Weight Gain and Weight Loss From Breast Cancer Treatment

Although some people lose weight, many people gain weight during and after breast cancer treatment, which can be frustrating.
 

The shock of a breast cancer diagnosis, the disruption of your life, getting through and beyond treatment, the strain of relationships at home and at work, and financial stress all can contribute to weight gain or loss during treatment.

Gaining or losing a few pounds is normal, but a considerable weight change — say 5% to 10% of your total body weight — could affect your health.

Did you know that exercise can help with many breast cancer treatment side effects? Our video series, Exercises to Relieve Side Effects, is designed especially for people living with breast cancer — no matter your fitness or energy level.

Watch the Exercises to Help With Weight Management video.

MBC Exercise podcast image
 

Weight gain

It’s very common for people to gain weight during and after breast cancer treatment. Treatment makes many people tired and nauseated, which makes them less active. Certain medicines can increase your appetite and cause you to store more fat.

Causes of weight gain

There are a number of reasons why people might gain weight during and after breast cancer treatment.

Chemotherapy can cause fatigue, and make you feel nauseated, all of which can make it hard for you to be active. If you’re receiving steroid medicines along with chemotherapy, they can trigger food cravings, so when you’re not feeling nauseated, you eat more. Chemotherapy also can bring on early menopause, which causes hormone changes that can lead to weight gain.

Hormonal therapy medicines lower the amount of estrogen in your body, which can cause weight gain. Hormonal therapy also can increase the amount of fat your body stores and decrease the amount of muscle you have. Both of these changes can lower your metabolism and lead to weight gain.

Many targeted therapy medicines also can cause fatigue and nausea, which can make you less active, which can lead to weight gain.

If you had lymph nodes removed during breast cancer surgery, you may develop lymphedema, the build-up of fluid in the arm, hand, trunk, or other part of the body. This excess fluid and swelling can make it hard to do daily activities or exercise using the affected area.

There is also the stress and anxiety that usually comes with a breast cancer diagnosis and treatment, which some people respond to by “eating their feelings” — indulging in less-than-healthy comfort food to try to feel better emotionally.

Managing weight gain

If you're concerned you've gained weight, it makes sense to talk to your cancer care team about it. They can help you figure out what may have caused the weight gain and give you advice on what you can do to manage it. They can also help you figure out a healthy weight for your body type. In general, eating a healthy diet and being more active can help you lose weight.

Weight gain also can play a role in breast cancer outcomes. In 2022, an expert panel reviewed 226 studies on how weight gain affects breast cancer outcomes. The experts concluded that there was strong evidence that gaining weight after being diagnosed with breast cancer was linked to a higher risk of developing a second primary breast cancer. Also, some studies have suggested that weight gain may increase the risk of recurrence or spread of breast cancer.

Create a healthy eating plan to lose weight

Healthy eating means eating a variety of foods that give you the nutrients you need to maintain your health, feel good, and have energy. These nutrients include protein, carbohydrates, fat, water, vitamins, and minerals. 

It also means limiting foods that don’t have many nutrients, including highly processed foods, like corn chips, candy, and hot dogs; foods full of sugar, like soda, cookies, and cupcakes; and alcohol.

If you’re not sure how to eat healthy, it makes sense to talk to a registered dietitian. You can search the Academy of Nutrition and Dietetics database for dietitians in your ZIP code.

 

Exercise to lose weight

Exercise is considered an important part of daily life, especially for people with a history of cancer. It has been shown to reduce the risk of recurrence (the cancer coming back), ease the severity of many side effects, and boost your mental and emotional health. And of course, exercise is key to managing your weight.

But exercise can be challenging to fit in during or after cancer treatment, even if you had a routine before your diagnosis. The American College of Sports Medicine’s general physical activity recommendations for people who’ve been diagnosed with cancer say, in part, that people should start slowly and build up to at least 150 minutes of moderate exercise (think brisk walking) or 75 minutes of vigorous exercise (think running, swimming, or biking uphill) each week. But if that feels daunting, don’t worry — just doing what you can is better than nothing. 

“If you’re struggling with your weight and body composition after breast cancer, you’re not alone,” says Sami Mansfield, a certified cancer trainer and founder of Cancer Wellness for Life. “The good news is, any amount of exercise can help improve your well-being and assist with weight loss.”

In this video, Mansfield, who has worked for more than 20 years with people in cancer treatment, demonstrates a short series of strength exercises designed to build muscle, which will boost metabolism and help you lose weight.

 

Weight loss

While less common, people do lose weight during and after breast cancer treatment. If you were carrying some extra weight when you were diagnosed, you might think losing weight is a good thing. But too much weight loss can make any treatment side effects you’re having more severe. If you lose more than 5% of your body weight in a year without trying, you also have a higher risk of developing an infection and fatigue.

Causes of weight loss

Nausea from chemotherapy or other treatments can make eating the last thing you want to do. Chemotherapy also can change your appetite and the way food tastes; many people report that food tastes metallic and unappealing when they’re receiving chemo.

Diarrhea and vomiting caused by chemotherapy and targeted therapy medicines also can make eating difficult and can make you dehydrated, which can cause weight loss.

Managing weight loss

As with weight gain, it makes sense to talk to your cancer care team about weight loss. They can help you figure out what may have caused you to drop weight and give you advice on what you can do to manage it. They can also help you figure out a healthy weight for your body type.

A registered dietitian who has experience working with people with cancer also can help you design an eating plan that can help you gain back what you’ve lost.

Other steps you can take:

  • Try to eat small meals or snacks every hour to make sure you're getting enough calories.

  • Make sure to drink enough water and other clear liquids (herbal tea, chicken broth, sports drinks)

  • Snack on foods high in protein, such as yogurt and cottage cheese, lean meat or fish, nuts or nut butter, trail mix, and lentils.

  • Add smoothies, milkshakes, or nutritional drinks or bars to your diet to get more calories and protein.

  • Use low-fat milk in some recipes instead of water.

— Last updated on August 14, 2025 at 5:45 PM

Join in the conversation about Managing Side Effects of Breast Cancer and Its Treatment
Connect with a supportive community of people discussing thousands of topics in hundreds of forums on our discussion boards. Our community welcomes anyone and everyone diagnosed with breast cancer, concerned about a breast condition, or caring for a loved one affected by breast cancer.
Learn more
https://images.ctfassets.net/zzorm7zihro2/6ShoWUym8XnS0MupfA4jMv/45647308616629a0bdae25eaffff16e4/Maria_D-Alleva-headshot-edited.png
https://images.ctfassets.net/zzorm7zihro2/4b1909b4-1e96-4fd4-a767-a1b67f442295/fae0a28dc0ce88b1cdf32727f330e276/tonya-sanders-headshot-treatment-breast-reconstruction-women-of-color-LARGE.jpg
https://images.ctfassets.net/zzorm7zihro2/5YSrYqg12AxKIjzjqoZttS/3ad1d98f4ce5b555b276f938fb1e43cc/Bmpntherd-headshots_400x400.png
https://images.ctfassets.net/zzorm7zihro2/e7fd5d11-c89f-49c8-8fda-8bef1706d057/12a65e1084743fd57cbe3c2a797c6116/Kite250x250.jpg_1402146457
https://images.ctfassets.net/zzorm7zihro2/d8a56a60-13ac-410f-9a1f-9fa65649b22c/d0ad41ff0d0fb853023e228b2695c2e7/omt_250x250.jpg_1487716813