Monitoring and managing your blood sugar levels

It is important to monitor your blood sugar levels during your cancer treatment. Your diabetes team can tell you what your safe range should be.

Blood sugar levels

While you are having cancer treatment, it is important to try to keep your blood sugar level in a safe range. The diabetes team can tell you what the safe range is for you. This may also be called your target range.

Keeping within this range has been shown to:

  • reduce the risk of infections and other side effects from cancer treatment
  • improve overall well-being
  • increase the effectiveness of your cancer treatment

During your cancer treatment, your blood sugar level may be higher. This can be caused by the treatment or by medication, such as steroids, which may be given as part of your cancer treatment. Less often, the treatment or side effects of treatment can cause a low blood sugar.

It is important to tell your diabetes team and your cancer doctor or nurse if your blood sugar levels are not within your target range. They will give you advice on how to manage this.

Monitoring your blood sugar levels

Blood sugar levels can be checked quickly and simply using a blood glucose monitor. It tests the sugar levels from a drop of blood from your finger. Some people may already use a continuous glucose monitor.

If you do not already monitor your blood sugar level, you will need to start while having cancer treatment. Your cancer doctor or diabetes team can give you advice before you start treatment. If you do not already have monitoring equipment, ask your diabetes team for some. The diabetes team can tell you:

  • how to check your blood sugar
  • how many times to check a day
  • the sugar levels you should be aiming for.

The aim is to keep your blood sugar level in an acceptable range during your cancer treatment. Your diabetes team or GP can advise what your personal target range is. You should tell your cancer doctor or nurse if you are getting any symptoms of high or low blood sugars. You can also get advice from your diabetes team. Your diabetes medicines should be reviewed as soon as possible.

Managing blood sugars during cancer treatment can sometimes be more difficult. Having a slightly higher blood sugar level (around 12 mmol/l) is not usually a big problem if it is only for a short time. But very high blood sugars (around 20 mmol/l or over) can be a problem. They can cause symptoms such as being more thirsty and needing to pee a lot. This can lead to dehydration.

You should contact the healthcare professional who usually helps you manage your diabetes. This may be a diabetes team or your GP. You can call them to talk about any worries you have about your diabetes.

Symptoms of a high blood sugar level (a hyper)

High blood sugar levels may also be called hyperglycaemia, or a hyper. A hyper can happen because the body either cannot produce enough insulin to process the sugar in the blood. Or it cannot use the insulin effectively.

Avoiding high blood sugar is important because it can make you feel unwell and cause other problems. Symptoms of high blood sugar include:
feeling very thirsty

  • passing urine more often or a new need to pee during the night
  • feeling very tired (fatigue)
  • blurred eyesight
  • headaches
  • confusion
  • weight loss
  • feeling sick
  • recurring infections, including bladder or skin infections and thrush.

Managing a high blood sugar level

If your blood sugar level is slightly high for a short time, emergency treatment is not usually needed.

Make sure you are taking your diabetes medicines as your doctor has told you to. Try to drink plenty of sugar-free fluids to prevent dehydration.

If you are feeling unwell, especially if you are vomiting, you should:

  • follow any ‘sick day rules’ you have been given
  • contact your diabetes team straight away for advice.

If your blood sugar level continues to rise, you may need to act fast to avoid developing diabetic ketoacidosis (DKA). You will need to check your blood sugar levels regularly.

If your blood sugar level is 15mmol/l or more, you should check your blood or urine for ketones. Or 13mmol/l or more if you use an insulin pump. But your diabetes team may have given you different targets so you should test for ketones no matter what your blood sugars are.

If there are ketones, it is likely that you do not have enough insulin in your body. If you find ketones, contact your diabetes team. You may need to increase your dose, or take an extra dose. They can advise you about what to do. If you are not able to speak to someone straight away and your ketone levels are high, go to your local accident and emergency department (A&E).

Symptoms of a low blood sugar level (a hypo)

Less often, treatment or its side effects can mean your blood sugar level goes too low. This is called hypoglycaemia, or a hypo.

