Officially known as anticoagulants, blood thinners themselves don't thin the blood at all. The drugs work by reducing levels of vitamin K in the blood.
The liver uses vitamin K to produce blood-clotting agents. When you mix blood clots and narrowed blood vessels, it can lead to strokes and heart attacks. By reducing the supply of vitamin K to the liver, blood-thinning drugs reduce the body's ability to form potentially troublesome blood clots.
Patients who receive man-made valve replacements or stents are often put on blood thinners to prevent clots from forming on the new component. No matter the reason for taking them, doctors must be very careful in determining the dosage since too much of a blood-thinning agent can lead to internal bleeding. A word to the wise -- tell your doctor and your dentist that you're on blood thinners before any medical or dental procedure.