Weight gain after cancer treatment: why and what helps

Weight gain after cancer usually has several causes at once: some treatments increase appetite or cause fluid build-up, tiredness makes people less active, and stress or low mood can change eating habits [1][2]. Treatment can also bring on an early menopause, which adds its own weight changes [2][6].

Macmillan names chemotherapy, steroids and hormonal therapies, including tamoxifen, anastrozole and hormone therapy for prostate cancer, among the treatments that may cause weight gain, for example by increasing appetite [1]. Some treatments cause fluid to build up in the ankles, legs, arms or face, which shows on the scales but is not body fat [1].

Everyday life plays a part too. Tiredness can make you less active, and some people eat more when stressed, anxious or low, when their routine changes, or after stopping smoking [1][2].

Cause	What happens
Steroids	Appetite can increase; weight may settle around the tummy [5]
Chemotherapy and hormonal therapies	May increase appetite [1]
Fluid build-up	Swelling in ankles, legs, arms or face [1]
Treatment-induced menopause	Weight gain is a common menopause symptom [2][7]
Tiredness, stress, low mood	Less activity, more eating [1]

For men on hormone therapy for prostate cancer, weight gain around the waist and loss of muscle are covered in our guide to exercise and prostate cancer.

Weight gain is commonly reported by people with breast cancer who take tamoxifen or aromatase inhibitors [3]. In large trials, women taking tamoxifen or anastrozole gained no more weight after 12 months than women taking a dummy pill (placebo), but those women did not have breast cancer [3].

These figures come from a 2012 analysis of three large trials in postmenopausal women (ATAC, IBIS-I and IBIS-II) [3]. IBIS-I and IBIS-II were prevention trials in women at high risk who did not have breast cancer: in IBIS-I, women on tamoxifen gained on average 0.1 kg in 12 months, against 0.3 kg on placebo; in IBIS-II, women on anastrozole gained 0.8 kg, against 0.5 kg on placebo. Neither difference was statistically significant [3]. Only ATAC involved women being treated for breast cancer: those on anastrozole gained 1.4 kg and those on tamoxifen 1.5 kg, with no statistically significant difference and no placebo group [3]. The authors found that weight gain occurred mainly in the first 12 months of treatment, in a subset of patients, and that women under 60, smokers and those who had had a mastectomy were more likely to gain more than 5 kg in the first 12 months [3].

Letrozole was not one of the drugs tested in those trials. Letrozole is one of three aromatase inhibitors, with anastrozole and exemestane, and Breast Cancer Now says all three "have similar effects" [4]. Breast Cancer Now's letrozole booklet lists swelling of the arms, hands, feet and ankles among its side effects and asks you to tell your GP if you notice it [4].

Whatever the cause, Macmillan says: "It is important to keep taking your medication even if you think it might be causing you to put on weight" [1]. If you are having hormonal therapy and have gained weight, talk to your cancer doctor or nurse [1]. Breast Cancer Now notes that a treatment team may suggest a different aromatase inhibitor or another hormone treatment if side effects are a problem [4]. For movement during and after breast cancer treatment, see exercise after breast cancer.

Yes. Steroids can increase your appetite and can cause weight gain around the tummy and fluid swelling, and Macmillan lists chemotherapy among treatments that may cause weight gain [1][5].

Cancer Research UK says that on steroids "You might put on weight around your tummy (abdomen)", and that appetite goes back to normal when the steroids stop, "but some people need to diet to lose the extra weight" [5]. Steroids can also cause a build-up of fluid, with swelling in the arms, hands, ankles, legs or face; Cancer Research UK advises contacting your healthcare team if this happens [5].

Chemotherapy can also cause weight gain indirectly. The NHS says early menopause can be caused by treatments such as radiotherapy or chemotherapy and surgery to remove your ovaries [6]. The NHS says weight gain during perimenopause and menopause is common and "often happens around the stomach and upper body" [7].

