The short answer
Cancer and some cancer treatments raise the risk of blood clots. Call 911 for trouble breathing, chest pain, coughing up blood, or fainting. Call your cancer care team right away about a newly swollen, painful leg or arm.
CDC says people with cancer, now or in the past, have a higher risk of a blood clot in a vein.
Call 911 for trouble breathing, chest pain, coughing up blood, a fast or irregular heartbeat, or fainting. These can be signs of a clot in the lung.
Tell your cancer doctor as soon as possible about swelling, pain, warmth or redness in a leg or arm.
Do not take an extra dose of a blood thinner on your own.
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This page is for people with cancer and their families. It explains why cancer raises the risk of blood clots, which signs to watch for, when to call 911, and what to ask your care team.
Call 911 now for these
These can be signs of a clot in the lung (a pulmonary embolism). CDC says a pulmonary embolism needs immediate medical attention because it can be deadly.
- Trouble breathing or sudden shortness of breath.
- Chest pain, often worse when you cough or take a deep breath.
- Coughing up blood.
- A fast or irregular heartbeat.
- Feeling lightheaded, or fainting.
Do not drive yourself. Do not take an extra dose of a blood thinner on your own.
Call your cancer care team right away for these
These can be signs of a clot in a leg or arm, called a deep vein thrombosis (DVT). CDC says to tell your cancer doctor as soon as possible.
- Swelling of a leg or arm.
- Pain or tenderness that is not from an injury.
- Skin that is warm, red, or a different color.
CDC notes that about half of people with a DVT have no symptoms at all. So a clot is possible even when nothing feels wrong.
If the swelling comes with any of the 911 signs above, call 911.
Why cancer raises clot risk
CDC says people who have cancer now, or had it in the past, have a higher risk of a blood clot in a vein. Some cancers carry more risk, including cancers of the pancreas, stomach, brain, lungs, uterus, ovaries, and kidneys. Some blood cancers, such as lymphoma and myeloma, also raise the risk.
Parts of cancer care add to the risk too. CDC lists:
- a hospital stay
- surgery
- chemotherapy
- hormone therapy
- catheters, such as a port or central line
Other things that raise risk include a past clot, a family history of clots, an inherited clotting disorder, not moving for long stretches, obesity, and smoking.
Some cancer drugs carry their own warning
Lenalidomide is one clear example. Its label has a boxed warning, the strongest kind, for clots in veins and arteries. In people with multiple myeloma taking it with dexamethasone, it raised the risk of DVT, pulmonary embolism, heart attack, and stroke. The label recommends medicine to prevent clots, chosen for each person's risks. Multiple myeloma covers where that combination is used.
If you are not sure whether your own treatment raises clot risk, ask your care team.
Arms count, not just legs
The National Heart, Lung, and Blood Institute (NHLBI) says a DVT usually forms in the lower leg, thigh, or pelvis. It can also form in the arm, especially when a large central line sits in the vein.
If you have a port or a PICC line, tell your team about new swelling, pain, or redness in that arm. Central line and PICC troubleshooting covers what else to watch for.
NHLBI says as many as 600,000 of these vein clots happen each year in the United States, from all causes.
Prevention and treatment
CDC's main advice is to talk with your cancer doctor about your risk. Tell them if you or anyone in your family has had a clot. Before surgery or before a catheter is placed, ask whether you need a blood thinner to prevent clots. CDC also mentions compression stockings, which are tight socks that help blood flow in the legs. Ask your team whether they suit you.
If a clot is found, the main treatment is a blood thinner, also called an anticoagulant. MedlinePlus says this can be a shot, such as heparin, or a pill, such as warfarin or a newer oral blood thinner. It says most people take a blood thinner for at least three months. Your team decides which one, and for how long.
Take a blood thinner exactly as prescribed. If you miss a dose, MedlinePlus says to call your provider for advice. Do not double up on your own.
Bleeding is the other risk
Blood thinners make clotting harder, so the warning signs run both ways. Call your care team right away for:
- Blood in your vomit, or bleeding from your mouth or nose.
- Red or pink urine, or black or bloody stools.
- Bleeding that does not stop after a few minutes.
- A bad headache, changes in how you see, or feeling confused or very sleepy.
- A serious fall, or hitting your head, while on a blood thinner.
Ask your team how long to wait for bleeding to stop before you call them or go to the emergency room. If you think it is an emergency, call 911.
Questions to ask your care team
- Does my cancer or my current treatment put me at higher risk for a clot?
- Do I need a blood thinner to prevent clots, for example before surgery or a new line?
- Which clot signs need a same-day call, and which need 911?
- If I am on a blood thinner, which bleeding signs bring me straight back?
- How long is my blood thinner planned for, and what would end it?
- Do I need to stop anything before my next procedure, and who tells the surgeon?
Creating a cancer symptom and call plan helps turn those answers into a written plan you can keep on the fridge.
Sources
- Centers for Disease Control and Prevention — Understanding Your Risk for Blood Clots with Cancer
- Centers for Disease Control and Prevention — Blood Clots (Deep Vein Thrombosis) and Cancer Survivors
- National Heart, Lung, and Blood Institute — Venous Thromboembolism
- MedlinePlus — Deep vein thrombosis
- MedlinePlus — Taking warfarin (Coumadin)
- National Cancer Institute — Bleeding and Bruising and Cancer Treatment
- DailyMed (National Library of Medicine) — REVLIMID (lenalidomide) prescribing information
Common questions
Which signs might mean a blood clot?
CDC lists swelling, pain or tenderness, and warm or red skin in a leg or arm for a clot in a limb. For a clot in the lung it lists trouble breathing, chest pain that is worse with a cough or deep breath, coughing up blood, a fast or irregular heartbeat, and lightheadedness or fainting. Other conditions can cause the same changes, so only your care team can tell what is happening.
When should I call an emergency number rather than the clinic?
Call 911 for signs of a clot in the lung: trouble breathing, chest pain, coughing up blood, a fast or irregular heartbeat, or fainting. For swelling or pain in a leg or arm without those signs, CDC says to tell your cancer doctor as soon as possible. Do not wait for a portal message when someone may be in danger.
What if I miss a dose of my blood thinner?
Do not take an extra dose on your own. MedlinePlus says to contact your provider for advice if you miss a dose.
What should my team know when we talk about clot risk?
Tell them about your cancer treatment, any central lines, recent surgery, time spent immobile, and any blood thinners you already take. If you are prescribed a blood thinner, ask for written instructions covering both bleeding and clot symptoms.
Questions to ask your doctor
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Sources last checked: 2026-09-27 what this meansLast updated: 2026-09-27Next planned review: 2027-01-22
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Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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