Medical Oncologist
Blood counts
Breathlessness during chemotherapy
Most often because the red cells carrying oxygen have fallen, so the body breathes harder to compensate. That builds gradually and shows first on stairs. But breathlessness has other causes during treatment that are urgent — a clot in the lung, a chest infection, fluid, or effects on the heart. Breathlessness that came on over hours, or is present sitting still, is a different situation.
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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The short answer
Why have you become breathless during chemotherapy?
Most often because the red cells that carry oxygen have fallen, so the body breathes harder to compensate. That builds gradually over cycles and shows up first on stairs or slopes. It is common, it is measurable on a blood test, and it is treatable.
But breathlessness has other causes during treatment that are urgent — a clot in the lung, a chest infection, fluid around the lungs, effects on the heart, or a reaction to a drug. Some of those need the same day rather than the next appointment.
The distinction that matters
Breathlessness that has crept up over weeks, only on effort, is usually the red cells. Breathlessness that came on over hours or days, or that is present sitting still, is a different situation and should be treated as urgent until someone has looked.
Never assume breathlessness during chemotherapy is just tiredness. It is one symptom that genuinely warrants being checked.
!
Go to the emergency department now
Breathlessness at rest, or that came on suddenly · chest pain or tightness, especially with breathing · coughing up blood · a racing heart that will not settle · pain and swelling in one leg · lips or fingertips looking blue · fainting or confusion · a fever with breathlessness · unable to speak a full sentence. Go to the nearest emergency department and say clearly that the person is on chemotherapy. A clot in the lung and a chest infection during low counts are both treatable and both get worse with waiting.
Not sure whether this applies to you?
Ask an oncologistWhat it can be
The causes your team will be thinking about
Listed so you understand why breathlessness gets investigated rather than simply attributed to treatment.
A low haemoglobin
The commonest cause and the least alarming. Fewer red cells means less oxygen carried, so effort feels harder. It builds gradually and is confirmed with a blood test taken before each cycle.
Usually
- Gradual, over weeks
- Only on effort at first
- With tiredness and dizziness
A clot in the lung
Cancer and chemotherapy both raise the risk of clots. Sudden breathlessness, chest pain worse on breathing in, or a swollen painful leg are the warning signs. This is an emergency and it is treatable.
Sudden breathlessness is a clot until proven otherwise.A chest infection
Far more likely and far more serious during the low-count days. A fever alongside breathlessness needs an emergency department the same day, not antibiotics from the medicine drawer.
Fluid around the lungs or the heart
Causes breathlessness that worsens lying flat, often with swelling elsewhere. It is drained or treated once identified, and it needs a scan rather than guesswork.
Effects on the heart
Some chemotherapy drugs can affect how well the heart pumps, which is why an echo test is done before and sometimes during treatment. Breathlessness with ankle swelling or difficulty lying flat belongs here.
Effects on the lungs, or a reaction
A few drugs can inflame the lung tissue, causing breathlessness with a dry cough. Reactions during an infusion also cause it. Both are recognised and both need reporting promptly.
Being useful
What to tell them, and what will happen
The most useful thing you can say is how it has changed. "I could walk to the gate a month ago and now I stop halfway" tells a doctor more than any description of how breathless you feel.
The five things worth reporting
When it started and how quickly. What brings it on — stairs, walking on the flat, or nothing at all. Whether it is worse lying down. Whether there is chest pain, cough, fever or leg swelling with it. And whether anything makes it better.
What the assessment involves
Usually oxygen levels measured on a finger, a listen to the chest, blood tests including the haemoglobin, and a chest X-ray. A scan of the lungs if a clot is suspected, and an echo if the heart is in question. None of it is unpleasant, and most of it is quick.
Do not adapt quietly
The commonest pattern is that people stop using the stairs, stop walking to the shop, and rearrange life around the symptom until nobody notices it. By the time it is mentioned the level has fallen a long way. Report the change while it is still a change.
