Blood thinners are medicines that prevent blood clots from forming. They do not break up clots that you already have. But they can stop those clots from getting bigger. It's important to treat blood clots, because clots in your blood vessels and heart can cause heart attacks, strokes, and blockages.

You may need a blood thinner if you have:

• Certain heart or blood vessel diseases
• An abnormal heart rhythm called atrial fibrillation
• A heart valve replacement
• A risk of blood clots after surgery
• Congenital heart defects

There are different types of blood thinners:

• Anticoagulants, such as heparin or warfarin (also called Coumadin), slow down your body's process of making clots.
• Antiplatelets, such as aspirin and clopidogrel, prevent blood cells called platelets from clumping together to form a clot. Antiplatelets are mainly taken by people who have had a heart attack or stroke.

When you take a blood thinner, follow the directions carefully. Blood thinners may interact with certain foods, medicines, vitamins, and alcohol. Make sure that your health care provider knows all of the medicines and supplements you are using.

You may need regular blood tests to check how well your blood is clotting. It is important to make sure that you're taking enough medicine to prevent clots, but not so much that it causes bleeding.

Bleeding is the most common side effect of blood thinners. They can also cause an upset stomach, nausea, and diarrhea.

Other possible side effects can depend on which type of blood thinner that you are taking.

Call your provider if you have any sign of serious bleeding, such as:

• Menstrual bleeding that is much heavier than normal
• Red or brown urine
• Bowel movements that are red or black
• Bleeding from the gums or nose that does not stop quickly
• Vomit that is brown or bright red
• Coughing up something red
• Severe pain, such as a headache or stomachache
• Unusual bruising
• A cut that does not stop bleeding
• A serious fall or bump on the head
• Dizziness or weakness