Unfractionated vs. Low-Molecular-Weight Heparin for DVT

Medically Reviewed by Melinda Ratini, MS, DO on April 29, 2022
4 min read

When you have deep vein thrombosis (DVT), a blood clot in a vein deep inside your body, doctors try to keep the clot from getting bigger and stop new clots from forming. One of their top priorities is to prevent a pulmonary embolism (PE), a serious condition in which part of the clot breaks off and travels to your lungs.

One drug they may use is heparin, an anticoagulant (sometimes called a blood thinner) that helps keep clots from forming. It reduces your risk of DVT and PE, and it treats these conditions once they begin to develop. Two types of heparins are commonly used for DVT: unfractionated heparin (UFH) and low-molecular-weight heparin (LMWH).

The two types of heparin work in a similar way. They don’t destroy existing blood clots. Instead, they keep them from getting bigger and prevent new ones. In the meantime, your body can slowly dissolve the blood clot you have.

Both UFH and LMWH do this by activating antithrombin. That’s a protein in your blood that limits the ability of thrombin (a blood enzyme that acts as a coagulant or clotting factor) to form clots.

But there are differences in:

  • How they’re given
  • How quickly they work
  • How long they keep working
  • Their costs

For these reasons and others, doctors may prefer one type over the other for certain people.

One of the main differences between the two types of heparins is how you get them. You get UFH through an IV, usually at a hospital, but you can inject yourself with LMWH at home.

UFH works right away – usually within a few minutes – and also wears off quickly. While treating you with this kind of heparin, your doctor will monitor you closely and give you frequent blood tests to see whether your dose needs to be adjusted. These tests, called activated partial thromboplastin time or aPTT, tell your doctor how long your blood takes to clot.

The fact that UFH wears off quickly once the infusion is stopped is a good thing if you have serious side effects. If you do, your doctor can reverse them quickly by giving you an antidote called protamine. UFH is also less expensive than LMWH and other types of heparin.

LMWH lasts longer than UFH and its effects are easier to predict. And you won’t need frequent blood tests. But because it stays in your body longer, any side effects you may have from LMWH are harder to reverse or remedy quickly.

Both types of heparin are effective at preventing blood clots. But there are a few reasons why your doctor may think one is a better option for you.

Because it stays in the body for less time and its effects are easier to reverse, doctors often prefer UFH for people who are more likely to have bleeding complications. It also puts less strain on your kidneys, so it’s a better choice if you’re very obese, significantly underweight, or have kidney disease or are at risk for it.

But since you get UFH via IV, it usually requires a hospital stay of 5-10 days, which may be challenging or impractical.

Because they don’t cross the placenta, heparins in general are safe when you’re pregnant. But obstetricians generally favor LMWH over UFH because there’s less risk of bleeding and other complications.

Possible side effects and complications are similar for UFH and LMWH. The most serious concern is uncontrolled bleeding. Other potential side effects for both include:

  • A reaction where the needle went in your skin
  • Decreased bone strength (this is more likely with UFH)
  • Elevated liver enzymes

Another complication with both UFH and LMWH is heparin-induced thrombocytopenia (HIT). This is a serious immune reaction that makes clots more likely in both your arteries and veins. HIT is life-threatening if not diagnosed and treated quickly. The risk of HIT is significantly higher with UFH. It’s also higher if you recently had surgery.

As always when you start a new medication, tell your doctor about any other medicines (both prescription and over-the-counter) you take. With both types of heparins, you need to be especially careful about taking:

  • Aspirin
  • Nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen and naproxen
  • Any other medication that can increase the risk of bleeding

Be careful to avoid injuries when you’re taking either kind of heparin. They could cause you to bleed too much. If you’re in a hospital, ask a nurse or aid to help you get out of bed to reduce your risk of a fall.