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Anticoagulants: Treatment of blood clots

Anticoagulant medications (blood thinners)

Doctors use anticoagulants, commonly called blood thinners, to treat blood clots. These medications slow the time it takes for blood to clot and also prevent growth of a clot. Therefore, anticoagulants prevent further clotting in most patients. The most common blood thinners used today are heparin, low molecular weight heparin, and warfarin.


Heparin is a strong, fast-acting anticoagulant (blood thinner). Heparin is given through a needle inserted in a vein (IV), but can also be given by injection under the skin. IV heparin works within minutes, and is usually given in the hospital.

Monitoring heparin with daily blood testing. Patients treated with heparin are monitored by a blood test daily to see if their dose is optimal. The blood test used to check a patient’s heparin level is the activated partial thromboplastin time (aPTT). The doctor adjusts the dose of heparin based on these blood test results. Levels can change, so frequent monitoring is necessary.

Advantages of heparin. Advantages of heparin are its low cost and fast action. This means blood is anticoagulated rapidly, so its action to prevent clotting is quick.

Disadvantages of heparin. The disadvantages of heparin include the need for frequent blood tests to check the levels of anticoagulation and the need for hospitalization to give heparin into the vein (IV). Patients are usually hospitalized for 5-10 days to treat a new clot.

Side effects of heparin. The side effects of heparin include:

  • Bleeding (most serious side effect)
  • Skin rash
  • Headache
  • Cold symptoms
  • Stomach upset
  • Loss of bone strength (less common; may occur when patients are on heparin for long periods of time, like several months)

A rare side effect of heparin is a condition called Heparin Induced Thrombocytopenia (HIT). HIT is sometimes incorrectly called “heparin allergy.” It occurs in a small number of patients, but is very serious, with symptoms that include increasing clotting or developing new clots, which can lead to deep vein thrombosis, pulmonary embolism, or death.

Low Molecular Weight Heparins (LMWH)

Low molecular weight heparins are similar to heparin, but can be taken at home through self-injection.
Names of LMWH. Those available in the United States are dalteparin, enoxaparin (Lovenox®) and tinzaparin (Innohep®).

Advantages of LMWH. Patients can be treated at home, because LMWH is given by injection under the skin rather than IV. This eliminates or reduces the time patients need to spend in the hospital to treat a clot. LMWH does not usually require blood tests.

Disadvantages of LMWH. LMWH is expensive. The side effects of LMWH are very similar to heparin, although HIT and loss of bone strength are much less common.


Warfarin (Coumadin®) is a pill for long-term anticoagulation. Heparin is usually given short-term, and warfarin is added in combination with heparin, before heparin is stopped. It can take 5-7 days (or longer) for the warfarin to reach an adequate level for it to be given alone. Once the warfarin dose is sufficient, heparin is stopped and the patient can go home from the hospital.

Advantages of warfarin. The advantage of warfarin is that it is inexpensive and is a pill.

Disadvantages of warfarin. Its most serious side effect is bleeding because it lengthens the time it takes for blood to clot.
Other side effects include headache, rash, hair loss, skin breakdown, purple toe syndrome, and elevated liver enzymes.
Sometimes these side effects will go away over time. It is important for you to discuss any side effects or unusual symptoms with your healthcare provider as soon as you notice them. If the side effects do not go away, your doctor may prescribe a different blood thinner.

Tips for taking warfarin

  • Same time each day. Take warfarin at the same time every evening. The evening is the best time to take warfarin because any needed dose change can be made the next day. If the evening is not a convenient time, take it at the same time that fits your schedule.
  • Consistency is key. Missed doses. If you miss taking a dose of warfarin, you have an 8 hour window when it is still safe to take it. Skip your dose if more than 8 hours goes by, and contact your doctor right away.
  • Warfarin and alcohol. Drinking a light or moderate amount of alcohol (1-2 glasses of wine or 1-2 beers per day) usually does not influence the INR. However, drinking a large amount can affect warfarin and increase your risk for bleeding.
  • Warfarin and food. It is important to keep intake of green leafy vegetables (high in vitamin K) steady.

Monitoring warfarin with regular blood tests.

Warfarin requires frequent monitoring, especially in the beginning. Once the correct dose of warfarin is established, monitoring frequency is about once a month. The blood test that monitors warfarin is called an INR (international normalized ratio). The INR measures how long it takes blood to clot. It standardizes results of prothrombin time, protime, or clotting time (names for different types of tests used in different labs). Most patients on warfarin do best with an INR between 2.0 and 3.0, considered the “therapeutic range.” A higher or lower INR range may be appropriate for certain patients.

  • INR too low. If an INR is below therapeutic range, the risk of clotting is higher.
  • INR too high. if an INR is above the therapeutic range, the risk of bleeding is higher. When patients first start warfarin, INR is tested two or three times a week. Once a patient is on a consistent dose of warfarin, four weeks is the usual time period between blood tests.

Effect of other prescribed or over-the-counter medications on warfarin.

Tell your doctor, nurse, or pharmacist whenever you start a new medication, as well as all those you take right now, both prescribed, over the counter, and herbal preparations. Some medications, especially antibiotics, can change your INR reading. Over-the-counter anti-inflammatory medications (such as aspirin or ibuprofen) may increase your risk of bleeding. You can usually take acetaminophen (Tylenol®). Tell your doctor if you take acetaminophen more than once a day or for longer than a week.

Tell your doctor all the medications you take

Herbal medicines can interact with warfarin and may change your INR. Some have anticoagulant effects and may put you at greater risk for bleeding. Herbal products do not always list all the ingredients and may not work as advertised. If you take herbal medication, tell your doctor which ones you take.

Reviewed 6/2017

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