the evidence that it works is far from definitive.
Garlic has a long folkloric history as a treatment for colds, and is commonly stated to strengthen the immune system. However, up until 2001, there was no evidence that this was truly the case. A recent study, however, does suggest that regular use of garlic extract can help prevent colds.95
Garlic may be an effective antibiotic when it contacts the tissue directly, but there is no evidence that it works like a standard antibiotic, spreading throughout the body and killing organisms everywhere.
Garlic has known antifungal properties,38–41 and there is preliminary evidence suggesting that ajoene, a compound derived from garlic, might help treat athlete's foot.42,43 Folklore suggesting that garlic ingestion can ward off insect bites may have some truth to it.29
Preliminary evidence suggests that regular intake of garlic as food may help prevent cancer.30–37
Based on extremely weak evidence, garlic has been proposed as a treatment for candida and vaginal infections.44,45 Garlic has also been proposed as a treatment for asthma and diabetes, but there is no supporting evidence for these uses.
Garlic is often combined with the herb mullein in oil products designed to reduce the pain of middle ear infections (otitis media – not external ear infections, known commonly as swimmer’s ear), and one study supports this use.97 Note: While these products may reduce pain, it is very unlikely that they have any actual effect on the infection because the eardrum is in the way.
Contrary to some reports, garlic does not appear to be a useful treatment for Helicobacter pylori, the stomach bacteria implicated as a major cause of ulcers.46,47
What Is the Scientific Evidence for Garlic?
Overall Effects on Hardening of the Arteries
Intriguing but far from definitive evidence suggests that garlic may help prevent hardening of the arteries.
Garlic preparations have been found to slow hardening of the arteries in animals, reducing the size of plaque deposits by nearly 50%.48,49
In a double-blind, placebo-controlled study that followed 152 individuals for 4 years, standardized garlic powder at a dosage of 900 mg daily significantly slowed the development of atherosclerosis as measured by ultrasound.50 Although this study suffered from some statistical problems, it nonetheless provides preliminary evidence that all of garlic's effects combine to protect against hardening of the arteries.
An observational study of 200 individuals measured the flexibility of the aorta, the main artery exiting the heart.51 Participants who took garlic showed more flexibility, indicating less atherosclerosis.
Heart Attack Prevention
In one study, 432 individuals who had suffered a heart attack were given either garlic oil extract or no treatment over a period of 3 years.52 The results showed a significant reduction of second heart attacks and about a 50% reduction in death rate among those taking garlic.
High Cholesterol
A number of studies published in the 1980s and 1990s found evidence that garlic preparations can lower cholesterol.53–57 However, several more recent and generally better-designed studies have found no benefit.58–63 The accumulating impact of these results has tended to reduce enthusiasm for using garlic as a cholesterol lowering agent.
Bucking this trend, one recent study suggests that garlic can work, providing it's the right form of garlic. 64 This 12 week, double-blind, placebo-controlled trial of 46 individuals with high cholesterol tested a special enteric-coated garlic product. (An enteric coated product is one designed to bypass stomach acid and release its contents in the intestines.) The results showed significant improvement in levels of total cholesterol and LDL cholesterol.
Besides using an enteric coating, researchers in this trial used almost twice the dose of allicin generally administered in clinical trials. This may explain the positive results. An alternative explanation may lie in the fact that the improvements seen in this trial were on the low side, ranging from 5 - 8%. Recent studies that failed to find garlic effective were not designed to reliably identify benefits as modest as these.
Hypertension (High Blood Pressure)
Numerous studies have found weak evidence that garlic lowers blood pressure slightly, perhaps in the neighborhood of 5 to 10% more than placebo.65,66,67 However, all of these studies suffered from significant flaws, and most were performed on people who did not have high blood pressure. At present, it is not clear whether garlic actually has any effects on blood pressure.
One study followed 47 subjects with an average starting blood pressure of 171/101.68 Over a period of 12 weeks, half were treated with 600 mg of garlic powder daily standardized to 1.3% alliin, the other half were given placebo. The results showed a statistically significant drop of 11% in the systolic blood pressure and 13% in the diastolic pressure. In comparison, blood pressure fell in the placebo group by 5% and 4%, respectively. However, this study suffers from a significant problem: the average starting blood pressure of the placebo and the treated groups were quite different, making comparisons unreliable.
Cold Prevention
The herb garlic has a long history of use for treating or preventing colds. However, up until 2001, there was no scientific evidence that it actually works for this purpose.
A recent study does provide meaningful evidence that garlic might possess cold-fighting powers.95 In this 12 week, double-blind, placebo-controlled trial, 146 individuals received either placebo or a garlic extract between November and February.
The results showed that participants receiving garlic were almost two-thirds less likely to catch cold than those receiving placebo. Furthermore, participants who did catch cold recovered about one day faster in the garlic group as compared to the placebo group.
Thus, regular use of garlic might help prevent colds. However, there is no evidence as yet that if you take garlic at the onset of a cold, you will recover more quickly.
Insect Repellent
A 20-week double-blind placebo-controlled crossover trial followed 80 Swedish soldiers and measured the number of tick bites received during the garlic and the placebo treatments.69 The results showed a modest but statistically significant reduction in tick bites when soldiers consumed 1,200 mg of garlic daily. Unfortunately, the type of garlic used in this study was not stated.
