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This fact sheet provides basic information about garlic—common names, what the science says, potential side effects and cautions, and resources for more information.
Garlic is the edible bulb from a plant in the lily family. It has been used as both a medicine and a spice for thousands of years. Garlic’s most common folk or traditional uses as a dietary supplement are for high cholesterol, heart disease, and high blood pressure. Other folk or traditional uses include prevention of certain types of cancer, including stomach and colon cancers. Garlic cloves can be eaten raw or cooked. They may also be dried or powdered and used in tablets and capsules. Raw garlic cloves can be used to make oils and liquid extracts.
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 NCCIH-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 stroke.
- Evidence suggests that taking garlic may slightly lower blood pressure, particularly in people with high 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 NCCIH-funded research includes studies on how garlic interacts with certain drugs; its effects on liver function and the dilation and constriction of blood vessels; and the bioavailability (how well a substance is absorbed by the body) of allicin, the main active compound of garlic.
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.
- 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 all your health care providers about any complementary health approaches you use. Give them a full picture of what you do to manage your health. This will help ensure coordinated and safe care.
For More Information
The NCCIH Clearinghouse provides information on NCCIH and complementary and integrative health approaches, including publications and searches of Federal databases of scientific and medical literature. The Clearinghouse does not provide medical advice, treatment recommendations, or referrals to practitioners.
A service of the National Library of Medicine (NLM), PubMed® contains publication information and (in most cases) brief summaries of articles from scientific and medical journals.
Office of Dietary Supplements (ODS), National Institutes of Health (NIH)
ODS seeks to strengthen knowledge and understanding of dietary supplements by evaluating scientific information, supporting research, sharing research results, and educating the public. Its resources include publications (such as Dietary Supplements: What You Need to Know), fact sheets on a variety of specific supplement ingredients and products (such as vitamin D and multivitamin/mineral supplements), and the PubMed Dietary Supplement Subset.
NIH National Library of Medicine's MedlinePlus
- Agency for Healthcare Research and Quality. Garlic: Effects on Cardiovascular Risks and Disease, Protective Effects Against Cancer, and Clinical Adverse Effects. Evidence Report/Technology Assessment no. 20. Rockville, MD: Agency for Healthcare Research and Quality; 2000. AHRQ publication no. 01-E023.
- 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.
- Garlic. In: Blumenthal M, Goldberg A, Brinckman J, eds. Herbal Medicine: Expanded Commission E Monographs. Newton, MA: Lippincott Williams & Wilkins; 2000:139–148.
- Garlic. Natural Medicines Comprehensive Database Web site. Accessed at www.naturaldatabase.com on June 16, 2009.
- Garlic (Allium sativum L.). Natural Standard Database Web site. Accessed at www.naturalstandard.com on June 16, 2009.
- 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.
- National Cancer Institute. Garlic and Cancer Prevention. National Cancer Institute Web site. Accessed at www.cancer.gov/cancertopics/factsheet/Prevention/garlic-and-cancer-prevention on June 3, 2010.
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