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Side Effects of Coumadin, Plavix and Other Blood Thinners

Blood Thinners And Their Side Effects


Updated May 28, 2014

Intravenous therapy
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Blood thinners are commonly used in the prevention of strokes. This is especially important for people who have already suffered one stroke, as they have an increased risk of suffering another. In fact, about 30% of all strokes in a given year are repeat strokes, so stroke survivors must be diligent about stroke prevention. However, even if you've never suffered a stroke, but are at risk of getting one, you're likely taking a blood thinner.

Here is a list of common blood thinners used for stroke prevention, and their common side effects. For a complete review of these medications, please visit the latest edition of the physician’s desk reference.



This medication is a combination of aspirin and extended-release dipyridamole. Almost 40% of patients on this medication report feeling a headache. Other common side effects include abdominal pain, indigestion and diarrhea.


What to watch out for: You should stop taking Aggrenox and go to your doctor or to an emergency room if you find black or tarry-appearing stools, as this is a sign of intestinal bleeding.



Aspirin can irritate the stomach and intestines and cause indigestion, nausea, and vomiting. The “enteric coated,” or EC, form of aspirin is gentler on the intestines and produces milder side effects. Other less common side effects of aspirin include difficulty breathing and intestinal bleeding.


What to watch out for: If you find black or tarry-looking stools this is a sign of intestinal bleeding. This, and any other forms of abnormal bleeding, should prompt you to stop taking aspirin and to go to the nearest emergency room. Also go to the emergency room if you develop difficulty breathing while on aspirin. You should never give your children aspirin as they can develop a serious and often fatal disease called Reye’s Syndrome.

Learn more about aspirin safety



Also known as warfarin, this medication is used to prevent strokes in people who suffer from atrial fibrillation, people who suffer blood clotting disorders, and people who have mechanical heart valves. Coumadin can cause serious bleeding. To avoid this, people who take this medication must have routine blood testing to monitor their INR, or International Normalized Ratio. This is an international measure of coagulation which attributes a value of 1.0 to people with a normal ability to clot. As the INR increases, it reflects that a person is less likely to form blood clots. Patients with atrial fibrillation must maintain an INR of 2-3 in order to effectively decrease their risk of stroke.


Coumadin works by decreasing the amount of vitamin K available for use in the body, which in turn reduces the efficiency of blood clot formation by the body. This is why you should monitor your intake of foods that are rich in vitamin K. Consuming too much of these foods can prevent Coumadin from working properly and may leave you temporarily at a high risk of stroke. Some foods with high vitamin K content include spinach, lettuce, alfalfa sprouts, asparagus, broccoli, cauliflower, and cabbage. Doctors recommend that you eat the same amounts of these food items every day in order to maintain a steady level of vitamin K in your body. This will both ensure that Coumadin works properly, and that you will have a low risk of dangerous bleeding while you take Coumadin.

What to watch out for: By far the most common side effect of Coumadin is abnormal and sometimes profuse bleeding. Often the abnormal bleeding can occur inside the eye, and in the intestines. Because of this, people who take Coumadin must monitor stool color and seek medical attention if stools become black or tarry-appearing. Of course, bleeding and easy bruising can occur anywhere in the body.

More about Coumadin



Heparin is usually given in the hospital directly into a blood vessel (i.e., intravenously) in order to prevent blood clot formation, and to enhance the body’s ability to break down existing blood clots. For heparin to work safely, blood must be drawn periodically in order to make sure that its levels fall within a safe margin. The blood test performed to do this is called the partial thromboplastin time or PTT. The main side effects of heparin are bleeding and easy bruising. Irritation at the site of the injection can also occur. In some rare instances heparin can cause an allergic reaction.


What to watch out for: The most common and dangerous side effect of heparin is abnormal bleeding. Therefore, you must be on the lookout for black stools, which reflect intestinal bleeding, or for orange, pinkish or smoke-colored urine, as this indicates there is blood in the urine.

More about heparin treatment


Lovenox, also called enoxaparin, is a form of heparin called fractionated heparin. Lovenox does not require monitoring of its blood levels and it can be injected intramuscularly. People with chronic kidney disease should not use Lovenox as poor kidney function makes Lovenox accumulate in the blood. The side effects of Lovenox include skin irritation at the site of injection and nausea.

What to watch out for: Rarely people develop an allergic reaction to Lovenox and develop a rash. If severe, the reaction can cause swelling on the hands and lips, and difficulty breathing. If you develop any of these symptoms while on Lovenox you should go to an emergency room. For further symptoms (bleeding) caused by abnormal reactions to Lovenox please refer to the “what to watch out for” section under heparin (above.)



Common side effects of Plavix include stomach pain, muscle aches, dizziness, and headache. Easy bruising and nose bleeds can also occur. People who have stomach ulcers might develop intestinal bleeding, which can be life threatening.


What to watch out for: if you find black or tarry-looking stools this is a sign of intestinal bleeding. This and any other forms of abnormal bleeding should prompt you to discontinue the medication and to go to the nearest emergency room.

Source: DRUG-REAX® System [Internet database]. Greenwood Village, Colo: Thomson Micromedex. Updated periodically.



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