Changes in weight

Some cancer treatments, side effects or even lifestyle changes can cause you to gain or lose weight.

What can cause weight changes?

Some cancer treatments, side effects or even lifestyle changes can cause you to gain or lose weight. Talk to your specialist doctor, nurse or GP. They can refer you to a dietitian. A dietitian can review your diet and consider any special dietary needs you may have, such as food intolerances or allergies. They can give you advice about which foods are best for you.

If you have lost weight or gained weight, you may have body image concerns.

If you have lost weight

You may have lost weight due to cancer, or the side effects of treatment. You may not feel hungry, or feel full soon after starting eating. Some people may feel sick, or find that foods taste different.

Some types of cancer make your body use up more energy, even if you are not very active. You may lose weight even if you are eating well. If you lose too much, it is important to talk to a dietitian.

The building-up diet is high in energy and protein. It can help you add extra energy and protein to your food without having to eat more. It is for people who have lost or are losing weight, or who can only eat small amounts.

Your doctor or dietitian may also recommend food supplements. These include nutritional drink, soups, powders, or puddings that can help you get extra nutrients and calories.

We have more information about the building up diet.

If you have gained weight

People do not usually expect to gain weight during cancer treatment. But some treatments, side effects or even lifestyle changes can cause you to gain weight.

Do not be too upset if you find you have gained weight. Sometimes knowing why it has happened can help you think of ways to manage it.

After treatment, most people need time to recover. But as you gradually get better, you may find that you are ready to make some changes. 

If you are having hormonal therapy as part of your treatment, it is important to keep taking this, even if you think it is causing weight gain. Talk to your cancer doctor or nurse if you are worried about this.

About our information

  • References

    Below is a sample of the sources used in our changes in weight information. If you would like more information about the sources we use, please contact us at cancerinformationteam@macmillan.org.uk

    European Society for Clinical Nutrition and Metabolism (ESPEN). ESPEN guidelines on nutrition in cancer patients. February 2017 www.espen.org [accessed Jan 2020] 

    NICE guidelines. Preventing excess weight gain. 2015. www.nice.org.uk (accessed August 2018).

    European Society for Clinical Nutrition and Metabolism (ESPEN). ESPEN expert group recommendations for action against cancer related malnutrition. June 2017 www.espen.org [accessed Jan 2020] 

    World Cancer Research Fund (WCRF). Healthy living after cancer. 2016. www.wcrf-uk.org [accessed Jan 2020]      


  • Reviewers

    This information has been written, revised and edited by Macmillan Cancer Support’s Cancer Information Development team. It has been reviewed by expert medical and health professionals and people living with cancer. It has been approved by Chief Medical Editor, Professor Tim Iveson, Consultant Medical Oncologist.

    Our cancer information has been awarded the PIF TICK. Created by the Patient Information Forum, this quality mark shows we meet PIF’s 10 criteria for trustworthy health information.