Blood thinners: Can I still get blood clots?

My doctor said I could still get a blood clot even though I'm taking blood thinners. Is this true? I thought blood thinners were supposed to prevent blood clots.

- Denise / Texas

Mayo Clinic hypertension specialist Sheldon Sheps, M.D., and colleagues answer select questions from readers.

Answer

Yes, it's true. Blood thinners — such as aspirin, warfarin (Coumadin) and heparin derivatives — decrease your risk of blood clotting. But these medications must be taken precisely as directed to work safely and effectively. Taking too little of these drugs may not be effective, and taking too much can lead to serious bleeding. Regular blood tests may be required to assure proper dosing.

Also, in some cases, blood thinners may not be able to counter the strong clotting tendency of an underlying disease, such as cancer. So, clots may still form.

If you take a blood thinner, be sure to follow your doctor's advice on dosing and ask about foods and other medications — including over-the-counter drugs and herbal supplements — that may interfere with the action of the blood thinner.