These drugs, also called anticoagulants, are the most common treatment for DVT. They can keep a clot from growing or breaking off, and they prevent new clots from forming. But they can't thin your blood, despite their name, or dissolve an existing clot.
In the hospital, your doctor may give you heparin at first, by a needle into your vein or as a shot. You may have to keep taking shots at home, once or twice daily. When you take heparin by IV, you'll need blood tests, too. But you won't need them when you're taking shots of low-molecular-weight heparin under your skin.
You may take warfarin (Coumadin) by pill once a day, starting while you're still on heparin, and then usually for 3 to 6 months or more. While you take it, you'll need regular blood tests to make sure you've got the right amount in your system -- too little won't prevent clots, too much makes dangerous bleeding more likely. It can also interact with other medicines, vitamins, and foods with a lot of vitamin K, which is another good reason to get your blood checked often.
Let your doctor know if you're pregnant, because warfarin can cause birth defects. You'll have to take something else.
Newer anti-clotting medicines, known as Xa inhibitors, work as well as warfarin for most people. You won't have to get blood tests, change your dose, or remember what foods you ate. These drugs may cause less bleeding than warfarin, but there's no medicine you can take to stop bleeding if it becomes a problem.