Blood clots during cancer treatment

Why cancer raises the chance of blood clots, the warning signs, and how clots are treated.

Rosa: Hi, I'm Rosa. Cancer and some treatments can raise the chance of blood clots. Let's learn the warning signs, so you know when to get help fast.

People with cancer are more likely to get blood clots. Cancer itself, some treatments, and surgery can raise the risk. So can a port or catheter, a hospital stay, and moving less.

The most common clots form in the deep veins of the leg. This is called a deep vein thrombosis, or DVT. A piece of a clot can travel to the lungs. This is called a pulmonary embolism, or PE. A PE can be very serious.

Clots are usually found with an ultrasound of the arm or leg, or a CT scan of the chest. Sometimes a clot is found by chance on a scan done to check the cancer.

Clots are treated with blood thinners. These come as pills or as shots under the skin. Blood thinners stop the clot from growing and help prevent new ones. Over time, your body slowly breaks down the old clot. People with cancer usually take them for at least several months. Many take them as long as the cancer is active.

Some people at high risk are offered a blood thinner to prevent clots during chemotherapy. Your care team will weigh the chance of a clot against the chance of bleeding.

If you cannot take blood thinners, a small filter may be placed in a large vein to catch clots. This is not used for most people.

Signs of a clot in the leg or arm

Swelling in one leg or arm, pain or tenderness (often in the calf), warmth, or redness. Call your care team the same day.

Signs of a clot in the lungs: call 911

Sudden shortness of breath or chest pain that gets worse with a deep breath. A fast heartbeat, feeling faint, or coughing up blood.

Living with blood thinners

Take them exactly as prescribed. Do not skip or stop them without talking to your team. Tell every doctor and dentist you take one, especially before a procedure.

Bleeding to watch for

Call your team for blood in your urine, black or bloody stools, or nosebleeds that will not stop. Go to the emergency room or call 911 for vomiting blood, a sudden bad headache, or after a fall or any hit to the head.

Medicine check

Some pain relievers, supplements, and cancer drugs can change how blood thinners work. Ask your pharmacist before starting anything new, including aspirin, ibuprofen, or fish oil.

Lowering your risk

Stay as active as you can. Walk often. On long car rides or flights, get up or move your legs every hour or two. Drink fluids unless your team tells you otherwise.

Questions to ask

Am I at high risk for clots? Should I take something to prevent them? Which blood thinner is right for me and for how long? What should I do if I miss a dose?

• Learn the signs of a clot in the leg and in the lungs.
• Keep a current list of all your medicines and supplements.
• Tell your dentist and every doctor if you take a blood thinner.
• Ask what to do before any surgery or procedure.

Last updated: 2026-10-27. Medically reviewed by Veli Bakalov, MD, medical oncologist (October 2026). General education from Step by Step, the patient portal of HemeOncBuddy.