Cancer Prevention
Evidence from observational studies suggests that garlic may help prevent cancer, particularly cancer of the stomach and colon.70–77 In one of the best of these trials, the Iowa Women's Study, a group of 41,837 women were questioned as to their lifestyle habits in 1986, and then followed continuously in subsequent years. At the 4-year follow-up, questionnaires showed that women whose diets included significant quantities of garlic were approximately 30% less likely to develop colon cancer.78
The interpretations of studies like this one are always a bit controversial. For example, it's possible that the women who ate a lot of garlic also made other healthful lifestyle choices. While researchers looked at this possibility very carefully and concluded that garlic was a common factor, it is not clear that they are right. What is really needed to settle the question is an intervention trial, where some people are given garlic and others are given a placebo. However, none has yet been performed.
Antimicrobial
There is no question that raw garlic can kill a wide variety of microorganisms by direct contact, including fungi, bacteria, viruses, and protozoa.79,80 A double-blind study reported in 1999 found that a cream made from the garlic constituent ajoene was just as effective for fungal skin infections as the standard drug terbinafine.81 These findings may explain why garlic was traditionally applied directly to wounds in order to prevent infection (but keep in mind that it can burn the skin). But there is no real evidence that taking garlic orally can kill organisms throughout the body. Thus, it's not an antibiotic in the usual sense. It's more like Bacitracin ointment.
Oral garlic could theoretically offer benefits against organisms in the stomach or intestines, because it can come into direct contact with them. However, there is only the scantiest evidence as yet that it works for any specific infection of this type.82,83,84 For example, despite test tube evidence that garlic can kill Helicobacter pylori (the cause of ulcers), studies in people have not been promising.85,86
Dosage
A typical dosage of garlic is 900 mg daily of a garlic powder extract standardized to contain 1.3% alliin, providing about 12,000 mcg of alliin daily. However, a great deal of controversy exists over the proper dosage and form of garlic. Most everyone agrees that 1 or 2 raw garlic cloves per day are adequate for most purposes, but virtual trade wars have taken place over the potency and effectiveness of various dried, aged, or deodorized garlic preparations. The problem has to do with the way garlic is naturally constructed.
A relatively odorless substance, alliin, is one of the most important compounds in garlic. When garlic is crushed or cut, an enzyme called allinase is brought in contact with alliin, turning it into allicin. The allicin itself then rapidly breaks down into entirely different compounds. Allicin is most responsible for garlic's strong odor. It can also blister the skin and kill bacteria, viruses, and fungi. Presumably the garlic plant uses allicin as a form of protection from pests and parasites. It also may provide much of the medicinal benefits of garlic.
When you powder garlic to put it in a capsule, it acts like cutting the bulb. The chain reaction starts: Alliin contacts allinase, yielding allicin, which then breaks down. Unless something is done to prevent this process, garlic powder won't have any alliin or allicin left by the time you buy it.
Some garlic producers declare that alliin and allicin have nothing to do with garlic's effectiveness and simply sell products without it. This is particularly true of aged powdered garlic and garlic oil. But others feel certain that allicin is absolutely essential. However, in order to make garlic relatively odorless, they must prevent the alliin from turning into allicin until the product is consumed. To accomplish this feat, they engage in marvelously complex manufacturing processes, each unique and proprietary. How well each of these methods work is a matter of finger-pointing controversy.
The best that can be said at this point is that in most of the clinical studies of garlic, the daily dosage supplied at least 10 mg of alliin. This is sometimes stated in terms of how much allicin will be created from that alliin. The number you should look for is 4 to 5 mg of "allicin potential."
Alliin-free aged garlic also appears to be effective when taken at a dose of 1 to 7.2 g daily.
Safety Issues
As a commonly used food, garlic is on the FDA's GRAS (generally recognized as safe) list. Rats have been fed gigantic doses of aged garlic (2,000 mg per kilogram body weight) for 6 months without any signs of negative effects.87 Long term treatment with standardized garlic powder at a dose equivalent to three times the usual dose, along with fish oil, produced no toxic effects in rats.96
The only common side effect of garlic is unpleasant breath odor. Even "odorless garlic" produces an offensive smell in up to 50% of those who use it.88
Other side effects occur only rarely. For example, a study that followed 1,997 people who were given a normal dose of deodorized garlic daily over a 16-week period showed a 6% incidence of nausea, a 1.3% incidence of dizziness on standing (perhaps a sign of low blood pressure), and a 1.1% incidence of allergic reactions.89 There were also a few reports of bloating, headaches, sweating, and dizziness.
When raw garlic is taken in excessive doses, it can cause numerous symptoms, such as stomach upset, heartburn, nausea, vomiting, diarrhea, flatulence, facial flushing, rapid pulse, and insomnia.
Topical garlic can cause skin irritation, blistering, and even third-degree burns, so be very careful about applying garlic directly to the skin.90
Since garlic "thins" the blood, it is not a good idea to take high-potency garlic pills immediately prior to or after surgery or labor and delivery, due to the risk of excessive bleeding.91,92 Similarly, garlic should not be combined with blood-thinning drugs, such as Coumadin (warfarin), heparin, aspirin, Plavix (clopidogrel), Ticlid (ticlopidine), or Trental (pentoxifylline). In addition, garlic could conceivably interact with natural products with blood-thinning properties, such as ginkgo, policosanol, or high-dose vitamin E.
Garlic may also combine poorly with certain HIV medications. Two people with HIV experienced severe gastrointestinal toxicity from the HIV drug ritonavir after taking garlic supplements.93 Garlic might also reduce the effectiveness of some drugs used for HIV.94
Garlic is presumed to be safe for pregnant women (except just before and immediately after delivery) and nursing mothers, although this has not been proven.
Interactions You Should Know About
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Last reviewed June 2002 by Medical Review Board
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