In in vitro studies, garlic has been found to have antibacterial, antiviral, and antifungal activity. However, these actions are less clear in vivo. Garlic is also claimed to help prevent heart disease (including atherosclerosis, high cholesterol, and high blood pressure) and cancer.^[1][30] Garlic is used to prevent certain types of cancer, including stomach and colon cancers. In fact, countries where garlic is consumed in higher amounts, because of traditional cuisine, have been found to have a lower prevalence of cancer.^[31] Animal studies, and some early research studies in humans, have suggested possible cardiovascular benefits of garlic. A Czech study found garlic supplementation reduced accumulation of cholesterol on the vascular walls of animals.^[32] Another study had similar results, with garlic supplementation significantly reducing aortic plaque deposits of cholesterol-fed rabbits.^[33] Another study showed supplementation with garlic extract inhibited vascular calcification in human patients with high blood cholesterol.^[34] The known vasodilative effect of garlic is possibly caused by catabolism of garlic-derived polysulfides to hydrogen sulfide in red blood cells (RBCs), a reaction that is dependent on reduced thiols in or on the RBC membrane. Hydrogen sulfide is an endogenous cardioprotective vascular cell-signaling molecule.^[35]

A randomized clinical trial funded by the National Institutes of Health (NIH) in the United States and published in the Archives of Internal Medicine in 2007 found the consumption of garlic in any form did not reduce blood cholesterol levels in patients with moderately high baseline cholesterol levels.^[36][37] According to Heart.org, "despite decades of research suggesting that garlic can improve cholesterol profiles, a new NIH-funded trial found absolutely no effects of raw garlic or garlic supplements on LDL, HDL, or triglycerides... The findings underscore the hazards of meta-analyses made up of small, flawed studies and the value of rigorously studying popular herbal remedies". In an editorial regarding the initial report's findings, two physicians from Weill Cornell Medical College of Cornell University, pointed out that there may "be effects of garlic on atherosclerosis specifically that were not picked up in the study".^[38]

Allium sativum has been found to reduce platelet aggregation^{[39][40][41][42]} and hyperlipidemia.^[42][43][44]

In 2007, the BBC reported Allium sativum may have other beneficial properties, such as preventing and fighting the common cold.^[45] This assertion has the backing of long tradition in herbal medicine, which has used garlic for hoarseness and coughs.^[46] The Cherokee also used it as an expectorant for coughs and croup.^[47] However, in contrast to these earlier claims concerning the cold-preventing properties of garlic, a 2012 report in the Cochrane Database of Systematic Reviews concludes that "there is insufficient clinical trial evidence regarding the effects of garlic in preventing or treating the common cold. A single trial suggested that garlic may prevent occurrences of the common cold but more studies are needed to validate this finding. Claims of effectiveness appear to rely largely on poor-quality evidence."^[48]