The short answer
Cancer and cancer treatment can change your weight in both directions. Side effects that affect taste, appetite, and eating can lead to weight loss and malnutrition, while some treatments, like hormone therapy, can cause weight gain. A registered dietitian and your care team can build a nutrition plan that fits you.
Cancer and its treatments can cause side effects that affect taste, smell, appetite, and the ability to eat or absorb nutrients.
When eating problems are not managed, they can lead to malnutrition and, in some cases, cancer cachexia — a wasting syndrome with weight, fat, and muscle loss.
Some treatments work the other way: hormone therapy drugs can cause weight gain and fluid retention.
People with cancer often need extra protein and calories — a diet that may look different from typical healthy eating.
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The full explanation.
The simple version
Cancer and cancer treatment can change your weight — in either direction. Side effects can make it hard to eat enough, which can lead to weight loss. Some treatments can cause weight gain instead. Either way, weight changes are something care teams watch closely, because good nutrition helps you stay strong through treatment.
Why weight loss can happen
Both cancer and its treatments may cause side effects that affect your taste, smell, appetite, and ability to eat enough food or absorb the nutrients from food. Common examples include:
- loss of appetite
- nausea and vomiting
- mouth sores, dry mouth, or trouble swallowing
- changes in the way food tastes
- constipation or diarrhea
- feeling full after eating a small amount of food (called early satiety)
People with cancers that directly affect the digestive system — such as cancers of the head and neck, esophagus, stomach, pancreas, liver, or colon — are more likely to have problems eating. But people with any type of cancer can find it hard to eat well during treatment.
When these problems are not managed, they can lead to malnutrition. Unmanaged malnutrition can lead to cancer cachexia, a wasting syndrome that causes weakness, weight loss, and fat and muscle loss — and it can occur even when you are eating well.
Why weight gain can happen
Some treatments have the opposite effect. Hormone therapy — used to slow or stop cancers that rely on hormones to grow, such as breast, ovarian, and prostate cancer — can cause weight gain. It can also cause fluid retention (edema) and high blood sugar.
Eating during treatment may look different
The National Cancer Institute explains that good nutrition for people with cancer may differ from what we usually think of as healthy. During treatment, you may need extra protein and calories to keep your strength up, prevent malnutrition, and keep your best possible quality of life. That can mean more meat, fish, eggs, dairy, fats, and plant-based proteins than someone without cancer would eat.
How care teams keep an eye on weight
If you have trouble eating and maintaining your weight, your nurse, doctor, or registered dietitian may ask about weight changes, changes in how much and what you eat, eating problems like nausea, and your ability to do daily activities.
If you are at risk of poor nutrition, your doctor can refer you to a registered dietitian, who reviews your food history, medications, weight, lab tests, and treatment side effects — and then creates a nutrition care plan with ways you and your family can improve your eating.
Things people discuss with their care team
For appetite loss, weight loss, or feeling full quickly, NCI lists ideas people often talk through with their team or dietitian:
- eating foods high in protein and calories, and eating high-protein foods first in a meal
- eating five to six smaller meals and snacks a day instead of three large ones
- eating the largest meal when appetite is strongest — breakfast, lunch, or dinner
- drinking milkshakes, smoothies, juices, or soups when solid food doesn't appeal
- drinking most fluids between meals rather than during them
- finding an enjoyable physical activity, which can help increase appetite
If eating stays very difficult, your team can discuss other options with you, such as medicines that increase appetite or nutrition support (tube feeding or IV nutrition). These are decisions made together with your doctor and dietitian.
Some treatments can also weaken the immune system, so your team may talk with you about food safety — handling and preparing food carefully to lower the risk of foodborne illness.
The takeaway
Weight changes during cancer treatment are common and treatable concerns, not personal failures. Telling your care team early about eating problems or a changing number on the scale gives them the best chance to help you stay nourished and strong.
Words to know
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Common questions
▸Why do people lose weight during cancer treatment?
Both cancer and cancer treatments can cause side effects that affect taste, smell, appetite, and the ability to eat enough food or absorb its nutrients. Problems like appetite loss, nausea, mouth sores, taste changes, and feeling full quickly can all make it hard to eat enough, which can lead to weight loss and malnutrition.
▸Can cancer treatment cause weight gain?
Yes. Hormone therapy drugs, used for cancers like breast, ovarian, and prostate cancer, can cause weight gain. They can also cause fluid retention (edema) and high blood sugar.
▸What is cancer cachexia?
Cancer cachexia is a wasting syndrome that can develop when malnutrition is not managed. It can cause weakness, weight loss, and loss of fat and muscle — and it can occur even when a person is eating well.
▸Is a 'healthy diet' during treatment the same as a normal healthy diet?
Not always. NCI explains that people with cancer may need extra protein and calories to keep their strength up, prevent malnutrition, and maintain quality of life. That may mean more meat, fish, eggs, dairy, fats, and plant-based proteins than someone without cancer would eat.
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