Medical Oncologist
Blood vessels and organ effects
Blood clots during chemotherapy
Cancer itself and some cancer treatments raise the risk of blood clots. A clot in a leg vein causes swelling, pain, warmth or redness, usually in one leg. A clot that travels to the lungs causes sudden breathlessness, chest pain or a racing heart, and is an emergency. Clots are treatable, especially when found early.
NG
Medically reviewed by Dr. Naresh GunduConsultant Medical Oncologist · MBBS, DNB (Internal Medicine), DM (Medical Oncology, AIIMS) · last reviewed September 2026, next review due September 2027
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Get emergency help immediately for any of these
Sudden breathlessness · chest pain that is worse when breathing in · coughing up blood · a racing heartbeat with dizziness or fainting · one leg or arm that is suddenly swollen, painful, warm or red, especially with breathlessness · sudden severe headache, weakness of one side, face drooping or slurred speech. Go to the nearest emergency department and say clearly that the person has cancer and is on or has had chemotherapy.
The short answer
Why are blood clots more likely during chemotherapy?
Cancer and its treatment both make the blood more likely to clot. Cancer cells release substances that activate clotting, some chemotherapy and hormone medicines add to this, and people having treatment often move less, become dehydrated, have surgery, or have a central line or port in a large vein. Together, these make blood clots one of the more common serious complications during cancer treatment, though most people never have one.
The most common clot forms in a deep vein of the leg, called a deep vein thrombosis or DVT. It usually causes swelling, pain, warmth or redness in one leg, often the calf. The danger is that part of the clot can break off and travel to the lungs, called a pulmonary embolism, which causes sudden breathlessness, chest pain or collapse. This is an emergency. Clots are treatable with blood-thinning medicines, and early treatment reduces the risk of serious harm.
Arm swelling matters too
Clots can form around a central line or port, causing swelling, pain or visible veins in the arm, neck or chest on that side.
Some cancers carry higher risk
Pancreatic, stomach, lung, brain, ovarian and blood cancers, and cancers that have spread, are more often linked with clots.
You may be offered prevention
People at higher risk, such as after surgery, may be given blood-thinning injections or tablets for a period.
Know the signs of a clot, and tell your team about any previous clot or family history of clotting before treatment starts.Not sure whether this applies to you?
Ask an oncologistKnow the signs
Signs of a blood clot and how urgent they are
- Swollen, painful leg: same day, urgently
- Swelling of one leg, calf pain or tenderness, warmth, redness or darker skin. Call your helpline or go to hospital the same day.
- Sudden breathlessness: emergency
- Unexplained breathlessness, especially if sudden, with or without chest pain, can mean a clot in the lungs.
- Chest pain on breathing in: emergency
- Sharp pain that worsens with a deep breath or cough needs emergency assessment.
- Arm, neck or face swelling near a line: same day
- Swelling, pain, heaviness or prominent veins on the side of a central line or port.
- Coughing up blood: emergency
- Blood-streaked phlegm with breathlessness or chest pain needs immediate care.
- Stroke signs: emergency
- Face drooping, arm weakness, slurred speech or sudden severe headache can mean a clot affecting the brain.
Practical
Ways to lower your risk
Everyday steps, alongside any prevention your team offers.
Keep moving
Short walks around the house, standing up regularly and ankle exercises when sitting keep blood flowing in the legs.
When sitting for long
- Point and flex your feet often
- Stand and walk every hour if you can
Drink enough fluid
Dehydration thickens the blood. Sip fluids through the day, especially with vomiting, diarrhoea or hot weather, unless advised to limit fluids.
Plan long journeys
On long bus, train, car or plane trips to treatment, take breaks, walk when you can and do leg exercises.
Also helpful
- Loose, comfortable clothing
- An aisle seat to stretch your legs
Use prevention as prescribed
If given blood-thinning injections, tablets or compression stockings, use them exactly as your team advises and ask how to handle missed doses.
Stop smoking
Smoking raises clot risk. Ask your team for support to quit during treatment.
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Being straight with you
What this page cannot tell you
It cannot tell you whether you have a clot. That needs examination and tests such as an ultrasound scan of the leg or a CT scan of the chest. It also cannot tell you whether you need blood-thinning medicine to prevent clots, which depends on your cancer, treatment, platelet count and bleeding risk.
