Why? Because they keep your blood from clotting the way it should. The culprits helping them do that are adenosine, allicin, and paraffinic polysulfides. Adenosine is familiar even to people who have only the haziest recollection of their high school biology classes. Adenosine triphosphate is the “energy molecule” of the cell. Cells rip off one of the phosphates and harvest energy from the process. On its own, adenosine plays a role in other bodily processes, including dilating the blood vessels, which is a no-no if you bleed too much.
Allicin and the polysulfides are responsible for that distinctive garlic and onion smell. Allicin is the smell we associate with chopped garlic. Scientists believe it’s created by garlic as a form of pest control, since the chemical discourages insects, funguses, and bacteria. Polysulfides are the long chains of sulfur that makes onions stinky. The fact that they also hack human blood is a coincidence.
The three compounds keep platelets from grouping together and forming clots. Under the right conditions, that’s a good thing — less clotting can prevent strokes, and anti-platelet drugs are often prescribed for people with a history of heart attacks, strokes, coronary artery disease, or chest pain. But once you’re on blood thinners, you don’t want your blood thinned any more. If you have hemophilia, or other clotting disorders, you also want your blood to be able to thicken as much as it can.