If you use an inhaler for asthma, keep using it and tell the team. Do not assume this is your asthma without checking.Living with it
What helps while the cause is being dealt with
Pace rather than push. Do the important things at the time of day when you have most in the tank, sit down for tasks you would normally stand for, and break journeys. Fighting it produces exhaustion without achieving more.
Position and air
Sitting upright, leaning slightly forward with your arms supported, is the position most people find easiest. A window open or a small fan directed at the face genuinely reduces the sensation of breathlessness, and costs nothing to try.
Breathe out slowly
When breathlessness builds, the instinct is to gasp in. Breathing out slowly through pursed lips, for longer than you breathe in, settles it faster. Ask for a physiotherapist if this is a regular problem — they teach it properly.
Keep moving within your limits
Complete rest weakens muscle and makes breathlessness worse over time. Short, gentle walks with rests are better than avoiding all effort. Ask your team what is reasonable for you, particularly during the low-count days.
Tell the household
Families read this as low mood or giving up and respond by pushing. Explaining that it is a measurable blood or lung problem rather than a state of mind usually changes how everyone behaves.
Do not start oxygen at home on anyone's advice but your team's. It is not a substitute for finding the cause.Leave a number, we will call you
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Being straight with you
What this page cannot tell you
It cannot tell you what is causing your breathlessness. The causes here range from the entirely manageable to the genuinely urgent, and they can look similar from the outside. Separating them needs oxygen levels, a chest examination, blood tests and usually imaging.
It also cannot tell you whether yours can wait. The general guide is that gradual, effort-related breathlessness can be raised at the next contact while sudden or resting breathlessness cannot — but if you are uncertain, treat it as urgent. That is the safer error.
What to do next
Write down what you could do a month ago and cannot do now. Ask what your haemoglobin is doing. Report any chest pain, cough, fever or leg swelling alongside it. And if it is present at rest, came on suddenly, or comes with a fever, go in today.
Questions we are asked
Common questions about breathlessness
Is breathlessness just part of chemotherapy?
No, and treating it that way is how serious causes get missed. It is common and often due to a low haemoglobin, which is itself treatable. But clots, chest infections, fluid and heart effects all present this way, so it warrants being checked rather than accepted.
How do I know if it is urgent?
Speed and setting. Gradual breathlessness only on effort can usually be raised at your next contact. Sudden breathlessness, breathlessness at rest, or breathlessness with chest pain, fever, a swollen leg or coughing blood needs an emergency department the same day.
Why would I get a clot?
Cancer itself raises the risk of clots, and chemotherapy, reduced activity and having a central line add to it. That is why sudden breathlessness or a swollen painful leg is taken so seriously. Clots are treatable, and the treatment works far better started early.
Will a transfusion fix it?
If a low haemoglobin is the cause, most people feel considerably less breathless within a day of a transfusion. If it does not help at all, that is important information — it suggests the breathlessness is coming from something else that needs investigating.
Should I use oxygen at home?
Not on your own initiative or on a relative's advice. Oxygen is not a general remedy for breathlessness and using it can delay finding the actual cause, which may be treatable. If your team prescribes it, they will arrange it properly with instructions.
Does a fan really help?
Yes, more than people expect. Cool air moving across the face reduces the sensation of breathlessness, even when the oxygen level is unchanged. A small handheld fan or an open window is worth trying, alongside sitting upright with your arms supported.
I have asthma. How do I tell the difference?
You often cannot, which is why it should be checked rather than assumed. Keep using your inhaler as prescribed and tell your oncology team you have asthma. Breathlessness that does not respond to your usual inhaler, or comes with a fever, needs assessment the same day.
Can anxiety cause it?
Anxiety genuinely causes breathlessness, and it is common during cancer treatment. But it is a conclusion to reach after the physical causes have been checked, not before. Ask for it to be looked into rather than accepting an explanation that has not been tested.
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Sources
- Cancer Research UK — Shortness of breath and cancer
- Macmillan Cancer Support — Breathlessness
- National Cancer Institute — Chemotherapy and You
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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