Garlic (Allium sativum) bulbs are extracted and made into tablet, powder, and oil forms; the major active ingredient is allicin or S-allylcysteine, an amino acid by-product. Garlic can also be eaten raw or cooked. Because the active ingredients are volatile and destroyed by the act of crushing, the amount of active ingredient in the various forms of garlic varies greatly. Supplements are best standardized by the amount of active compound. Aged garlic extract (AGE), made from garlic allowed to age for at least 20 months, has more stable active compounds than most forms. Consuming garlic supplements in this form appears to confer the greatest health benefits and freedom from adverse effects.
(See also Overview of Dietary Supplements Overview of Dietary Supplements Dietary supplements are the most commonly used of all integrative, complementary, and alternative therapies, primarily because they are widely available, relatively inexpensive, and can be bought... read more and National Institutes of Health (NIH): Garlic.)
Claims
Garlic is said to have favorable effects on several cardiac risk factors, including reduction of blood pressure and serum lipid and glucose levels; garlic inhibits platelets in vitro. Garlic is also said to protect against laryngeal, gastric, colorectal, and endometrial cancer and adenomatous colorectal polyps. Garlic is also said to prevent the common cold and to treat nonalcoholic fatty liver disease (NAFLD).
Evidence
The strongest evidence available for garlic supplementation, specifically AGE, is lowering blood pressure. A 2016 meta-analysis evaluated 20 trials (970 subjects) in which study duration ranged from 2 to 24 weeks. A variety of garlic preparations included garlic powder and AGE. Mean systolic and diastolic pressure decreases were 5.1 mm Hg and 2.5 mm Hg, respectively (1 References Garlic (Allium sativum) bulbs are extracted and made into tablet, powder, and oil forms; the major active ingredient is allicin or S-allylcysteine, an amino acid by-product. Garlic can... read more ). A 2020 meta-analysis of 12 randomized control trials (553 hypertensive subjects) lasting at least 2 months found a mean decrease of 8.3 mm Hg in systolic pressure and 5.5 mm Hg in diastolic pressure (2 References Garlic (Allium sativum) bulbs are extracted and made into tablet, powder, and oil forms; the major active ingredient is allicin or S-allylcysteine, an amino acid by-product. Garlic can... read more ).
Results of the lipid-lowering effects of garlic supplementation have been quite inconsistent. A 2013 meta-analysis of 39 randomized controlled studies (2298 participants) found that garlic lowered total cholesterol by 17 mg/dL (0.4 mmol/L) and LDL (low-density lipoprotein) cholesterol by 9 mg/dL (0.2 mmol/L) (3 References Garlic (Allium sativum) bulbs are extracted and made into tablet, powder, and oil forms; the major active ingredient is allicin or S-allylcysteine, an amino acid by-product. Garlic can... read more ).
A 2015 meta-analysis of 7 studies (513 subjects) has verified efficacy of garlic in lowering fasting glucose (4 References Garlic (Allium sativum) bulbs are extracted and made into tablet, powder, and oil forms; the major active ingredient is allicin or S-allylcysteine, an amino acid by-product. Garlic can... read more ). Other studies are needed to assess impact on hemoglobin A1C.
A randomized, placebo controlled trial found that garlic can improve hepatic steatosis in patients with NAFLD, evidenced by changes in the ultrasound findings. Garlic also significantly decreased weight and liver enzymes (5 References Garlic (Allium sativum) bulbs are extracted and made into tablet, powder, and oil forms; the major active ingredient is allicin or S-allylcysteine, an amino acid by-product. Garlic can... read more ).
Scientific evidence of either garlic intake or garlic supplement use shows limited or no protection against cancer. A 2016 systematic review and meta-analysis evaluating garlic intake in relation to colorectal cancer incidence did not find any protective effect (6 References Garlic (Allium sativum) bulbs are extracted and made into tablet, powder, and oil forms; the major active ingredient is allicin or S-allylcysteine, an amino acid by-product. Garlic can... read more ).
Garlic consumed in high doses has general antimicrobial effects in vitro (7 References Garlic (Allium sativum) bulbs are extracted and made into tablet, powder, and oil forms; the major active ingredient is allicin or S-allylcysteine, an amino acid by-product. Garlic can... read more ). In a randomized controlled trial, 146 participants were treated with an allicin-containing garlic supplement or placebo, one capsule a day, for 12 weeks. During the 12 weeks of the study, those treated with garlic had significantly fewer colds than the placebo group (24 versus 65, P < .001). The authors concluded that an allicin-containing garlic supplement can prevent attack by the common cold virus (8 References Garlic (Allium sativum) bulbs are extracted and made into tablet, powder, and oil forms; the major active ingredient is allicin or S-allylcysteine, an amino acid by-product. Garlic can... read more ).
Most of these studies lack the specific details with regard to the supplement and/or concentration of active ingredients in the supplement, which may account for the variable results.
Adverse Effects
Breath and body smell and nausea may occur; high doses may cause burning in the mouth, esophagus, and stomach.
Drug Interactions
Theoretically, garlic is contraindicated in patients who have bleeding diatheses or who take antihypertensives, antiplatelet drugs, or warfarin. Garlic can reduce serum saquinavir levels. Garlic may interact with drugs that decrease blood sugar levels, causing possible hypoglycemia. (See also table .)
References
1. Ried K: Garlic lowers blood pressure in hypertensive individuals, regulates serum cholesterol, and stimulates immunity: an updated meta-analysis and review. J Nutr 146(2):389S-396S, 2016. doi: 10.3945/jn.114.202192
2. Ried K: Garlic lowers blood pressure in hypertensive subjects, improves arterial stiffness and gut microbiota: a review and meta-analysis. Exp Ther Med 19(2):1472-1478, 2020. doi:10.3892/etm.2019.8374
3. Ried K, Toben C, Fakler P: Effect of garlic on serum lipids: an updated meta-analysis. Nutr Rev 71(5):282-299, 2013. doi: 10.1111/nure.12012
4. Hou LQ, Liu YH, Zhang YY: Garlic intake lowers fasting blood glucose: meta-analysis of randomized controlled trials. Asia Pac J Clin Nutr 24(4):575-582, 2015. doi: 10.6133/apjcn.2015.24.4.15
5. Soleimani D, Paknahad Z, Rouhani MH: Therapeutic effects of garlic on hepatic steatosis in nonalcoholic fatty liver disease patients: a randomized clinical trial. Diabetes Metab Syndr Obes 13:2389-2397, 2020. doi:10.2147/DMSO.S254555
6. Chiavarini M, Minelli L, Fabiani R: Garlic consumption and colorectal cancer risk in man: a systematic review and meta-analysis. Public Health Nutr 19(2):308-317, 2016. doi: 10.1017/S1368980015001263
7. Filocamo A, Nueno-Palop C, Bisignano C, et al: Effect of garlic powder on the growth of commensal bacteria from the gastrointestinal tract. Phytomedicine 19(8-9):707-711, 2012. doi: 10.1016/j.phymed.2012.02.018
8. Josling P: Preventing the common cold with a garlic supplement: a double-blind, placebo-controlled survey. Adv Ther 18(4):189-193, 2001. doi:10.1007/BF02850113
More Information
The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
National Institutes of Health (NIH), National Center for Complementary and Integrative Health: General information on the use of garlic as a dietary supplement
Drugs Mentioned In This Article
| Drug Name | Select Trade |
|---|---|
garlic |
No brand name available |
warfarin |
Coumadin, Jantoven |
saquinavir |
Fortovase, Invirase |
