Dipyridamole is used to prevent platelets from forming blood clots. It works by blocking the action of an enzyme found in platelets called phosphodiesterase. Inside the platelets phosphodiesterase normally breaks down a chemical called cyclic AMP. Cyclic AMP plays a key role in blood clotting. If the level of cyclic AMP in the platelets is high this prevents the platelets from clumping together. Dipyridamole causes the levels of cyclic AMP in the platelets to rise, because it stops phosphodiesterase from breaking it down. This means that dipyridamole stops the platelets from clumping together and causing a blood clot.