Managing side effects

Cancer and cancer treatment can cause different side effects. These side effects can affect your diabetes. You may feel sick, have diarrhoea or not manage to eat like you usually do.

It is important to contact to your diabetes team and your cancer team about managing your diabetes when you are having cancer treatment. Tell them if you are unwell. They will give you advice to help you manage your blood sugars. The advice is often called ‘sick day rules’. Sick day rules apply when you are feeling unwell for different reasons, not just feeling sick.

Sick day rules

Sick day rules include:

  • contact your diabetes team – they will give you advice on how to manage your blood sugar level
  • stay hydrated
  • keep eating and drinking
  • check your blood sugar level regularly
  • check for ketones.

You may be told to keep taking your diabetes medication even if you do not feel like eating. If you are unwell, there is some medication that you should not take as much of. And there are some you should stop taking. It is important to contact your diabetes team straight away if you are unwell.

If you take insulin, your dose may need to be changed when you are unwell. Talk to your diabetes team for more advice on how to manage your insulin doses when you are unwell.

We have more information about what to do when you are unwell. Diabetes UK also has more information about sick day rules.

When you have diabetes, it is important to be prepared. Follow the advice your diabetes team gives you. You might want to give this information to a friend or family member, so they can help you. Contact your diabetes team, who will help you if you have any queries or are unsure about what to do.

It is important that you know the symptoms of both high and low blood sugar levels, and how to manage them.

Feeling sick and being sick

Cancer or its treatments can make you feel sick (nausea) or be sick (vomit). Physical changes caused by the cancer can cause these side effects too. For example, this might be because of damage to the liver or pressure on the brain. Sometimes anxiety can cause nausea.

Feeling and being sick can be a problem if you have diabetes. It can make your blood sugar level drop. This is because you may be eating less and not absorbing food as usual.

Being sick can lead to dehydration. In some cases, severe dehydration and very high blood sugars can mean you need to go into hospital. Tell your diabetes team or cancer team straight away if you have any symptoms of high or low blood sugar levels.

Managing sickness

Tell your cancer doctor or specialist nurse straight away if you are feeling sick and cannot eat or drink anything. They can give you medication to help the sickness. There are different types of drugs that help treat sickness. These are called anti-emetics. They can be given in different ways. Your cancer doctor, specialist nurse or pharmacist will give you more information.

If you are having cancer treatment that can cause sickness, you should start taking anti-emetics before your treatment starts. Take them as they are prescribed, even if you are not feeling sick. It is easier to prevent sickness before it starts than to stop it. Tell the doctor if the anti-emetics are not working. They can give you different medicines that may work better.

Tips to help with eating and drinking when you are feeling sick

  • 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.
  • Ginger can help reduce sickness. Try drinking ginger tea or eating ginger biscuits.
  • Try to keep hydrated even if you cannot eat. Try drinking little and often.
  • Fizzy drinks that have gone flat may be easier to drink.

Managing your blood sugar level when feeling or being sick

You will need to take extra care of your diabetes if you are feeling or being sick. Your diabetes team can give you advice to help you manage your blood sugars. They may give you sick day rules to follow.

If you check your blood sugar level at home, you will need to check it more often. This could be every 2 to 4 hours, including during the night. If you do not usually check it at home, it is important that you know the symptoms of a very high blood sugar level.

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

If you cannot eat without being sick, try to sip sugary drinks, such as fruit juice or non-diet cola or lemonade. You could also suck glucose tablets or sweets like jelly-beans. If you cannot drink without being sick, contact the hospital as soon as possible. It is important to prevent dehydration.

If your blood sugar level is high and you use insulin to control your diabetes, it is important to check your blood or urine for ketones and follow the advice on managing high blood sugar levels. You may be at risk of ketoacidosis (DKA). DKA is an emergency. We have more information about the symptoms of DKA and what to do if you think you have it. People with type 2 diabetes can develop hyperosmolar hyperglycaemic state (HHS).

Your diabetes medication

Keep taking your diabetes medication even if you do not feel like eating. But there are some medicines that you should not take as much of or stop taking altogether. Make sure you contact your diabetes team or a local pharmacist as soon as you are feeling ill. They will give you advice about what to do.

You should stop taking SGLT2i tablets if you become unwell or are in hospital for major surgery. If you take these tablets, you should check your blood sugar level. You should also check for ketones – in your blood rather than your urine, if possible. Contact your healthcare team straight away if you become unwell when taking SGLT2i tablets.