Weight gain after cancer can matter for long-term health. Macmillan says keeping to a healthy weight and being active reduces the risk of heart disease, strokes, diabetes and other health conditions [1].

For breast cancer, a 2015 review of 12 studies with 23,832 women found that gaining 10% or more of body weight after diagnosis was linked with a higher risk of death from any cause; gains of 5% to 10% were not [8]. These were observational studies, so the review shows a link, not proof that the weight gain was the cause [8].

On cancer coming back, the evidence is cautious. Breast Cancer Now says maintaining a healthy weight "can reduce the risk of breast cancer coming back" [14], and Cancer Research UK lists "how much body fat you have" among factors that may reduce the risk of cancer coming back [9]. A Cochrane review of weight loss programmes after breast cancer found there is "a lack of data" on whether losing weight changes survival or recurrence [10].

Lose weight slowly, with small changes to eating and activity. Macmillan and Breast Cancer Now both suggest aiming for about 0.5 to 1 kg (1 to 2 pounds) a week [1][2], and Macmillan advises speaking to your GP, cancer doctor or nurse before you start [1].

Practical eating changes from UK cancer charities include:

• Follow the Eatwell Guide and eat a varied, balanced diet [2].
• Have smaller portions of high-calorie foods and more vegetables instead [1].
• Try a small treat on 2 days of the week rather than every day [1].
• Avoid skipping meals or extreme dieting [2].
• Remember that we need fewer calories as we get older [1].

A dietitian can help with food choices that are healthier but still filling, and your doctor or another health professional can refer you [1]. Our guide to nutrition during cancer treatment explains when to ask for one and how a dietitian differs from a nutritionist, and our guide to eating well after cancer treatment covers a balanced diet, protein and alcohol.

The evidence for planned weight loss comes mostly from breast cancer survivors. A 2020 Cochrane review of 20 studies with 2,028 women found that weight loss programmes reduced body weight by about 2.25 kg on average, with smaller waist measurements and better quality of life, and no increase in harmful effects [10]. Programmes that combined diet, exercise and psychological support appeared to work better, though the evidence was rated low quality [10]. During active treatment, the American Society of Clinical Oncology (ASCO) guideline found evidence for diet and weight loss programmes "very limited", so wait for your team's advice before trying to lose weight mid-treatment [11].

Both everyday activity such as walking and strength training are recommended. Cancer Research UK suggests building towards 150 minutes of moderate activity or 75 minutes of vigorous activity a week, plus strength training twice a week [9].

Cancer Research UK recommends starting "with short chunks (10 to 15 minutes) of gentle activity" and building up gradually [9]. Walking and gardening count, and more energetic options include swimming, cycling or the gym [1]. Macmillan says any increase in physical activity is good for your health, and your cancer team may be able to refer you to an exercise programme [1].

Some treatments can cause muscle loss: Prostate Cancer UK says "Hormone therapy can cause you to lose some muscle tissue" [15], so it is worth including strength work in your week. For building fitness back up after treatment ends, see exercise after cancer treatment, and if tiredness is holding you back, cancer-related fatigue.

Clinical exercise physiologists (CEPs) are university-trained exercise professionals. In oncology, NHS Health Careers says they work with people "with different types of cancer and at different stages of treatment", and "prescribe and deliver a range of exercise programmes tailored to the patient's needs" [12]. Read more about cancer rehabilitation and cancer rehabilitation in London.

Request a match with a verified clinical exercise physiologist through The Health Nav.

Ask for help if weight keeps rising despite changes, if you are struggling on hormonal therapy, or if low mood is driving your eating. Your GP, cancer doctor or nurse can check your weight and refer you to a dietitian or an exercise programme [1][2].

Breast Cancer Now says your breast care nurse or GP can assess whether your current weight is healthy and work out your body mass index (BMI) [2]. Macmillan advises talking to your GP or nurse if you think weight gain is linked to feeling anxious or depressed, and to your cancer doctor or nurse if you have gained weight on hormonal therapy [1].