A Comparison of Blood Thinners Warfarin and Heparin

Medically Reviewed by Minesh Khatri, MD on August 09, 2022
4 min read

Both are anticoagulant medications, or "blood thinners," that help stop your blood from thickening (clotting.) Natural clots help seal wounds on the inside and outside of your body. But unnecessary clots can cause problems. This is especially true when they form inside veins in your leg (deep vein thrombosis) or travel to:

Blood thinners don’t really thin your blood. They just interrupt your body’s natural clotting process. Warfarin and heparin act on different parts of this process.

Warfarin, also known by the brand name Coumadin, is a vitamin K antagonist. That means it works to stop your liver from processing vitamin K into substances, or "factors," that normally help clot your blood.

You get vitamin K from eating green leafy vegetables like broccoli, kale, Brussels sprouts, and spinach. Bacteria in your gut also produce it naturally.

Heparin drugs interfere with certain factors that activate thrombin, an enzyme that helps your blood clot. That means your blood clots less easily.


Doctors usually prescribe warfarin as a daily pill. Your dosage is based on the results of a blood test called a prothrombin time (PT) test. You may also hear this called international normalized ratio (INR) testing, because that ratio is the standard way the test results are reported.

The test tracks how quickly your blood clots. Your doctor will give you this test every few weeks and adjust your dose as needed.


Heparin is taken as a shot, and it works more quickly than Warfarin.

You get the type called unfractionated heparin (UFH) through an IV, usually in a hospital. The more you weigh, the more your doctor will give you. The hospital staff will check your blood regularly to make sure your dosage is correct.

You can inject low molecular weight heparin (LMWH) into your skin at home. You might know these drugs as dalteparin (Fragmin) or enoxaparin. As with UFH, your dose is based on your weight. These drugs work in a similar way to UFH. But they're more stable and don't require hospital staff to monitor you.

Pros: Warfarin has a long history of success going back to the 1950s. It’s inexpensive and easy to take in pill form. In case of injury or emergency surgery, doctors can easily reverse its effects with an antidote drug.

Cons: You and your doctor will need to monitor your blood to track the effects of your dose and your diet. Even so, sometimes it's hard to keep just the right amount of warfarin in your bloodstream. You may need to keep track of the vitamin K in your diet to help your doctor estimate the right dose for you. Warfarin may also interact with some prescription and over-the-counter medications and supplements.

Possible Side Effects of Warfarin:

As with all blood thinners, the most serious possible side effect of warfarin is bleeding that won’t stop.

UFH Pros: This kind of heparin works quickly to stop clots, and it wears off rapidly when medical staff stop giving it. You can reverse its effects with an antidote in case of emergency.

UFH Cons: You have to go to the hospital. And medical staff have to watch you closely and test your blood several times a day to make sure you're safe. When you get UFH through an IV for blood clots, you usually need to be in the hospital for 5-10 days.

LMWH Pros: You can inject this kind of heparin into your skin at home. You don't need constant blood testing. It's far more predictable than UFH, and you won't have to change or keep track of what you eat. It’s also safe for pregnant women because it doesn’t pass through to the baby.

LMWH Cons: You have to poke yourself with a needle every day, which can be uncomfortable and scary to some people. This type of heparin can also be expensive. And it’s not as easy to reverse its effects with an antidote as for some other types of anticoagulants.

Potential Side Effects of Heparin:

  • Swelling, redness, and irritation where you get the shot
  • Weaker bones
  • High potassium levels
  • High liver enzyme levels
  • Thrombocytopenia, a condition in which your blood has too few platelets. Platelets are cells that help your blood clot.

As with all blood thinners, the most serious possible side effect of heparin is bleeding that won’t stop.