There are different types of SGLT2i tablets. Diabetes UK has a list of all the brand names. Taking these tablets when you are unwell could increase your risk of developing DKA. It is important to know the symptoms to look out for.

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

If you are being sick, your insulin dose may need to be increased or decreased. The type of insulin you use may need to be change. When you are unwell, contact your diabetes team straight away for advice on how to manage your insulin.

If you are worried about coping with sickness, talk to your cancer doctor and your diabetes team. They can give you more advice.


Diarrhoea means that you need to go to the toilet more often than is normal for you. The stools (poo) you pass are also looser than normal. It can be caused by chemotherapy, radiotherapy, targeted therapy, immunotherapy and surgery. Sometimes other medicines, such as antibiotics, or an infection can cause diarrhoea. Diarrhoea may be worse if you already have bowel problems.

Diarrhoea can be a problem if you have diabetes, and can make your blood sugar levels drop. This is because you are not absorbing food as usual. Your cancer doctor, specialist nurse or other members of your cancer team can tell you if your cancer treatment is likely to cause diarrhoea. They will tell you what to do if this happens and when to contact them for advice.

Diarrhoea caused by cancer treatment can be mild. But, for some people, it can be severe and may lead to dehydration. It is important to avoid dehydration. Your diabetes team may suggest you follow the sick day rules. If you are severely dehydrated and have high blood sugars, you may need to go into hospital.

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 is causing it and may give you anti-diarrhoea medicines.

Check it is safe to keep taking your usual medication you have been prescribed. Some people may need to stop taking some medication for a while if you are very dehydrated. Try to keep drinking fluid, and follow the advice from your cancer team.

If your diarrhoea is caused by radiotherapy or chemotherapy, changing your diet may not help. It is important to take the anti-diarrhoea medicines you are given and follow the advice you are given. If the diarrhoea is caused by radiotherapy, your radiographer will advise you about how to manage it. You can also be referred 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

Cancer and cancer treatments can affect your appetite. This may include losing your appetite or having a bigger appetite than usual.

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 a small appetite, healthy eating for diabetes may not be appropriate. You may need to have high-energy, high-protein food and drinks during this time.

While you are not eating your normal diet, you will need to check your blood sugar level more often than normal. If you cannot eat enough, they may drop too low. This can lead to hypoglycaemia (a hypo). A good way to prevent a hypo is to eat regularly. If you cannot eat solid food, you could sip carbohydrate-containing drinks, such as whole milk, milky tea and coffees and fruit juice.

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. We have more information about the symptoms of a hypo and how to manage it.

While you have a poor appetite, your dose of insulin or your diabetes medicine may need to be changed to help prevent hypos. Your diabetes team or healthcare team will give you advice on this.

If you are struggling to eat, you can be referred to a dietitian for support and advice to help with your eating. There are meal replacement drinks that are available on prescription. Ask your doctor or dietitian whether they are appropriate for you. We have more information about eating problems and how to cope with them.

Tips to help improve your appetite

  • Eat small amounts as often as possible. If your appetite is better at certain times of the day, try to plan your meals for then.
  • If you change your mealtimes to help manage your blood sugar level, you may need to change when you take your diabetes medicines. Contact your healthcare team for advice about this.
  • Keep snacks with you, such as bags of nuts, crisps or dried fruit. If these are hard to swallow, try creamy Greek 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 milk drinks made with whole milk, or savoury nourishing drinks. These can replace small meals and can be sipped slowly through the day. Your doctor or dietitian can prescribe special nutritional drinks that contain carbohydrates.
  • Eat your meals slowly. Chew the food well and relax for a bit after each meal. We have more information about coping if you are struggling to eat because of a sore mouth.
  • Have drinks after meals rather than before if you feel full easily.

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. A balanced and healthy diet will help you manage your blood sugar level and avoid putting on too much weight.

Try to limit sugary foods and drinks, such as biscuits, chocolates, cakes and ice cream. These are high in calories and can raise blood sugar levels. Try to avoid drinking sugary drinks such as full sugar fizzy drinks and squashes. Instead choose ‘diet’, ‘zero’, or ‘no added sugar’ type drinks. Water is the best drink.

We have more information to help you cope with eating problems. Diabetes UK also has information and recipes to help you try to maintain a healthy weight.

