Blood sugar changes during chemotherapy

The most common reason is steroids. Dexamethasone and similar steroids are given with many chemotherapy treatments to prevent sickness and allergic reactions, and they push blood sugar up by making the liver release more sugar and making the body less sensitive to insulin. The rise usually starts within hours, is often highest in the afternoon and evening, and settles a day or two after the steroid stops. It can happen in people who have never had diabetes, and in people with diabetes sugars can climb sharply.

Other causes include some targeted medicines and immunotherapy, infection, stress, reduced activity, and eating more sugary or starchy foods to cope with nausea. Very rarely, immunotherapy triggers a sudden type of diabetes that needs urgent treatment. High sugars matter because they increase infection risk, cause thirst, tiredness and frequent urination, and can lead to dehydration. They are common and manageable, but they are often poorly planned for. This page explains why sugars change. It cannot interpret your readings or tell you how to adjust diabetes medicines, which your team must do.

People with diabetes need a plan before starting

Ask for a clear plan for steroid days, including when to check sugars and who to call, before your first cycle.

Sugars may be checked at visits

Your team may check blood sugar before treatment and ask about readings at home.

It usually settles

Steroid-related rises often return towards normal once steroids are finished, though some people need ongoing treatment.

It cannot tell you what your blood sugar readings mean or what targets are right for you. Targets during cancer treatment may be different from usual diabetes targets, and your team sets them based on your health and treatment.

It also cannot tell you how to change insulin or diabetes tablets. Doses must be adjusted by your doctors. Never stop your steroids to lower sugars, because they protect against sickness and reactions.

Involve your diabetes doctor

If you see a diabetologist or family doctor for diabetes, tell them your chemotherapy and steroid schedule so they can plan with your oncologist.

New diabetes may appear

Some people develop high sugars for the first time during treatment. It may settle, but sometimes needs longer-term care.

HbA1c may be misleading

The three-month sugar test can be affected by blood transfusions and low haemoglobin. Your team may rely more on daily readings.

High sugars raise infection risk

Keeping sugars controlled helps protect against infections when counts are low.

Sugar and cancer myths

Eating sugar does not directly make cancer grow faster. Controlling sugars is about safety and wellbeing, not starving the cancer.

Steroid timing matters

Steroids taken in the morning cause fewer night-time sugar spikes and less sleeplessness. Ask whether morning dosing is suitable.

Eye and foot care

People with diabetes should keep up with foot checks, especially if nerve symptoms develop from chemotherapy.

Carers can help monitor

A relative can help check and record readings on steroid days when you feel unwell or tired.

Ask about glucose meters

If you do not have diabetes but sugars are rising, ask whether a home meter is needed.

Sick days need a separate plan

If you have diabetes and cannot eat because of nausea, vomiting or mouth sores, sugars can swing high or low. Ask your diabetes and oncology teams for sick-day rules, including when to check sugars more often, how to keep drinking, and when to call.

Infections can show up as high sugars

A sudden rise in sugars without a change in steroids or diet can be an early sign of infection. Check your temperature and tell your team if readings jump unexpectedly.

What to do next

Tell your team if you have diabetes, ask for a sugar plan for steroid days, check readings as advised and bring them to appointments, choose steady foods, never change diabetes medicines or stop steroids on your own, and seek urgent help for very high or low sugars with symptoms.