People with cancer are more at risk of blood clots. Here we explain who is at risk and why, what you should look out for and ways of preventing a blood clot.

What is a blood clot?

A blood clot is a plug of solidified blood that is the normal way the body controls bleeding. There is a complicated mechanism in the body for controlling blood clotting called the ‘clotting cascade’. This helps to make sure that a blood clot only forms when and where it needs to. But sometimes things go wrong and a clot can form when it isn’t supposed to.

The medical name for a blood clot is a thrombosis. A deep vein thrombosis (or DVT) is a blood clot that forms in the deep veins of the body, most often in the leg. It is dangerous because it can break up and bits of the clot can circulate in the blood. A small piece may then lodge in the smallest blood vessels in the body – in the heart, brain or lungs – and cause a heart attack, stroke or pulmonary embolus (a clot in the lungs). 

Cancer and blood clots

Cancer cells are capable of making a variety of natural body chemicals but in abnormal amounts. These can affect some of the clotting factors your body naturally makes as part of the clotting cascade. It can make your blood more likely to clot when it shouldn’t.

Some cancers increase the risk of blood clots more than others. They include lung, bowel, stomach, womb, pancreatic and ovarian cancers.

There is another rarer clotting condition called Disseminated Intravascular Coagulation or DIC. This is when multiple tiny clots can form throughout the body. So many clots are formed that the clotting factors in the blood are used up, so abnormal bleeding also occurs, such as nosebleeds, bleeding gums, blood in the urine and bruising. DIC can be triggered by severe infection (septicaemia) but it can also happen in particular types of cancer. It is mostly associated with acute leukaemia but can also happen in prostate cancer and cancers of the digestive system. DIC is an emergency so if you think you or someone you know has developed it, you need to contact your doctor or hospital.

Why does cancer treatment increase risk of blood clots?

Cancer treatment can also increase your risk of blood clots. Usually, if you have surgery, you’re asked to put on white elastic socks or stockings. These are to reduce your risk of a blood clot, caused because you will not be moving around as much.

Chemotherapy can make the blood more likely to clot. When it kills cancer cells, they release chemicals that affect blood clotting. 

Some hormone therapies can also increase risk of blood clots, particularly tamoxifen. Doctors don’t fully understand why this is, but we do know that it can affect levels of some clotting factors in the blood, increasing some and decreasing others.

What to look out for

The symptoms of a blood clot can include:

  • Pain in your calf or elsewhere in your leg for no apparent reason
  • Redness and swelling in your leg - just in the calf or in the whole leg
  • Feeling breathless - this might start suddenly or increase over time
  • Chest or back pain that gets worse on breathing in
  • Coughing up blood
  • Feeling faint

If you have symptoms of a clot in your lung, this is an emergency – call an ambulance.

Ways to help prevent a blood clot

Any of us are at increased risk of a blood clot if we’re not moving around as much as usual. So firstly, try and move around more by doing some gentle exercise like taking short walks regularly. If you’re less mobile, do some leg exercises as you would on a long plane flight – ankle circling or bending and straightening your legs. Try and do these every hour.

Drink plenty of fluids. If you’re dehydrated your circulation can slow and mean you’re more prone to clotting.

If you have an increased risk of blood clots because of another condition, or because you’ve had one before your doctor may recommend medicines to reduce your risk of a blood clot while you are having chemotherapy.

If you spot any signs that could be a DVT, tell your doctor. They would prefer you to bother them unnecessarily than fail to report a blood clot!


Related article: Why does cancer treatment increase your risk of bruising and bleeding? (Plus tips on how to manage)