Diabetes and side effects of cancer

Information and tips to help cope with side effects when you have diabetes.

Feeling sick and being sick

Cancer or its treatments can sometimes make you feel sick (nausea) or be sick (vomit).

Being sick can be a problem when you have diabetes. Because you may not be able to eat or drink, you could become dehydrated. Your blood sugar level can get very high (hyperglycaemia – a hyper) if you are dehydrated. This can be more serious for people with diabetes.

Before you start any cancer treatment, talk to your cancer doctor or specialist nurse. Explain that you have diabetes and that you need to prevent sickness as much as possible. Ask your diabetes team for advice about managing sickness. They can tell you how to manage your diabetes when you are ill. These are known as the 'sick-day rules'. It is helpful for family or friends to know how to manage things too.

Tell your cancer doctor or specialist nurse straight away if you are feeling sick and cannot eat or drink anything.


Controlling your blood sugar when feeling or being sick

You will need to take extra care of your diabetes if you are feeling or being sick.

You will need to check your blood sugar more often. This could be every 2 to 4 hours, including during the night.

You should also be aware of the symptoms of a very high blood sugar level. These symptoms include:

  • passing a lot of urine, especially at night
  • being very thirsty
  • headaches
  • being very tired.

If your blood sugar is very high and you use insulin to control your diabetes, you may need to check your blood or urine for ketones. Ketones are chemicals that can sometimes 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. If you have ketones in your blood or urine, you should contact your diabetes team for advice straight away.

Try to keep eating and drinking as normal. It is important to drink enough, so have plenty of unsweetened drinks. If you cannot eat much, try snacks or drinks with carbohydrates to give you energy.

If you cannot eat without being sick, it is okay to sip sugary drinks, such as fruit juice or non-diet cola or lemonade. You could also suck on glucose tablets or sweets like jelly beans. Contact your doctor as soon as possible if you are vomiting and cannot keep any fluids down.


Your diabetes medicines

Try to keep taking your diabetes medicines as normal.

If you use tablets to manage your diabetes, you may need to stop taking them while you are being sick. Your diabetes team can give you more advice.

If you use insulin, you will usually have this before eating. But if you are sick, you may not absorb enough food and your blood sugar may drop too low. If you are sick after eating, check your blood sugar and try to eat something to stop it getting too low.

It is a good idea to have a fast-acting carbohydrate nearby, such as glucose tablets, sweets or fruit juice. You should try to follow this with a snack containing slower-acting carbohydrates, such as a slice of toast or a couple of digestive biscuits. The fast-acting carbohydrates will quickly increase your blood sugar level. The slow-acting carbohydrates will stop it dropping again.

If you cannot eat solid foods, you could have a milky drink. Your doctor can also prescribe special nourishing drinks containing carbohydrates.

If you are being sick, you may need to change the dose of your insulin or change the type of insulin you use. You should not stop taking it. Your diabetes specialist can give you advice.

Tips to help with eating

  • Try to stick to what you usually eat. Try to eat the same amount of carbohydrates as normal. But if this is difficult, it is okay to eat foods you would not normally eat.
  • You may need to eat little and often.
  • If feeling sick is putting you off food or you cannot eat without being sick, replace meals with snacks or drinks that contain carbohydrates. These could include glucose tablets, fruit juice or non-diet fizzy drinks. Fizzy drinks that have gone flat may be easier to drink.
  • Try to keep drinking even if you cannot eat. Aim to drink a cup of fluid every hour.

We also have general information about nausea and vomiting, including more tips to help with eating.

If you cannot drink without being sick, contact the hospital as soon as possible. It is important to prevent dehydration.

You should talk to your diabetes team if you are worried about coping with sickness. They will be able to give you more advice.



Diarrhoea means that you need to go to the toilet more often than is normal for you, and the stools (poo) you pass are looser than normal. It can be caused by some cancer treatments. Sometimes other medicines, such as antibiotics, or an infection can also cause diarrhoea.

Some people with diabetes may already have diarrhoea. It can happen because the nerves that control bowel movements are damaged by high blood sugar levels (gastroparesis), but this is rare. If you already have diarrhoea, your symptoms could get worse during cancer treatment.

Your cancer doctor or specialist nurse can tell you if you are likely to get diarrhoea because of your cancer treatment. They will tell you what you need to do if this happens and when you should contact them for advice.

Most diarrhoea caused by treatment is mild. But for some people, it can be severe and may lead to dehydration. It is important to avoid dehydration.

If you have diarrhoea or if it is getting worse, contact the hospital on the number they have given you and speak to a doctor or nurse. They can find out what might be causing it and may give you anti-diarrhoea medicines. Check it is safe to keep taking the medicines you have been prescribed. Some may need to be stopped for a while if you are very dehydrated.

We have tips for coping with diarrhoea.

If your diarrhoea is caused by radiotherapy or chemotherapy, changing your diet may not help. It is important to take the anti-diarrhoea medicines your doctor gives you. Your doctor can refer you to a dietitian for more advice.

If you have diarrhoea after surgery for bowel cancer, tell your cancer doctor, specialist nurse or a dietitian. They can talk to you about what may help.

Eating problems

Loss of appetite

During cancer treatment, you may lose your appetite. This could be because you feel sick, you are too tired to eat, or foods taste different. This usually does not last long.

If you have diabetes and cannot eat enough, your blood sugar may drop too low. This can lead to hypoglycaemia (a hypo). It is important to know the early signs of a hypo, so you can treat it quickly. Make sure your family and friends also know the symptoms, so they can help you.

