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Viral Upper Respiratory Infections (Colds and Influenza) and Garlic

Written by ColleenO, FoundHealth.

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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. Many people joked that garlic merely makes you smell so bad people stay away from you, and so you don't catch their cold.

However, there is now some evidence that garlic--specifically, garlic extract--may really help keep you from getting sick. There is not yet any evidence to suggest that garlic or garlic extracts reduce symptoms or shorten the duration of an existing cold.

Effect of Garlic on Viral Upper Respiratory Infections (Colds and Influenza)

Garlic may have a number of properties that help with colds and flu. For instance, we know that garlic contains compounds that help the plant defend itself from microbes, and these anti-microbial qualities can help fight infections in humans.

Read more details about Garlic.

Research Evidence on Garlic

In one 12-week, double-blind, placebo-controlled trial, 146 individuals received either placebo or a garlic extract between November and February.96 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.

Benefits were also seen in a smaller double-blind study.124

How to Use Garlic

Raw garlic can kill a wide variety of microorganisms by direct contact, including fungi, bacteria, viruses, and protozoa.79,80 There is not yet any research evidence that taking garlic orally can kill microbes throughout the body.

The garlic discussed in this article was not whole garlic, but garlic extract. 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, or 4-5 mg of “allicin potential.” Alliin-free aged garlic is taken at a dose of 1 to 7.2 g daily.

Alliin is a relatively odorless substance found in garlic. When garlic is crushed or cut, an enzyme called allinase is brought in contact with alliin, turning it into allicin. Allicin is responsible for much of the typical odor of garlic. It is very active chemically and probably helps the garlic bulb defend itself from attack by insects and other threats. However, allicin is unstable, and soon breaks down into a variety of other substances. When garlic is ground up and encapsulated, the effect is similar to cutting the bulb: 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 it is purchased.

Some garlic producers believe that alliin and allicin are not essential for garlic's effectiveness and do not worry about this. Aged garlic, for example, has very little of either compound. But other manufacturers believe that allicin is the primary active ingredient in garlic. Because allicin is an unstable chemical, these manufacturers are faced with a challenge.

One solution might be to chemically stabilize allicin so that it doesn’t break down. However, allicin has a strong garlic smell, and a relatively odorless product is preferable. Many manufacturers of garlic powder products seek to stabilize the alliin in the product, and to do so in such a way that the alliin converts to allicin after it is consumed. How well their methods work remain a matter of controversy.

Note: Do not confuse essential oil of garlic with garlic oils. The term "garlic oil" refers to garlic extracted by means of oil. Garlic essential oil is the pure oily component of the herb, and, like other essential oils, it is potentially toxic.

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. 1 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. 2 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. 3 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. 4 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. 5 Since garlic might "thin" the blood, it is probably imprudent to take garlic pills immediately prior to or after surgery or labor and delivery, because of the risk of excessive bleeding. 6 Similarly, garlic should not be combined with blood-thinning drugs, such as warfarin (Coumadin), heparin , aspirin , clopidogrel (Plavix), ticlopidine (Ticlid), or pentoxifylline (Trental). In addition, garlic could conceivably interact with natural products with blood-thinning properties, such as ginkgo , policosanol , or high-dose vitamin E . However, a placebo-controlled study found that actual raw garlic consumed in food at the fairly high dose of 4.2 mg once daily did not impair platelet function. In addition, volunteers who continued to consume the dietary garlic for a week did not show any change in their normal platelet function. 7 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. 8 Garlic might also reduce the effectiveness of some drugs used for HIV. 9 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

If you are taking:

  • Ginkgo , policosanol , or high-dose vitamin E : Taking garlic at the same time might conceivably cause a risk of bleeding problems.
  • Medications for HIV : Do not use garlic.


  1. Sumiyoshi H, Kanezawa A, Masamoto K, et al. Chronic toxicity test of garlic extract in rats [in Japanese; English abstract]. J Toxicol Sci. 1984;9:61-75.
  2. Morcos NC, Camilo K. Acute and chronic toxicity study of fish oil and garlic combination. Int J Vitam Nutr Res. 71(5):306-12.
  3. Schulz V, Hansel R, Tyler VE. Rational Phytotherapy: A Physicians' Guide to Herbal Medicine. 3rd ed. Berlin, Germany: Springer-Verlag; 1998:121.
  4. Schulz V, Hansel R, Tyler VE. Rational Phytotherapy: A Physicians' Guide to Herbal Medicine. 3rd ed. Berlin, Germany: Springer-Verlag; 1998:121.
  5. Garty BZ. Garlic burns. Pediatrics. 91(3):658-9.
  6. Burnham BE. Garlic as a possible risk for postoperative bleeding. Plast Reconstr Surg. 95(1):213.
  7. Scharbert G, Kalb ML, Duris M, et al. Garlic at dietary doses does not impair platelet function. Anesth Analg. 2007;105:1214-1218
  8. Piscitelli SC. Use of complementary medicines by patients with HIV: Full sail into uncharted waters. Medscape HIV/AIDS. 2000;6.
  9. Piscitelli SC, Burstein AH, Welden N, Gallicano KD, Falloon J. The effect of garlic supplements on the pharmacokinetics of saquinavir. Clin Infect Dis. 34(2):234-8.
  1. Agarwal KC. Therapeutic actions of garlic constituents. Med Res Rev. 1996;16:111-124.
  2. Hughes BG, Lawson LD. Antimicrobial effects of Allium sativum L. (garlic), Allium ampeloprasum L. (elephant garlic), and Allium cepa L. (onion), garlic compounds and commercial garlic supplement products. Phytother Res. 1991;5:154-158.
  3. Josling P. Preventing the common cold with a garlic supplement: a double-blind, placebo-controlled survey. Adv Ther. 2001;18:189-93.
  4. Andrianova IV, Sobenin IA, Sereda EV, et al. Effect of long-acting garlic tablets "allicor" on the incidence of acute respiratory viral infections in children. Ter Arkh. 2003;75:53-56.

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