http://www.gregjoneslaw.com The most serious Plavix side effect is gastrointestinal hemorrhage or bleeding ulcers. These sores can be caused or irritated by medications. Plavix prevents the formation of platelets in the blood, the part of the blood that forms clots, so that the risk of bleeding in the gastrointestinal tract is increased. Gastrointestinal bleeding can be fatal. Researchers believe that blood-thinning drugs like Plavix trigger an antibody in susceptible people that causes the formation of platelet and protein clots. If you are taking Plavix, you might want to discuss the risk of Plavix side effects with your health care provider. If you or a family member has been injured from using Plavix, you may be able to recover damages for that injury.