Anticoagulants, commonly known as blood thinners, prevent or treat blood clot formation. These drugs can be administered to prevent strokes and heart attacks caused by blood clots, or they can be used to treat deep vein thrombosis or pulmonary embolism (blood clot in the lungs). These drugs can cause serious side effects, so it's important to follow all instructions and read the package labels carefully for warnings and important information.

Warfarin

Warfarin reduces blood clot formation by blocking clotting factors from forming in the blood. This drug is used to prevent strokes, heart attacks and blood clot formation in the lungs, arteries and veins. Pregnant women should not take warfarin. According to Cerner Multum, the drug is known to cause birth defects, miscarriages, stillbirths and fetal bleeding. Possible side effects of warfarin include hair loss, vomiting, nausea, stomach pain, bloating and gas. Seek immediate medical attention for bleeding that will not stop, easy bruising or signs of internal bleeding, such as black, tarry stools or blood in the urine.

Heparin

Heparin reduces the blood's ability to clot so that clots cannot form in the veins, arteries or lungs. This drug can be used during surgical procedures that create a high risk of blood clot formation. It can also be used to treat heart, lung and blood vessels conditions that increase the likelihood of dangerous clots. Patients taking heparin should not take drugs that contain aspirin or ibuprofen, as these drugs can change the way heparin works and cause serious side effects. Easy bruising and bleeding are the most common side effects of this drug. According to the Mayo Clinic, heparin can also cause abdominal pain, bleeding from the gums, constipation, dizziness, headaches, joint pain, unexplained nosebleeds and heavy menstrual periods. If bleeding cannot be controlled or if there is blood in the urine or stool, seek immediate medical treatment.

Clopidogrel

Clopidogrel is used in the prevention or management of strokes and heart attacks. This drug is an anti-platelet drug, which means that it prevents platelets from sticking together and forming blood clots that can lead to strokes and heart attacks. People with bleeding ulcers, brain bleeds or other bleeding conditions may not be able to take this drug. According to the National Institutes of Health, clopidogrel can cause headache, dizziness, stomach pain, nosebleed, diarrhea, nausea and excessive tiredness. Seek immediate medical attention for allergic reactions to clopidogrel, uncontrollable bleeding or signs of internal bleeding, such as bleeding under the skin or blood in the urine or stool.

Ticlopidine

Ticlopidine is a medication used to prevent strokes by preventing excessive blood clotting. This drug should be used with caution in people who have liver disease, low blood cell counts, high cholesterol, kidney disease, high triglycerides and bleeding disorders. Patients should inform their doctors about all prescription and over-the-counter medications they are taking, as well as any vitamins or supplements. Possible side effects of this drug include diarrhea, stomach pain, gas, itching, upset stomach, vomiting, loss of appetite and headache. Seek immediate medical attention for unusual bleeding and bruising, skin rash, light-colored stools and signs of infection.

Enoxaparin

Enoxaparin is an anticoagulant used to prevent blood clots in people who are immobile because of major surgery or the need for bed rest. It can also be used with aspirin to prevent heart attacks or with warfarin to treat the formation of blood clots in the legs. This drug should be used with caution in people who have or have had heart infections, bleeding disorders, kidney disease, strokes, low platelet counts or ulcers. Enoxaparin may cause side effects, such as upset stomach, fever and burning or irritation at the injection site. Seek immediate medical attention for black or bloody bowel movements, unusual bruising, swollen feet and ankles, blood in the urine or unusual bleeding.