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Why cotrimoxazole is given during blood cancer treatment | CION Cancer Clinics
Cotrimoxazole is given to prevent PJP, a lung infection that can become serious in people whose immune system is weakened by blood cancer treatment. It is common with lymphoblastic leukaemia treatment, some chemotherapy medicines, long steroid courses and after a transplant. This page explains who gets it, what to watch for, what replaces it when it does not suit, and what only your haematologist can decide. At CION Cancer Clinics, our haematology team supports patients and families through treatment, cost questions and life at home.
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Medically reviewed by Dr. Basudev PokhrelConsultant Haematologist · last reviewed September 2026, next review due September 2027
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On this page
- Why is an antibiotic given when you do not have an infection?
- Which treatments usually come with cotrimoxazole?
- What side effects should you tell the team about?
- What is used instead when cotrimoxazole is not possible?
- What do families often misunderstand about this tablet?
- What can this page not tell you?
- Common questions about cotrimoxazole in blood cancer
The short answer
Why is an antibiotic given when you do not have an infection?
Cotrimoxazole is given to stop one particular lung infection, called PJP, before it can start. Many blood cancer treatments weaken the part of the immune system that normally keeps this germ quiet, and the infection can become serious quickly once it takes hold.
What PJP is
PJP stands for Pneumocystis jirovecii pneumonia. Pneumocystis is a fungus-like germ that most people carry harmlessly in their lungs from childhood. It only causes illness when a type of white cell called the T-cell, or CD4 cell, is badly weakened. Then it can cause a dry cough, rising breathlessness and fever that build over days.
Why this medicine
Cotrimoxazole is an old, well-studied combination of two antibiotics, sulfamethoxazole and trimethoprim. Taken as prevention, it is very good at keeping PJP away, and it also gives some protection against a few other infections. It is taken on set days as your team prescribes, often not every day. The schedule is written on your prescription.
This is prevention, not treatment. You take it while you feel well, and that is exactly when it is doing its job.Who it is for
Which treatments usually come with cotrimoxazole?
The decision rests on how much a treatment weakens T-cells, not on the name of the cancer. These are common situations, not a complete list.
Acute lymphoblastic leukaemia
Treatment runs for a long time and uses steroids repeatedly, so cover is usually continued through most of it, in children and adults.
Certain chemotherapy medicines
Some medicines hit T-cells hard and the effect lasts well after the last dose.
Examples you may see
- Fludarabine and bendamustine
- Alemtuzumab
- Some targeted tablets, such as idelalisib
Long courses of steroids
High-dose steroids given for weeks, as in some lymphoma and myeloma plans, raise the risk on their own.
Stem cell transplant
People who have had a transplant usually take PJP cover for months afterwards, as set by the transplant centre, while the immune system rebuilds.
Not sure whether this applies to you?
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When a rash cannot wait
A spreading rash with blisters, peeling skin, sores in the mouth or eyes, or a rash with fever, can be a rare but serious reaction to cotrimoxazole. Go to the nearest emergency department the same day or call 108, and take the medicine strip with you. Do the same for new breathlessness with a dry cough. Do not wait for your next clinic appointment.
What to watch
What side effects should you tell the team about?
Most people take cotrimoxazole for months with little trouble. The problems that do happen are usually picked up on routine blood tests, which is why those tests matter even when you feel fine.
What you might notice
Feeling sick, loss of appetite and a mild itchy rash are the common complaints. A mild rash still needs mentioning at your next contact with the team, because some rashes get worse.
What the blood tests look for
Cotrimoxazole can lower blood counts, which matters when leukaemia treatment is already doing the same. It can raise the potassium level and affect kidney tests. It can also clash with methotrexate, a common medicine in lymphoblastic leukaemia treatment. For this reason your haematologist sometimes pauses it around certain treatment days. That is a planned decision, and it is not something to copy on your own.
Why the team may keep going through a mild problem
A small rise in a kidney test or a slight dip in counts does not always mean the tablet must stop. The risk of PJP during these treatments is real, and the alternatives have their own drawbacks. Your haematologist weighs how large the change is, how long you still need cover and what else could explain it, and then decides whether to continue, lower, pause or switch.
If you have ever reacted to a sulfa medicine, tell the team before the first dose.If it does not suit you
What is used instead when cotrimoxazole is not possible?
Alternative
What to know
Dapsone tablets
A G6PD blood test is usually checked first, as low G6PD raises the risk of red cells breaking down
Atovaquone liquid
Taken with food, gentler on blood counts, and usually more costly
Pentamidine by nebuliser
Breathed in at the hospital at intervals, and protects the lungs only
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Commonly believed
What do families often misunderstand about this tablet?
