Drug Database - Garlic
Garlic's most common uses as a dietary supplement are for high cholesterol, heart disease,
and high blood pressure. Garlic is also used to prevent certain types of cancer, including
stomach and colon cancers.
What the Science Says
Some evidence indicates that taking garlic can slightly lower blood cholesterol levels;
studies have shown positive effects for short-term (1 to 3 months) use. However, an
NCCAM-funded study on the safety and effectiveness of three garlic preparations (fresh
garlic, dried powdered garlic tablets, and aged garlic extract tablets) for lowering blood
cholesterol levels found no effect.
Preliminary research suggests that taking garlic may slow the development of
atherosclerosis (hardening of the arteries), a condition that can lead to heart disease or
Evidence is mixed on whether taking garlic can slightly lower blood pressure.
Some studies suggest consuming garlic as a regular part of the diet may lower the risk of
certain cancers. However, no clinical trials have examined this. A clinical trial on the
long-term use of garlic supplements to prevent stomach cancer found no effect.
Recent NCCAM-funded research includes studies on how garlic interacts with certain drugs
and how it can thin the blood.
Side Effects and Cautions
Garlic appears to be safe for most adults.
Side effects include breath and body odor, heartburn, upset stomach, and allergic
reactions. These side effects are more common with raw garlic.
Garlic can thin the blood (reduce the ability of blood to clot) in a manner similar to
aspirin. This effect may be a problem during or after surgery. Use garlic with caution if
you are planning to have surgery or dental work, or if you have a bleeding disorder. A
cautious approach is to avoid garlic in your diet or as a supplement for at least 1 week
Chemical compounds in garlic may inhibit blood clotting. Don't use garlic supplements if
you are already taking anticoagulants or antiplatelet drugs such as clopidogrel
(Plavix®).Garlic can also interfere with the action of the antiviral drug saquinavir
(Invirase), which is used to treat HIV infection.
Garlic has been found to interfere with the effectiveness of saquinavir, a drug used to
treat HIV infection. Its effect on other drugs has not been well studied.
Tell your health care providers about any complementary and alternative practices you use.
Give them a full picture of what you do to manage your health. This will help ensure
coordinated and safe care.
Adverse effects of oral ingestion of garlic are "smelly" breath and body odor. Other
possible, but not proven, adverse effects include flatulence, esophageal and abdominal
pain, small intestinal obstruction, contact dermatitis, rhinitis, asthma, bleeding, and
There are two reports of patients taking warfarin who experienced increases in
International Normalized Ratio (INR) when taking garlic pearls or tablets. The content and
method of preparation of the pearls and tablets were not given. The frequency of adverse
effects with oral ingestion of garlic and whether they vary by particular preparations are
Adverse effects of inhaled garlic dust include allergic reactions such as asthma,
rhinitis, urticaria, angioedema, and anaphylaxis. Adverse effects of topical exposure to
raw garlic include contact dermatitis, skin blisters, and ulcero-necrotic lesions.
Frequency of reactions to inhaled garlic dust or topical exposures of garlic is not
Garlic may alter the function of certain prescription medications. If you are being
treated with any of the following medications, you should not use garlic supplements
without first talking to your health care provider.
Garlic may exaggerate the activity of medications that inhibit the action of platelets
in the body. Examples of such medications include:
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 not
use garlic supplements unless you are under the supervision of a doctor.
Garlic may reduce blood levels of protease inhibitors, a medication used to treat people
with the human immunodeficiency virus (HIV). Protease inhibitors inclue:
Possible Interactions with: Garlic, © 2011. Rush University Medical Center.
- Agency for Healthcare Research and Quality, Evidence Report/Technology Assessment:
Number 20, Effects on Cardiovascular Risks and Disease, Protective Effects Against Cancer,
and Clinical Adverse Effects, Accessed at Agency for Healthcare Research and Quality
website on October 30, 2009.
- Gardner CD, Lawson LD, Block E, et al. Effect of raw garlic vs. commercial garlic
supplements on plasma lipid concentrations in adults with moderate hypercholesterolemia: a
randomized clinical trial. Archives of Internal Medicine. 2007;167(4):346–353.
- National Cancer Institute. Garlic and Cancer Prevention: Questions and Answers.
National Cancer Institute Web site. Accessed at cancer.gov/newscenter/pressreleases/garlic
on July 9, 2007.
- Agency for Healthcare Research and Quality. Garlic: Effects on Cardiovascular Risks and
Disease, Protective Effects Against Cancer, and Clinical Adverse Effects. Accessed at
ahrq.gov/clinic/epcsums/garlicsum.htm on July 9, 2007.
- Garlic. Natural Medicines Comprehensive Database Web site. Accessed July 3, 2007.
- Garlic (Allium sativum L.). Natural Standard Database Web site. Accessed June 28, 2007.
- Garlic. In: Blumenthal M, Goldberg A, Brinckman J, eds. Herbal Medicine: Expanded
Commission E Monographs. Newton, MA: Lippincott Williams & Wilkins; 2000:139–148.
- Milner JA. Garlic (Allium sativum). In: Coates P, Blackman M, Cragg G, et al., eds.
Encyclopedia of Dietary Supplements. New York: NY: Marcel Dekker; 2005:229–240.