The short answer
Scientists identify carcinogens by combining three kinds of evidence: studies of exposed people, laboratory animal studies, and research on how an agent behaves in cells. Expert groups such as NTP weigh all relevant information together.
Evidence comes from human (epidemiological) studies, animal studies, and cell/mechanism research.
NTP lists a substance as a known human carcinogen based on sufficient evidence from studies in humans.
NTP can list a substance as reasonably anticipated to be a human carcinogen when human evidence is limited but animal evidence is sufficient.
NTP says its conclusions rest on scientific judgment, with consideration given to all relevant information.
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This page explains how scientists decide whether something can cause cancer. It is for anyone trying to judge a new cancer headline.
Three kinds of evidence
Deciding whether something can cause cancer is detective work. Scientists gather three main kinds of evidence and weigh them together.
1. Human studies. NTP lists a substance as a known human carcinogen when there is sufficient evidence of carcinogenicity from studies in humans.
2. Animal studies. NTP can list a substance as reasonably anticipated to be a human carcinogen when there is limited human evidence but sufficient evidence in experimental animals.
3. Mechanism (how it works). NTP says it also considers information such as dose response, exposure route, chemical structure, metabolism, and mechanism of action.
No single study decides
NTP says its conclusions about carcinogenicity are based on scientific judgment, with consideration given to all relevant information. Expert groups look at the whole body of evidence.
Who does the judging
Two of the most influential groups are:
- The International Agency for Research on Cancer (IARC), which NCI says convenes expert scientists to evaluate the evidence that an agent can increase the risk of cancer. See IARC groups.
- The U.S. National Toxicology Program (NTP), which publishes the Report on Carcinogens.
The NTP Report on Carcinogens identifies substances known or reasonably anticipated to cause cancer in humans. NCI cautions that a substance being listed does not mean it will necessarily cause cancer: the amount and duration of exposure and your genetic background also matter.
Why classifications change over time
NCI's page does not describe how lists change over time. Check the current NTP Report on Carcinogens for up-to-date listings.
What this means for you
When you see a new claim that something causes cancer, ask whether it comes from an expert group such as NTP or IARC reviewing the evidence. Ask your care team how it applies to you.
Common questions
How do scientists prove something causes cancer?
NTP bases its conclusions on scientific judgment, considering all relevant information, including studies in humans, studies in experimental animals, and how the agent works (mechanism).
Why use animal studies at all?
NTP can list a substance as reasonably anticipated to be a human carcinogen when there is sufficient evidence in animals but only limited evidence in humans.
Do the lists change?
This page's source, NCI, does not describe how often lists change. Check the current NTP Report on Carcinogens for the latest listings.
Who decides?
NCI names two groups: the U.S. National Toxicology Program (NTP), an interagency program of HHS, and the International Agency for Research on Cancer (IARC). NTP publishes the Report on Carcinogens.
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Sources last checked: 2026-07-21 what this meansLast updated: 2026-10-16Next planned review: 2028-07-21
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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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