Symptoms of a hypo include:

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

A good way to prevent a hypo is to eat regularly. If you cannot eat solid food, you could sip sugary drinks.

While you are not eating your normal diet, you will need to check your blood sugar more often than normal. If your blood sugar level starts to get lower or you develop symptoms of a hypo, try eating or drinking a fast-acting carbohydrate. This could be glucose tablets, sweets or fruit juice. Try to follow this with a snack containing slower-acting carbohydrates, such as a slice of toast or a couple of digestive biscuits. The fast-acting carbohydrates will increase your blood sugar level. And the slow-acting carbohydrates will stop them dropping again.

While you have a poor appetite, you may need to change your dose of insulin or tablets to help prevent hypos.

Your diabetes team can give you advice about preventing hypos and how to manage them if they happen.

Tips to help improve your appetite

  • Eat small amounts as often as possible. If you find your appetite is better at certain times of the day, try to plan your meals for then. You may need to change when you take your diabetes medicines.
  • Keep snacks with you. Bags of nuts, crisps or dried fruit, or a bowl of grated cheese, are light and tasty. If these are hard to swallow, try yoghurt or fromage frais. If you have recently had surgery or radiotherapy for bowel cancer, you may need advice about the best foods for you. Talk about this with your specialist nurse or cancer doctor, or a dietitian.
  • Try sweet or savoury nourishing drinks. These can replace small meals and can be sipped slowly through the day.
  • Eat your meals slowly. Chew the food well and relax for a bit after each meal.


Bigger appetite than normal

Some medicines, such as steroids, may make you want to eat much more than usual. It is important to try to eat healthy foods as much as possible. Try to avoid foods that are high in carbohydrates and sugar, such as biscuits and sweets.

A balanced and healthy diet will help you avoid putting on too much weight. Having a healthy weight is important for managing your blood sugar levels.

We have more information to help you cope with eating problems.

Diabetes UK has more information and recipes to help you try to maintain a healthy weight.

Lack of activity

Being active is an important part of a healthy lifestyle. It can also help with your diabetes by:

  • helping the body use insulin more effectively
  • increasing the amount of sugar used by the body
  • improving how you manage your diabetes
  • helping you manage your weight.

During cancer treatment, there may be times when you do not feel like being active and that is okay. You may feel very tired (fatigued) or not have much energy. You may also have side effects, such as sickness or pain, that stop you being active.

If you have diabetes, not being active can change your blood sugar level. This will depend on your situation. But you may need to test your blood sugar more often if you are not active.

Your diabetes team can give you advice about managing your blood sugar while you are less active.


Getting more active

Choose activities you enjoy and try to do a mix of activities. During cancer treatment, it is a good idea talk to your specialist nurse, cancer doctor or a physiotherapist before starting any new physical activity.

We have more information and a range of tools to help you get more active. Our Move More pack includes lots of tips of how to get more active in ways that are safe for you.

Risk of infection

The immune system protects the body from harmful bacteria and other organisms. Some types of cancer and its treatment can weaken your immune system and increase your risk of infection.

People with diabetes may be more at risk of an infection or a weaker immune system. This is usually if their blood sugar level is often too high. Your diabetes team can give you advice if you have problems keeping your blood sugar level under control.

The body tries to fight infection by releasing extra sugar into the blood. If you do not have diabetes, the pancreas will make more insulin to cope with the extra sugar. But if you have diabetes, the extra sugar will cause high blood sugar levels (a hyper). This can make you feel more thirsty and pass more urine (pee), which can lead to dehydration.

If you develop any symptoms of an infection, it is important to talk to your doctors as soon as possible. They can give you antibiotics to fight the infection. They can also help you to control your blood sugar level.

If you have an infection, you will need to check your blood sugar level more often. If you usually control your diabetes with diet, you may need to start taking medicines. If you already take medicines, you may need to change the dose. Talk to your diabetes team if you have an infection and your blood sugar level is high.

Slow wound healing

After an operation, your wound will be closed using stitches or clips. These are usually removed after about 7 to 10 days. Some stitches are designed to slowly dissolve as the wound heals and will not need removing.

How long the wound takes to heal depends on the operation you have had. If you have only had a small area of tissue removed, your wound will usually heal quickly. If you have had a bigger operation, it may take a few weeks to heal properly.

Wound healing can be slower if you:

  • are older
  • do not eat a varied diet or are not eating enough
  • smoke or use nicotine replacements
  • are having cancer treatment, such as chemotherapy or radiotherapy.

If you have diabetes and your blood sugar level is high, your wound can take longer to heal. Wound healing may also be slower in people who have had diabetes for many years. Over time, high levels of blood sugar can affect the nerves and lead to poor blood circulation and nerve damage. Wounds need a good blood supply to heal.

The longer a wound takes to heal, the more risk there is that it will become infected.

It is important to keep the wound clean and dry, to help it heal and prevent infection. After an operation, the wound will be covered with a dressing for a day or two. The ward nurses will change the dressing before you go home. They will tell you how to look after the wound when you are at home. The ward nurses can make you an appointment with your practice nurse. Or they can arrange for a district nurse to visit you at home if you need it.

Contact the hospital doctor if you develop any symptoms of a wound infection. Symptoms to look out for include:

  • your wound becoming hot or painful
  • your wound starting to bleed or leak any fluids.


Tips to help with wound healing

  • Keep your blood sugar levels under control. Ask your diabetes team for help if you are having problems.
  • Get plenty of rest and sleep.
  • Do not smoke.
  • Eat a well-balanced diet.


This information was produced in partnership with Diabetes UK.
In partnership with Diabetes UK. Know diabetes. Fight diabetes.
Image: Diabetes UK