Approved by the Cancer.Net Editorial Board, 05/2018

Listen to the Cancer.Net Podcast: What is Chemoprevention?, adapted from this content.

Cancer begins when healthy cells change and grow uncontrollably. This forms a mass called a tumor. The process of a healthy cell transitioning to a cancerous one usually takes years. And many genetic, dietary, and lifestyle factors, such as smoking, may influence this process.

Cancer chemoprevention uses substances to stop cancer from developing. These substances may be natural or made in a laboratory. A doctor uses chemoprevention to lower a person’s risk of developing cancer, especially for:

  • People who are at a high risk of developing cancer. This includes those with an inherited cancer syndrome or a family history of cancer.

  • People who have already had cancer. Chemoprevention can lower the risk of a cancer recurrence or a new cancer. A recurrence is when the cancer comes back after treatment.

Chemoprevention drugs are typically not used to treat cancer. And taking them does not fully protect a person from developing cancer in the future.

Examples of chemoprevention drugs

  • Tamoxifen (Nolvadex, Soltamox) and raloxifene (Evista). Researchers have studied these medicines as a way to reduce breast cancer risk. They are most effective in lowering the risk of estrogen receptor-positive breast cancer. This type of breast cancer depends on the hormone estrogen to grow.

    Tamoxifen blocks the effects of estrogen on tumor growth. It has also been shown to lower the risk of a breast cancer recurrence. Tamoxifen is also effective in lengthening the lives of women with early-stage breast cancer.

    Raloxifene has been shown to lower the risk of breast cancer in women who have gone through menopause.

  • Aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs). NSAIDs may lower the risk of many types of cancer in people with an average risk of cancer.

Risks and benefits of chemoprevention

The risks and benefits of chemoprevention are different for each person. For example:

  • Drugs that may lower the risk of cancer can increase the risk of side effects. People with a higher risk of developing cancer may be willing to accept specific side effects. But others may not want to use a drug that gives them side effects when they are not sick.

  • The effect of chemoprevention seen in research studies may be different for you.

Ask your health care team about the risks and benefits of chemoprevention specific to you.

Chemoprevention in clinical trials

Researchers use clinical trials to evaluate whether chemoprevention substances are safe and effective in delaying or preventing cancer. Clinical trials are research studies involving volunteers. Some clinical trials have shown that certain types of chemoprevention cause harm. For example:

  • Beta carotene, a substance found in carrots, squash, and similar vegetables, was thought to help prevent lung cancer. But clinical trials showed that it raised the risk of lung cancer in people who smoked.

  • A clinical trial of selenium and vitamin E for prostate cancer showed that neither lowered the risk of the disease. And there was evidence that men who took vitamin E were at a higher risk for prostate cancer. Learn more about vitamins and minerals.

Clinical trials can also show that some substances used for chemoprevention do not work for everyone. When evaluating the results of chemoprevention clinical trials, make sure to look at the group of participants. Often, people participating in these types of clinical trials have known, increased risks for cancer. These may include smoking or having a family history of cancer. Therefore, the results of the study may not be applicable to everyone.

Questions to ask your health care team

  • What is my risk of developing cancer?

  • How does my current health status affect my risk of developing cancer?

  • How much will chemoprevention lower my cancer risk?

  • What side effects could I experience?

  • What are the risks and benefits of chemoprevention for me?

  • Is a chemoprevention clinical trial an option for me?

More Information

Cancer Screening

Understanding Cancer Risk

Additional Resource

National Cancer Institute: Prevention