Aspirin is called a blood thinner because it prevents blood platelets from sticking together and forming clots, according to MedlinePlus. The medicine does not actually thin blood.
There are two types of blood thinners: anticoagulant and antiplatelet. Anticoagulants, such as warfarin, cause blood to take longer to clot, according to Healthline. Antiplatelets, such as aspirin, interfere with blood proteins that promote clotting. These drugs are commonly taken to keep blood from clotting in an artery, which can cause a heart attack. They can also be used to prevent blood clots from forming in the brain, which may cause a stroke.