Side Effects of Coumadin, Plavix and Other Blood Thinners

Blood thinners are commonly used in the prevention of strokes. This is especially important for people who have already experienced a stroke, because a stroke a risk factor for another stroke. In fact, about 30% of all strokes are repeat strokes. If you have had a stroke or a TIA, there is a chance that you will need to take a blood thinner.

Here is a list of common blood thinners used for stroke prevention, as well as some of their common side effects.


This medication is a combination of aspirin and extended-release dipyridamole. Almost 40% of people who use 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 is a blood thinner that can prevent stroke for people who have certain risk factors. Aspirin can irritate the stomach and intestines and may 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. You should get urgent medical attention if you develop difficulty breathing while on aspirin. You should never give children aspirin because children can develop a serious and often fatal disease called Reye’s Syndrome.


Also known as warfarin, this medication is used to prevent strokes for people who have heart disease such as atrial fibrillation or heart valve problems and people who suffer blood clotting disorders.

Coumadin can cause serious bleeding. If you are taking coumaden, you need to have routine blood testing to monitor you 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 the 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.  Consuming too much vitamin K can prevent Coumadin from working properly and may leave you temporarily at a high risk of stroke.

This is why you should monitor your intake of foods that are rich in vitamin K if you are taking coumaden. 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 to 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. The bleeding is most likely to occur inside the eye or 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. It is important to note that the bleeding and easy bruising can occur anywhere in the body.


Heparin is usually given in the hospital. Heparin has to be injected directly into a blood vessel (i.e., intravenously.) It is used to prevent blood clot formation and to enhance the body’s ability to break down existing blood clots.

For heparin to work safely, blood levels must be checked periodically in order to make sure that heparin 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.


Lovenox, also called enoxaparin, is a form of heparin called fractionated heparin. Lovenox does not require monitoring of 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 seek urgent medical attention. 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.


Safety of Direct Oral Anticoagulants: Insights from Postmarketing Studies, Villines TC, Peacock WF, Am J Emerg Med. 2016 Sep 28. pii: S0735-6757(16)30648-9

Edited by Heidi Moawad MD

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