Medical Oncologist
Supportive medicines
Diabetes medicines during chemotherapy
They continue, but expect the doses to need changing. Many chemotherapy plans include steroids, and steroids push blood sugar up sharply for a few days around each cycle. This happens to almost everybody with diabetes, it is entirely predictable, and it is still routinely left unmanaged because nobody warns people it is coming.
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
What happens to my diabetes medicines during chemotherapy?
They continue, but expect the doses to need changing. Many chemotherapy plans include steroids, and steroids push blood sugar up sharply for a few days around each cycle. This happens to almost everybody with diabetes and it is entirely predictable.
Predictable, and still routinely unmanaged. People are given the steroids, their sugar climbs, nobody has warned them, and they either panic or assume it will settle on its own. It usually does settle, but the days in between are worth planning for.
The pattern repeats every cycle
Sugar rises during and just after the steroid days, then drifts back down before the next cycle. Once you have measured it through one cycle, you and your doctor can plan for the same shape in every cycle that follows.
Test more often than you used to
This is the single most useful thing you can do. Without readings there is nothing for your physician to adjust against, and the adjustment is what keeps you out of trouble. Write the numbers down with the date and the time.
Ask at the planning appointment whether your plan includes steroids, and on which days. That is the whole question.What changes and why
What is actually going on
- Steroids raise blood sugar
- They are given with many chemotherapy plans to reduce sickness and reactions, and they make the body less responsive to insulin for as long as they are in the system. The rise starts within hours and settles over the following days.
- Eating becomes irregular
- Sickness, taste changes and poor appetite mean meals are skipped or much smaller. That pulls sugar in the opposite direction, which is why both high and low readings can appear within the same week.
- Vomiting and loose motions
- These change how much of a tablet is absorbed and how much fluid you are carrying. Both make sugar readings less predictable and both are worth reporting early rather than after several days.
- Kidney function matters more
- Some diabetes medicines depend on the kidneys, and chemotherapy can affect kidney function. This is one reason your physician may change which medicine you are on rather than only the dose.
- Infections are harder to spot
- High sugar makes infection more likely, and chemotherapy lowers the defences against it. A rising sugar with no obvious cause is sometimes the first sign that an infection is developing.
- Wounds and feet need watching
- Healing is slower with both diabetes and chemotherapy. Check your feet daily, treat any small cut seriously, and mention anything that is not healing rather than waiting for the next appointment.
Not sure whether this applies to you?
Ask an oncologistPractical
How to manage the steroid days
All four need your physician's agreement first. None of them is something to start on your own.
Get a written plan before cycle one
Ask your oncologist and your physician together for instructions covering the steroid days: what to test, when, and what to do at different readings. Written down, in your file, before you need it.
It should say
- Which reading means calling someone
- Who to call, and on which number
Test more often, and write it down
More frequently on and after the steroid days than you normally would. A written record with dates and times is what allows a doctor to adjust anything; a remembered impression is not.
Keep eating, even in small amounts
Skipping meals while still taking diabetes medicines is how readings swing low. Small, frequent, plain food works better than three proper meals on the days when appetite is poor.
If you cannot eat
- Tell the team, do not simply skip doses
- Ask what to do about that day's medicine
Take glucose and a meter everywhere
Including to treatment day, where you may wait for hours with no food. Carry something sugary for a low reading, and tell the day-care nurse that you have diabetes when you arrive.
!
Get medical help the same day for these
A reading far above your usual range that will not come down · readings you cannot check because you are vomiting and cannot keep anything down · severe thirst with passing large amounts of urine · drowsiness, confusion or difficulty waking someone · fruity-smelling breath · sweating, shaking or confusion that does not improve after something sugary · a fever alongside any of these. Say that the person has diabetes and is on chemotherapy, and take the meter readings and both prescriptions with you.
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Who decides what
Getting your two doctors to work together
Your oncologist runs the chemotherapy and the steroids. Your physician or diabetologist runs the diabetes. The gap between them is where people come unstuck, because each assumes the other is handling the sugar during treatment.
Ask directly who is adjusting the doses
Put the question in the room: "When my sugar rises on the steroid days, who changes my medicine, and how do I reach them?" A named person and a number is the answer you want.
Take the readings to every appointment
A page of dated readings across a full cycle is worth more than any description. It shows the pattern, and the pattern is what the plan for later cycles is built on.
Expect the plan to change more than once
Doses adjusted, a medicine swapped, insulin started temporarily or increased around the steroid days. None of this means your diabetes has suddenly worsened; it means the treatment is being matched to what your body is doing that week.
Ask about coming back down
Whatever is increased for the steroid days usually needs reducing again afterwards, and continuing a raised dose into the low-sugar part of the cycle is how readings go dangerously low. Ask for that instruction in writing too.
Never change an insulin dose or a diabetes tablet on your own because of a reading. Ask, even if it seems obvious.Commonly believed
Four beliefs that cause trouble here
"My sugar has been well controlled for years, so it will stay that way."
Steroids raise blood sugar regardless of how good your control has been. People with excellent control for a decade see sharp rises on the steroid days. It is the medicine, not a failure of your management.
"The sugar is high, so I will stop eating."
Skipping meals while still taking your medicines is how readings swing dangerously low, and eating too little during treatment causes its own problems. Keep eating small plain amounts and let the doctor adjust the medicine instead.
"I will just increase my own dose on the steroid days."
The size and timing of any change has to match your particular medicine and the steroid schedule, and a raised dose continued into the wrong part of the cycle causes a dangerous low. Get the instruction written down first.
"I should stop the diabetes tablets while on chemotherapy."
Uncontrolled sugar during treatment makes infections more likely, healing slower and cycles more likely to be postponed. Diabetes medicines are continued and adjusted, not dropped. Any change comes from your physician.
Questions we are asked
Common questions about diabetes and chemotherapy
Why does my sugar rise after every cycle?
Because of the steroids given with many chemotherapy plans. They make the body less responsive to insulin for as long as they are in the system. The rise starts quickly and settles over the following days, then repeats next cycle.
How often should I test?
More often than usual, particularly on and after the steroid days. Ask your physician for a specific instruction and write every reading down with the date and time. The record is what allows the dose to be adjusted.
Will I need insulin?
Some people do, temporarily, around the steroid days, even if tablets have always been enough before. It is usually a short-term measure rather than a permanent change. Your physician decides this, not the oncologist alone.
What should I do on treatment day?
Ask in advance about your morning dose, since it depends on whether you will be eating. Take your meter, something sugary and your medicines with you, and tell the day-care nurse you have diabetes when you arrive.
I cannot eat because of the sickness. What about my tablets?
Call and ask rather than simply skipping them. The right answer differs between medicines. Also report the sickness itself, because the anti-sickness cover can usually be improved for the next cycle.
Does high sugar affect how well chemotherapy works?
It affects the course indirectly, by making infections more likely, healing slower and postponed cycles more common. That is reason enough to manage it properly rather than treating it as a secondary problem.
Should I check my feet more often?
Yes, daily. Healing is slower with both diabetes and chemotherapy, and a small cut or blister matters more than it usually would. Report anything that is not healing rather than waiting for the next appointment.
Will my diabetes go back to normal after treatment?
The steroid-driven rises stop once the steroids do, and for most people control returns to roughly where it was. Some people need a lasting adjustment. Ask your physician to review it once the course is finished.
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Sources
- Cancer Research UK — Steroids and cancer treatment
- Macmillan Cancer Support — Diabetes and cancer treatment
- 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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