"Taking antibiotics when you are well causes resistance, so we stopped it."
That worry is sensible for colds and loose motions. Here, the tablet protects against a lung infection that can become severe in people on these treatments. Stopping it is a decision for your haematologist, not a household choice.
"It is only on some days, so a missed day does not matter."
Because it is taken on set days, each dose covers more ground. If you miss one, call the ward or the helpline and ask what to do next.
"Chemotherapy has finished, so the tablet can stop too."
T-cells take time to recover after some treatments. Cover often continues for weeks or months after the last cycle. Your haematologist will tell you when it ends.
"It is a cheap old medicine, so it cannot be important."
It is old and cheap because it has been used and studied for decades. That is a strength. Price says nothing about how much it matters.
Being straight with you
What can this page not tell you?
It cannot tell you whether you need PJP cover, which medicine suits you, or for how long. Those depend on your exact treatment protocol, your blood counts, your kidney function and your other medicines.
Who it is not for
People on gentler treatment that does not weaken T-cells much are often not given it at all. People with a proven sulfa allergy, severe kidney problems or blood counts that keep dropping may be moved to an alternative instead.
What to ask your team
Ask why it was prescribed, which days to take it, when it will stop, and what to do if you miss a dose. If you are seen at more than one hospital, make sure each one knows you take it. Keep the strip or a photo of the prescription on your phone, so any doctor who sees you in an emergency knows straight away that you are on PJP cover and why. CION's haematology team can go through the plan with you and help you understand the reasons.
Questions we are asked
Common questions about cotrimoxazole in blood cancer
Is PJP the same as a normal chest infection?
No. It is caused by a different germ, it does not respond to the usual chest-infection antibiotics, and it often starts with a dry cough and breathlessness rather than a lot of phlegm. Anyone on a blood cancer treatment who becomes breathless should be seen urgently, whatever the cause turns out to be.
Why is it prescribed only on certain days of the week?
For prevention, a steady level is not needed every day. Taking it on set days protects well and lowers the chance of side effects on the blood counts. Your prescription gives the exact days. Put a reminder on your phone, as these doses are easy to forget.
My child is on leukaemia treatment. Does a child need it too?
Most children on lymphoblastic leukaemia treatment take it for much of the treatment, often as a liquid with the amount set by weight. The paediatric team works out the amount. Ask them how to give it if your child spits out medicines or struggles with the taste.
Can I take it with my other medicines?
Usually yes, but some combinations need care, including methotrexate, some blood pressure tablets that raise potassium, and warfarin. Show every prescription and every over-the-counter or herbal product to the team or the pharmacist. Do not change anything on your own.
What if the pharmacy gives a different brand?
The medicine inside is the same, sulfamethoxazole with trimethoprim. What matters is that the strength and the days match your prescription. If the tablet looks different or the strength on the strip is not what you expected, check with the pharmacist before taking it.
Should I drink more water while taking it?
Drinking enough fluid is sensible during any blood cancer treatment and helps the kidneys. Unless your doctor has limited your fluids for a heart or kidney reason, keep well hydrated. If your urine becomes very dark or much less, tell the team.
Does it lower my blood counts?
It can, particularly the white cells and platelets, and that is one reason your counts are checked often. A drop does not always mean the tablet is to blame, because the leukaemia treatment does the same. Your haematologist weighs this up and decides whether to continue, pause or switch.
Is it covered by schemes or insurance?
It is an inexpensive medicine and is often included when it forms part of an approved cancer treatment plan. Rules for Aarogyasri, CGHS, ECHS, EHS and private insurers change, so check the current position with your scheme. The alternatives cost more, which is worth asking about if a switch is suggested.
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Dr. Basudev Pokhrel reviews blood counts, transfusion needs and blood disorders, and works with the CION tumour board on blood cancers.
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Sources
- National Cancer Institute — Infection and cancer treatment
- American Cancer Society — Infections in people with cancer
- Macmillan Cancer Support — Treatments and drugs
- Leukaemia & Lymphoma Society — Leukaemia & Lymphoma Society
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.
Keep reading
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Antifungal medicine during leukaemia treatmentThe other preventive medicine often on the same chartBlood counts after treatment endsHow the immune system recovers, and when cover stopsSupportive care in haematologyAll the supportive care guides in one placeBlood cancer guideThe bigger picture of blood cancer treatment
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