Allicin is an organosulfur compound obtained from garlic, a species in the family Alliaceae. It was first isolated and studied in the laboratory by Chester J. Cavallito and John Hays Bailey in 1944. When fresh garlic is chopped or crushed, the enzyme alliinase converts alliin into allicin, which is responsible for the aroma of fresh garlic. The allicin generated is very unstable and quickly changes into a series of other sulfur containing compounds such as diallyl disulfide. It exhibits antibacterial, antifungal, antiviral, and antiprotozoal activity. Allicin is garlic's defense mechanism against attacks by pests.

Allicin, one of the sulfur compounds of garlic, possesses antioxidant activity and is shown to cause a variety of actions potentially useful for human health. Allicin exhibits hypolipidemic, antiplatelet, and procirculatory effects. It demonstrates antibacterial, anticancer and chemopreventive activities. In addition, aged garlic extract possesses hepatoprotective and neuroprotective. But a factor that will limit the biological activity of allicin is its instability. Fresh crushed garlic cloves generated antibacterial activity and chemically detectable allicin, but this activity declines on a daily basis in aqueous and ethanol solutions. Allicin is also not bioavailable and will not get absorbed in the blood, even after ingesting large amounts of allicin.

Antimicrobial
The antimicrobial effect of allicin is due to its chemical reaction with thiol groups of various enzymes. The phytochemical inhibits bacteria and viruses, but also yeasts such as candida. By its antimicrobial activity, allicin may be an effective therapeutic candidate to promote ulcer healing. In vitro-studies have demonstrated the antimicrobial activity against various pathogens, such as Helicobacter pylori, Staphylococcus aureaus, Escherichia coli and Lancefield group B streptococci.

Anticancer
In vitro studies show that allicin inhibits the invasion and metastasis of human colon carcinoma cells. The phytochemical also exhibits antigenotoxic action. But the anticancer effect of allicin in humans remains uncertain, because of its low stability and poor bioavailability.

Heart health
Garlic has been suggested to improve heart health by lowering blood pressure, but scientific studies have provided conflicting results. One study showed that the protective role of allicin against atherosclerosis, is not only the direct result of its antioxidant activity but also of other mechanisms, such as lipoprotein modification, inhibition of LDL uptake and degradation by macrophages.