Can Cancer Be Prevented? What the Research Actually Shows

Three tiers, not one list

Prevention coverage tends to flatten everything into one pile. A food "fights cancer." A supplement "cuts risk." A habit "causes" it.

NCI does not work that way. Its prevention summary sorts evidence into tiers, and knowing which tier a claim sits in tells you almost everything.

There are factors known to increase risk. There are factors that may affect risk. There are interventions known to lower risk. And there is a fourth category people rarely see quoted: interventions not known to lower risk.

What is known to raise risk

NCI lists four things in this tier.

Cigarette smoking and tobacco use. NCI names smoking as the leading cause of acute myelogenous leukemia, bladder, cervical, esophageal, kidney, lung, oral cavity, pancreatic and stomach cancer. It states that scientists believe cigarette smoking causes about 30 percent of all cancer deaths in the United States. Not smoking, or stopping, lowers both the risk of getting cancer and the risk of dying from it.

Certain infections. Human papillomavirus raises the risk of cancers of the cervix, penis, vagina, anus and oropharynx. Hepatitis B and hepatitis C raise the risk of liver cancer. Epstein-Barr virus and Helicobacter pylori appear on the same list.

Radiation, including ultraviolet light from the sun and ionizing radiation.

Immunosuppressive medicines taken after an organ transplant.

That is the whole tier. Four items.

What may affect risk

This second tier is where most headlines live, and the difference in wording is deliberate.

NCI lists diet, alcohol, physical activity, obesity, diabetes, and environmental exposures as factors that may affect the risk of cancer.

"May affect" is not a hedge for politeness. It means the evidence is real but weaker, harder to isolate, or inconsistent across studies. People who eat well also tend to exercise, smoke less, and see doctors more. Untangling one factor from the others is genuinely difficult.

Our pages on alcohol and cancer and physical activity and cancer go into what the evidence supports for each.

What is known to lower risk

Here the list is short and specific, and mostly aimed at people already at high risk.

Chemoprevention means using a substance to lower risk. NCI reports that selective estrogen receptor modulators, such as tamoxifen or raloxifene, reduce breast cancer risk in women at high risk. Side effects like hot flashes mean they are not often used for prevention. Finasteride lowers prostate cancer risk. COX-2 inhibitors may prevent colon and breast cancer, but possible heart problems have limited study of them.

Weight-loss surgery, also called bariatric surgery, has been shown to lower cancer risk, through the weight loss it produces.

Risk-reducing surgery is offered to some people with changes in BRCA1 or BRCA2, an inherited syndrome, or a strong personal or family history. NCI is explicit that a cancer risk assessment and counseling should come before that decision. Our page on genetic testing for cancer risk explains what that assessment involves.

Vaccination against HPV and hepatitis B belongs here too, because it removes a known cause rather than adjusting a probability.

What has not been shown to work

This is the tier that almost never makes it into a headline, and NCI states it plainly.

Aspirin has not been shown to prevent most cancers. Studies show mixed results, and most show no prevention effect. There is evidence that long-term aspirin may prevent colorectal cancer in certain people. NCI also notes a randomized trial suggesting aspirin may make cancer grow more quickly in older people, with longer follow-up needed. Bleeding in the gut or brain is a known side effect.

Vitamin and dietary supplements have not been shown to prevent cancer. NCI names the ones studied without benefit: vitamin B6, vitamin B12, vitamin E, vitamin C, beta carotene, folic acid, selenium, and vitamin D.

One of those went further than useless. The Selenium and Vitamin E Cancer Prevention Trial found that vitamin E taken alone increased the risk of prostate cancer, and the raised risk continued after the men stopped taking it.

Where screening fits, and where it does not

Screening is not prevention, with one exception. Removing a colon polyp prevents a cancer that has not formed yet. Most screening finds cancer that already exists.

NCI's screening summary is candid about the costs. Some procedures cause serious problems; colonoscopy and sigmoidoscopy can tear the lining of the colon. False positives cause anxiety and lead to further tests that carry their own risks. False negatives can delay care for someone who does have symptoms. And for some cancers, finding and treating early does not improve the chance of cure or extend life.

That is why NCI frames screening as informed and shared decision-making rather than a checklist.

What to bring to an appointment

• Which of the four known risk factors apply to me, and what can be done about each?
• Is my family history strong enough to warrant a formal risk assessment?
• Am I up to date on HPV and hepatitis B vaccination, and are my children?
• Which screening tests fit my age and history, and what are the harms of each?
• I read a claim about a supplement or food. Which evidence tier does it sit in?

What this does not mean

• Prevention lowers odds. It does not issue guarantees, in either direction.
• Some risk factors cannot be changed. Age, inherited genes, and past exposures are fixed.
• "No evidence of benefit" is not the same as "proven harmful," though vitamin E in the SELECT trial was both.
• Chemoprevention drugs carry real side effects and are aimed at specific high-risk groups, not the general population.
• Nothing here is a personal recommendation. The list of what applies to one person comes from that person's own history.

Sources

• NCI PDQ: Cancer Prevention Overview (Patient Version)
• NCI PDQ: Cancer Screening Overview (Patient Version)
• NCI: Cancer Causes and Prevention

How this article was prepared

An AI-assisted editorial system helped prepare this page. No named medical reviewer has reviewed it unless one is listed.

Cancer Explained is published by the National Cancer Information Foundation. It is not medical advice and does not suggest a test or treatment.

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