Garlic is a popular medicinal herb in Europe and the United States. It has been used since antiquity for the treatment of cardiovascular and infectious diseases. Several mechanisms for garlic's beneficial effects on cardiovascular disease have been proposed: decreased platelet aggregation and adhesion, increased fibrinolysis, antioxidant activity, blood pressure lowering, and decreased low-density lipoprotein (LDL) cholesterol synthesis.
Garlic is available in many different formulations (dried, powdered, oils). The most frequently studied form is dehydrated garlic powder extract. Many products are standardized to a certain percentage of allicin, the compound historically believed responsible for both its odor and its therapeutic benefit, although it is likely that the many other compounds in garlic contribute to the therapeutic effects. Garlic is well tolerated and generally safe for long-term use. In addition to the well-known breath and body odor, common side effects include gastrointestinal upset, nausea, and flatulence.
There is inconsistent information about the effects of garlic on cytochrome P450 3A4 (CYP3A4) isoenzymes. Some garlic preparations containing allicin appear to induce activity of CYP3A4. This was demonstrated in one small trial that showed a significant reduction in saquinavir levels, a CYP3A4 substrate. Because garlic has some antiplatelet activation activity, there is a theoretical risk of increased bleeding, especially if taken with aspirin, anticoagulants, or NSAIDs. However, a 2006 study of 48 patients taking warfarin randomized to aged garlic extract or placebo for 12 weeks did not report any increased bleeding in those taking garlic. In addition, a 2008 study showed no effect of garlic on warfarin pharmacokinetics in healthy volunteers. The effect of garlic on bleeding risks in patients taking direct-acting oral anticoagulants is unknown. No changes in platelet function were noted in healthy volunteers after ingesting raw garlic and garlic oil, but nevertheless, some clinicians recommend stopping garlic 1–2 weeks prior to undergoing elective surgery.
Prior studies have suggested that garlic has the capacity to lower blood pressure in hypertensive patients, but systematic reviews are unable to determine its true impact. In 2018, a 12-week randomized double-blinded placebo-controlled trial of 49 patients showed that an aged garlic extract lowered systolic blood pressure by 10 mm Hg and diastolic blood pressure by 5 mm Hg.
An interesting study attesting to its vascular effects looked at garlic's effect in patients with hepatopulmonary syndrome. In this study, 41 patients were randomized to either garlic supplement or placebo and monitored for 18 months. In the experimental group, baseline PaO2 increased and A-a gradient decreased significantly, two-thirds of patients experienced resolution of their hepatopulmonary syndrome, and mortality was reduced.
There is conflicting evidence of the effect of garlic on serum lipids. A 2018 meta-analysis concluded that garlic produces a modest but significant benefit for lowering total and LDL cholesterol. Explanations for inconsistent results include the lack of standardization of garlic preparations, as well as how humans metabolize them.
There are several small studies showing reduced progression of atherosclerotic plaques with garlic use. The most recent is a 2018 randomized controlled trial of 92 patients with obesity in which arterial stiffness and markers of endothelial function (high-sensitivity C-reactive protein, LDL, plasminogen activator inhibitor 1) were significantly reduced in those taking a garlic extract. In contrast, a small 2016 randomized controlled trial of 26 individuals with type 2 diabetes did not find improvements in endothelial function, vascular inflammation, oxidative stress, or insulin resistance after a 4-week treatment with an aged garlic extract.
Numerous observational studies have suggested that regular consumption of garlic might reduce the risk of developing certain malignancies, but conclusions from the few prospective trials are unclear, given the poor quality of studies, as suggested by a 2018 meta-analysis and systematic review. Another 2018 analysis of two large prospective US cohort studies found no association between high garlic intake and gastric cancer as well as Helicobacter pylori infection. Given the number of in vitro studies that have suggested its antitumor and apoptotic potential, garlic continues to be an HDS of significant interest in the treatment of different cancers.
There are small and few studies in this domain. One well-designed study of 75 people showed that daily ingestion of a fermented garlic extract for 12 weeks resulted in improvements in liver biochemical tests (serum GGT and ALT levels in adults) without reported adverse side effects. These findings may have implications for individuals with mild hepatic dysfunction, such as that seen with alcohol liver disease and nonalcoholic fatty liver disease.
et al. Garlic intake and gastric cancer risk: results from two large prospective US cohort studies. Int J Cancer. 2018 Sep 1;143(5):1047–53.
et al. Efficacy and safety of fermented garlic extract on hepatic function in adults with elevated serum gamma-glutamyl transpeptidase levels: a double-blind, randomized, placebo-controlled trial. Eur J Nutr. 2017 Aug;56(5):1993–2002.
et al. Allicin bioavailability and bioequivalence from garlic supplements and garlic foods. Nutrients. 2018 Jun 24;10(7):E812.
et al. The association of garlic with Helicobacter pylori
infection and gastric cancer risk: a systematic review and meta-analysis. Helicobacter. 2018 Oct;23(5):e12532.
et al. The effect of kyolic aged garlic extract on gut microbiota, inflammation, and cardiovascular markers in hypertensives: The GarGIC Trial. Front Nutr. 2018 Dec 11;5:122.
et al. Anti-hyperlipidemia of garlic by reducing the level of total cholesterol and low-density lipoprotein: a meta-analysis. Medicine (Baltimore). 2018 May;97(18):e0255.