Being less active

During cancer treatment, there may be times when you do not feel like being active. 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. You may need to check your blood sugars more often if you are not active. Your diabetes team can give you advice about managing your blood sugars while you are less active.

Being active can 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.

Being active can also help reduce stress levels and improve low mood.

Tips to help you get more active

  • Join an activity group.
  • Walk or cycle to the shops.
  • Keep a record of how active you have been.
  • Set goals you can achieve.
  • Do activities you enjoy.
  • Tell your friends about being active. They may want to join you.

We have more information and tools to help you get more active. Diabetes UK has information about exercise and blood sugar levels.

Risk of infection

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

  • Chemotherapy, radiotherapy, targeted therapy, immunotherapy and some types of cancer can reduce the number of white blood cells your body produces. White blood cells fight infection.
  • Surgery may make you more at risk of infection. The skin acts as a barrier to infection, so any cut in the skin increases the risk of infection.

People with diabetes have more risk of an infection or a weaker immune system. This is usually if their blood sugar level is often too high.

Infections can raise your blood sugars to dangerously high levels. As part of the body’s way of fighting illness and infection, more sugar is released into the blood stream. This can happen even if you are not eating food or eating less than usual.

People who do not have diabetes produce more insulin to cope with the extra sugar. But if you have diabetes, your body cannot do this. The extra sugar will cause a high blood sugar level. This can make you feel more thirsty and pass more urine (pee), which can lead to dehydration.

Symptoms of an infection include:

  • a high temperature
  • suddenly feeling unwell, even with a normal temperature
  • a painful, swollen or hot wound
  • feeling shaky or shivering
  • a sore throat
  • a cough, especially if there is yellow or green phlegm
  • diarrhoea
  • needing to pass urine a lot, or pain when you pass urine.

If you develop any of these symptoms, tell your doctors straight away. They can give you antibiotics to fight the infection. They can also help you to manage your blood sugar level.

If you have an infection, you will need to check your blood sugar level more often. Talk to your diabetes team for advice on managing your blood sugar levels.

Tips to help you avoid infection

  • Wash your hands regularly with soap and hot water, especially after going to the toilet or before preparing a meal.
  • Have a shower or bath every day, and do not share towels with other people.
  • Avoid people with sore throats, viruses, colds, flu, diarrhoea or vomiting, or infections such as chickenpox. If you have been around someone who has an infection, ask your cancer doctor or specialist nurse for advice.
  • Stay away from crowded places as much as possible.
  • Avoid using public swimming pools and jacuzzies. If you are doing sports or social activities, try to go when it is quiet.
  • Be careful if you have pets or work with animals. If you can, try not to handle any animal waste, such as litter trays or manure.

We have more information about avoiding infection.

Slow wound healing

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
  • are having cancer treatment, such as chemotherapy or radiotherapy.
  • smoke, vape or use nicotine replacements
  • are not able to eat a varied diet or are not managing to eat enough.

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, there is more risk it will become infected. It is important to keep the wound clean and dry. After an operation, the wound will be covered with a dressing for 1 to 2 days. The ward nurses will change the dressing before you go home. They will tell you how to look after the wound. They can make you an appointment with your practice nurse to check the wounds and remove stitches if needed. Or they can arrange for a district nurse to visit you at home.

Contact the hospital doctor if you develop any symptoms of a wound infection. These include:

  • your wound becoming hot or painful
  • your wound looking red or swollen
  • having a high temperature
  • your wound starting to bleed or leak any fluids.

Tips to help with wound healing

  • Keep your blood sugar level well managed. Ask your diabetes team for support to do this.
  • Get plenty of rest and sleep.
  • Do not smoke. If you need support, you can ask your local stop smoking service.

It is important to eat a well-balanced diet. This includes foods that contain lots of:

  • protein – meats, fish, pulses, beans, eggs, nuts and dairy products such as milk, cheese and yoghurt
  • vitamin A – cheese, eggs, oily fish, liver, dark-green leafy vegetables, and orange and yellow vegetables
  • vitamin C – peppers, broccoli, spinach, tomatoes, potatoes and citrus fruits such as oranges
  • zinc – meats, some shellfish, cheese, lentils, beans, nuts, seeds, wholegrain cereals and breads.
This information was produced in partnership with Diabetes UK.
In partnership with Diabetes UK. Know diabetes. Fight diabetes.
Image: Diabetes UK

Date reviewed

Reviewed: 01 September 2023
Next review: 01 September 2026
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