In test tube studies garlic has been found to have antibacterial, antiviral, and antifungal activity. However, these actions are less clear in humans. Garlic is also claimed to help prevent heart disease (including atherosclerosis, high cholesterol, and high blood pressure) and cancer.[16] Animal studies, and some early investigational studies in humans, have suggested possible cardiovascular benefits of garlic. A Czech study found that garlic supplementation reduced accumulation of cholesterol on the vascular walls of animals.[17] Another study had similar results, with garlic supplementation significantly reducing aortic plaque deposits of cholesterol-fed rabbits.[18] Another study showed that supplementation with garlic extract inhibited vascular calcification in human patients with high blood cholesterol.[19] The known vasodilative effect of garlic is possibly caused by catabolism of garlic-derived polysulfides to hydrogen sulfide in red blood cells, a reaction that is dependent on reduced thiols in or on the RBC membrane. Hydrogen sulfide is an endogenous cardioprotective vascular cell-signaling molecule.[20]

Although these studies showed protective vascular changes in garlic-fed subjects, 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 that the consumption of garlic in any form did not reduce blood cholesterol levels in patients with moderately high baseline cholesterol levels.[21][22]

According to the 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."[23]

In 2007, the BBC reported that Allium sativum may have other beneficial properties, such as preventing and fighting the common cold.[24] This assertion has the backing of long tradition in herbal medicine, which has used garlic for hoarseness and coughs.[25] The Cherokee also used it as an expectorant for coughs and croup.[26]

Allium sativum has been found to reduce platelet aggregation[27][28][29][30] and hyperlipidemia.[30][31][32]

Garlic is also alleged to help regulate blood sugar levels. Regular and prolonged use of therapeutic amounts of aged garlic extracts lower blood homocysteine levels and has shown to prevent some complications of diabetes mellitus.[33][34] People taking insulin should not consume medicinal amounts of garlic without consulting a physician.

In 1858, Louis Pasteur observed garlic's antibacterial activity, and it was used as an antiseptic to prevent gangrene during World War I and World War II.[35] More recently, it has been found from a clinical trial that a mouthwash containing 2.5% fresh garlic shows good antimicrobial activity, although the majority of the participants reported an unpleasant taste and halitosis.[36]

In modern naturopathy, garlic is used as a treatment for intestinal worms and other intestinal parasites, both orally and as an anal suppository. Garlic cloves are used as a remedy for infections (especially chest problems), digestive disorders, and fungal infections such as thrush.[37][38]

Garlic has been used reasonably successfully in AIDS patients to treat cryptosporidium in an uncontrolled study in China.[39] It has also been used by at least one AIDS patient to treat toxoplasmosis, another protozoal disease.[40]

Garlic supplementation in rats, along with a high protein diet, has been shown to boost testosterone levels.[41]