Allicin is the predominant thiosulfinate in garlic (Allium sativum). Allicin is the chemical responsible for the typical and offensive odour of garlic.

Allicin is garlic's defence mechanism against attacks by pests. When the garlic plant is attacked or injured it produces allicin by an enzymatic reaction. The enzyme alliinase, converts the chemical alliin to allicin, which is toxic to insects and micro-organisms.

The antimicrobial effect of allicin is due to its chemical reaction with thiol groups of various enzymes.
The incidence of gastric ulcers is lower in populations with high garlic intake. Studies have confirmed that allicin has inhibitory activity on Helicobacter pylori, a bacteria responsible for the development of gastric ulcers.