It cannot advise on blood thinners. They need careful management during chemotherapy, especially when platelet counts fall. Never start, stop or change a blood thinner without advice from your team.
Blood thinners and bleeding
If you take a blood thinner, report nosebleeds, bleeding gums, black stools, blood in urine or unusual bruising promptly.
Low platelets change the plan
When platelets are low, your team may pause or reduce blood thinners. Keep appointments for blood tests.
Some medicines raise clot risk
Thalidomide and lenalidomide, some hormone treatments such as tamoxifen, certain targeted medicines and growth factors can add to the risk.
Previous clots matter
A past clot, a family history of clots, or a known clotting condition should be shared before treatment starts.
Surgery raises the risk
After cancer surgery, the risk is higher for several weeks. Prevention may be continued after you go home.
Leg swelling has other causes
Swelling in both legs can come from fluid, heart problems or medicines. One-sided swelling is more suspicious of a clot. Both need checking.
Tell every doctor about your clot history
Dentists, surgeons and other doctors need to know about clots and blood thinners before procedures.
Clots after treatment ends
The risk falls after treatment, but remains higher than normal for a while. Keep watching for signs.
Family should know the signs
Relatives who know what a clot looks like can act quickly, especially if the patient is too unwell or breathless to explain.
Injections at home can be learned
If you are prescribed blood-thinning injections, a nurse can teach you or a relative how to give them into the skin of the tummy. Rotate the site, keep a record of each dose, and ask what to do if you miss one or notice large bruises.
Compression stockings must fit well
Stockings that are too tight can cut into the skin and cause harm. Have them measured, put them on smoothly without rolling, and remove them daily to check the skin, especially if your legs are numb.
What to do next
Learn the signs of a clot with your family and act quickly on them. Tell your team about any previous clots, keep moving and drink enough fluid, use any prevention exactly as prescribed, and never change blood thinners without advice.
Commonly believed
Four beliefs about blood clots
"A painful calf is just a muscle cramp."
It might be, but during cancer treatment one-sided calf pain or swelling needs checking for a clot.
"Breathlessness is just weakness from chemotherapy."
Sudden or unexplained breathlessness can mean a clot in the lungs and needs emergency assessment.
"Resting in bed is safest during treatment."
Long periods of bed rest increase clot risk. Gentle, regular movement helps prevent clots.
"Blood thinners can be stopped when I feel better."
Stopping early can allow clots to return. Only your team should decide when to stop.
Questions we are asked
Common questions about blood clots during chemotherapy
Why does chemotherapy increase clot risk?
Cancer activates clotting, some medicines add to the effect, and reduced movement, dehydration, surgery and central lines all raise the risk.
What are the signs of a clot in the leg?
Swelling of one leg, calf pain or tenderness, warmth, redness or darker skin. Report it the same day.
What are the signs of a clot in the lungs?
Sudden breathlessness, chest pain on breathing in, a racing heart, coughing up blood or fainting. This is an emergency.
How urgent is a suspected clot?
Leg or arm swelling needs same-day assessment. Breathlessness, chest pain or fainting need emergency care immediately.
Can a port or central line cause a clot?
Yes. Swelling, pain or visible veins in the arm, neck or chest on the line side should be reported the same day.
Will I be given blood thinners to prevent clots?
Some people at higher risk are, for example after surgery. Your team weighs clot risk against bleeding risk.
How are clots treated?
Usually with blood-thinning injections or tablets, adjusted for your platelet count and treatment. Your team will monitor you closely.
How can I lower my risk?
Keep moving, drink enough fluid, take breaks on long journeys, stop smoking, and use any prescribed prevention exactly as advised.
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MBBS, MD(General Medicine), DM(Medical Oncology)(Adyar,Chennai), ECMO, MRCP SCE(UK)
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MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)
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MBBS (AIIMS), MS (Surgery) (AIIMS), DNB (Surgical Oncology), MRCS (Edinburgh)
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Dr. Vinay Mamidala
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MBBS, MS (General Surgery), DrNB (Surgical Oncology), FALS Oncology
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Sources
- Cancer.Net (ASCO) — Blood Clots and Cancer
- Macmillan Cancer Support — Blood clots
- NHS — Deep vein thrombosis
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
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