The short answer
Losing weight without trying can be a sign of some cancers, though it has many other causes. MedlinePlus says to contact your provider if you lose more than 10 pounds or 5% of your body weight over 6 to 12 months or less and do not know why.
Weight loss without trying can be a sign of cancer, sometimes even when there are no other symptoms.
MedlinePlus says to contact your provider if you lose more than 10 pounds or 5% of your body weight over 6 to 12 months or less without knowing why.
For someone with cancer, ACS says to tell the care team about a loss of more than 3 pounds in a week.
Many things other than cancer can cause weight loss, such as an overactive thyroid, diabetes, depression, or stress.
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The simple version
Losing weight without trying can be a sign of some cancers. MedlinePlus defines unintentional weight loss as losing 10 pounds (4.5 kilograms) or 5% of your normal body weight over 6 to 12 months or less without knowing the reason. It says to contact your provider if that happens, or if you have other symptoms along with the weight loss. Weight loss has many causes that are not cancer. But it should be checked.
Why cancer can cause it
MedlinePlus lists cancer as a cause of unintended weight loss, even when other symptoms are not present. ACS says cancer cells use a lot of the body's energy. It also says some cancers release substances that change how the body uses food for energy.
For someone who already has cancer
ACS says someone with cancer should tell their cancer care team about any of these:
- Losing more than 3 pounds in a week, or in less than a week. ACS says quick weight loss can be a sign of dehydration, which can be serious.
- Losing more than 5% of body weight in a year without trying.
- Feeling weak, very tired, or dizzy.
- Not being able to do usual activities.
- Not being able to eat or drink as much as usual.
Other causes
MedlinePlus lists many other causes. They include depression, stress or anxiety, chronic infection, chronic illness, some medicines, long-lasting digestive problems, undiagnosed diabetes, and an overactive thyroid. ACS says most signs and symptoms, even ones that could point to cancer, are more often caused by other conditions.
What a doctor checks
MedlinePlus says your provider will do a physical exam and check your weight. You will be asked how much weight you lost, when it began, and how fast. You will be asked about your appetite, diet, exercise, medicines, stress, and other symptoms. Your provider decides which tests you need. You may also see a dietitian.
If tests come back normal
If your weight loss continues after a first check, say so at your next visit. Tell your provider too if you have new symptoms, like pain, fatigue, or a change in bowel habits.
When to get help sooner
- Call 911 or go to the nearest emergency department if you vomit blood or something that looks like coffee grounds, your stools are black and tarry, or you have heavy bleeding. Do not drive yourself.
- Call your care team right away if you are having cancer treatment and you cannot eat or drink, you feel dizzy, or vomiting will not settle. If you are having chemotherapy, call day or night.
- Call your care team right away if solid food is catching in your throat or chest on the way down.
- Call your care team within a day or two if you have lost weight without trying and it meets the numbers above, or it comes with belly pain, unshakeable tiredness, or bowels behaving differently. For someone with cancer, do not wait after a loss of more than 3 pounds in a week.
What to ask your doctor
- Is my weight loss significant enough to investigate?
- Could it be caused by something other than cancer?
- What tests would help find the cause?
- Should I track my weight and report back?
Sources
Common questions
When is weight loss a concern?
MedlinePlus says to contact your provider if you have lost more than 10 pounds or 5% of your normal body weight over 6 to 12 months or less, and you do not know the reason. Also contact your provider if you have other symptoms along with the weight loss.
Which cancers cause weight loss?
Many cancers can cause weight loss. MedlinePlus lists cancer as a cause of unintended weight loss, even when other symptoms are not present. ACS says cancer cells use a lot of the body's energy, and some cancers release substances that change how the body uses food for energy.
Does weight loss always mean cancer?
No. MedlinePlus lists many other causes, including depression, stress or anxiety, an overactive thyroid, diabetes that has not been diagnosed, chronic infections, chronic illness, some medicines, and long-lasting digestive problems. A doctor can help find the cause.
What should I do?
See a doctor if you have lost weight without trying, especially if it meets the numbers above or comes with other symptoms. If you are having cancer treatment, tell your care team about weight loss and call right away if you cannot eat or drink or you feel dizzy.
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Last updated: 2026-09-29Next planned review: 2027-07-07
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Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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