Symptoms of a low blood sugar level include:

  • sweating
  • feeling anxious and irritable
  • fatigue
  • feeling weak and shaky
  • looking pale
  • being hungry
  • having a faster heart rate than normal.

Managing a low blood sugar level

You must do something as soon as you notice symptoms of a hypo, or if a blood test has shown your blood sugar levels are too low. You should treat a hypo straight away. You can do this by eating or drinking 15 to 20g of a fast-acting carbohydrate such as:

  • 5 glucose or dextrose tablets
  • 4 jelly babies
  • a small carton or glass or pure fruit juice
  • a small glass of a sugary (non-diet) drink
  • glucose gel such as GlucoGel® or Rapilose® gel
  • glucose juice such as Lift Shots.

After 10 to 15 minutes, check your blood sugars again to make sure it is back above 4mmol/l. If it is still less than 4, take some more fast-acting carbohydrate and recheck it after 10 minutes.

After a hypo, you may need to eat or drink a bit more. This is to stop your sugar levels going down again. Try to eat 15 to 20g of a slower-acting carbohydrate. This could be a sandwich or 2 digestive biscuits.

You should also tell your diabetes team that you have had a hypo. Your diabetes team can give you advice about how to manage hypos and will review your diabetes medicines.


If your blood sugar level is very high, your doctor may advise you to check your blood or urine for ketones.

Ketones are chemicals that can build up in the body when there is a severe lack of insulin. They are toxic to the body, and large amounts can be very serious. A high level of ketones in the body is called diabetic ketoacidosis (DKA). If you have type 1 diabetes, you are more at risk of DKA than people with other types of diabetes. But if you have type 2 and use insulin, you should still look out for the signs of DKA. DKA is very serious, and needs to be treated straight away.

Your doctor may suggest you check your blood or urine for ketones while having treatments that increase your blood sugar levels, such as steroids. Contact your doctor straight away if you have ketones in your blood or urine.

If you suspect DKA, go to your nearest A&E straight away. You will need hospital treatment. If you are not sure whether you need to go to hospital, call your GP or diabetes team straight away.

Symptoms of diabetic ketoacidosis (DKA)

It is important to be aware of the symptoms of DKA. Contact your doctor straight away if you have these symptoms:

  • high blood sugar levels
  • being very thirsty
  • needing to pee more often
  • feeling tired and sleepy
  • feeling confused
  • blurred eyesight
  • stomach pain
  • feeling or being sick
  • sweet or fruity-smelling breath (like nail polish remover or pear drop sweets)
  • passing out (fainting).

Testing for ketones

The best way to check your ketone levels is using a ketone monitor to check your blood. Checking blood for ketones will give you real-time results, just like when you check your blood sugar level. A urine check will tell you what your ketone levels were a few hours ago. A ketone monitor works in a similar way to a blood sugar monitor.

There are different types of ketone monitors available. Speak to your doctor about which one is best for you. If you have type 1 diabetes, you usually get a ketone monitor for free on the NHS.

Hyperosmolar Hyperglycaemic State (HHS)

This can happen to people with type 2 diabetes who have very high blood glucose levels (often over 40mmol/l). It can develop over weeks. It can be caused by a combination of factors – for example, having an infection and being dehydrated.

Symptoms of HHS include:

  • peeing often
  • thirst
  • nausea
  • dry skin
  • feeling very confused
  • in later stages, drowsiness and a gradual loss of consciousness.

Contact your diabetes team if your blood glucose levels remain high (more than 15mmol/l). They can give you advice about what to do, including managing your medication. Try to drink plenty of unsweetened fluids. You may need to test your blood sugar level more regularly.

If you are unable to eat, replace meals with snacks and drinks containing carbohydrates. HHS can be a life-threatening emergency that needs to be treated in hospital.

Date reviewed

Reviewed: 01 September 2023
Next review: 01 September 2026
Trusted Information Creator - Patient Information Forum
Trusted Information Creator - Patient Information Forum

Our cancer information meets the PIF TICK quality mark.

This means it is easy to use, up-to-date and based on the latest evidence. Learn more about how we produce our information.