Changes in weight
Cancer and side effects of treatments, or lifestyle changes may cause you to gain or lose weight.
What can cause weight changes?
Cancer and side effects of cancer treatments, or lifestyle changes can cause you to gain or lose weight. If you have lost or gained weight, your cancer doctor, specialist nurse or GP or other healthcare professional may be able to refer you to a dietitian. A dietitian can review your diet and give you advice about which foods are best for you.
If your weight has changed a lot, you may be concerned about your body image. Your healthcare team can give you advice about how to cope.
If you have lost weight
Cancer or the side effects of treatment can cause weight loss. 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. You may need foods that are high in energy and protein. This is sometimes called a building up diet. It can help you add extra energy and protein to your food without having to eat more. If you are worried about your weight, talk to your doctor, nurse, or other healthcare professional.
They may be able to help you manage your symptoms so that you can eat and drink more. They may be able to refer you to a dietitian.
Your cancer doctor, GP, dietitian or other healthcare professional may also recommend and prescribe nutritional supplements. These include nutritional drinks, milkshake or smoothie powders, and puddings or soups that can help you get extra nutrients and calories.
If you have gained weight
Some cancer treatments, side effects or lifestyle changes may cause you to gain weight. After a cancer diagnosis it can be harder to have a healthy diet. You may also become less active.
Hormonal therapy as part of your cancer treatment can cause weight gain. Knowing why you may have gained weight can help you think of ways to manage it. But it is important to keep taking your medication.
Talk to your cancer doctor, nurse or healthcare professional involved in your care if you are worried about weight gain. They may be able to refer you to a dietitian.
About our information
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.
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References
Below is a sample of the sources used in our information about changes in weight. If you would like more information about the sources we use, please contact us at informationproductionteam@macmillan.org.uk
National Institute for Health and Care Excellence. NICE. Obesity: identification, assessment and management. CG189. Last updated: 26 July 2023. Available from www.nice.org.uk/guidance/cg189/chapter/Recommendations#identifying-and-assessing-overweight-obesity-and-central-adiposity [accessed July 2025]
World Cancer Research Fund WCRF. Weight, Obesity and Cancer. Available from Be a healthy weight - Cancer prevention recommendations | World Cancer Research Fund (wcrf-uk.org) [accessed July 2025]
European Society for Clinical Nutrition and Metabolism. ESPEN. Muscaritoli M, Arends J et al. ESPEN practical guideline: Clinical Nutrition in Cancer. Clinical Nutrition 40 (2021). 2898-2913. [TG1] Available from www.espen.org/files/ESPEN-Guidelines/ESPEN-practical-guideline-clinical-nutrition-in-cancer.pdf ESPEN practical guideline: Clinical Nutrition in cancer [accessed July 2025)
British Dietetic Association BDA. Obesity and overweight. Available from www.bda.uk.com/food-health/your-health/obesity-and-overweight.html [ accessed July 2025]
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