Free Radicals: Definition, Cause, and Role in Cancer

What Exactly Are Free Radicals and Why Are They Important?

Free radicals are highly reactive and unstable molecules that are made by the body naturally as a byproduct of normal metabolism. Free radicals can also be made by the body after exposure to toxins in the environment such as tobacco smoke and ultraviolet (UV) light.

Free radicals have a lifespan of only a fraction of a second, but during that time can damage DNA, sometimes causing mutations that can increase your risk of getting health conditions like heart disease and cancer.

Antioxidants in the foods we eat can neutralize the unstable molecules and reduce the chances of them causing damage.

This article will explain what free radicals are. You will also learn the kind of harm free radicals cause, which supplements or vitamins may protect against free radicals, and what foods to include in your diet to reduce oxidative stress.

Many different berries held in hands
​Peter Close / / Stock Photo

What Are Free Radicals?

Free radicals are atoms that have one electron that is not paired. Sometimes they have more than one unpaired electron.

Electrons need to be paired to be stable. Free radicals constantly look to bind with another atom or molecule to become stable.

As they're looking around, free radicals can damage human cells. The consequences of that damage include effects like speeding up the aging process and even playing a role in the development of cancer and other diseases.

Types of Free Radicals

There are many types of free radicals. The oxygen free radicals (reactive oxygen species) are the most important in humans.

Examples of oxygen free radicals include:

  • Singlet oxygen (when oxygen is "split" into single atoms with unpaired electrons)
  • Hydrogen peroxide
  • Superoxides
  • Hydroxyl anions

Sources of Free Radicals

Free radicals can come from normal metabolic processes in the body or from exposure to cancer-causing substances (carcinogens) or other harmful substances in the environment.

Free Radicals From Normal Body Processes

The body makes free radicals while breaking down nutrients to give us the energy we need to function.

The production of free radicals during these normal processes in the body is one reason why our risk for many diseases goes up as we age even if we haven't been exposed to toxic substances.

Free Radicals From Carcinogens

Free radicals can also be made when we're exposed to carcinogens such as:

  • Tobacco smoke
  • Ultraviolet radiation
  • Radon
  • Environmental and occupational substances and chemicals (e.g., asbestos and vinyl chloride)
  • Some viruses
  • Medical radiation
  • Air pollution

Effect of Free Radicals on the Body: Oxidative Stress

Once free radicals are made, they're free to do damage to the body—whether they came from exposure to a carcinogen or the normal processes of the body.

The availability of free radicals creates something called oxidative stress in the body. It's called "stress" because the chemical reactions that let free radicals get an electron occur in the presence of oxygen.

There are several parts to this process. When one free radical "steals" an electron from a molecule, that molecule becomes a free radical because it's missing an electron. That cycle continues and makes more free radicals.

Free radicals can damage the body's DNA. Our DNA contains our genes, proteins, lipids, cell membranes, and other important substances. Damaged DNA can lead to disease.

The Free Radical Theory of Aging

There are several theories about why our bodies age and free radicals are a key player in many of them. Free radicals are not considered fully responsible for aging-related changes, though; it's more likely that normal aging is related to many processes in the body.

How Free Radicals Can Cause Cancer

Damage to genes in the DNA can cause them to make ineffective proteins. Some of those proteins are an important part of making sure the DNA is working right.

A key area where damage can cause problems is in tumor suppressor genes. These genes direct the proteins that repair damaged DNA or cause cells that are damaged so badly they can't be fixed to be removed through "programmed cell death" (apoptosis).

Usually, it's a series of mutations in tumor suppressor genes and other genes that lead a cancer cell to form.

Antioxidants and Free Radicals

Many of the plant chemicals (phytochemicals) in our foods are antioxidants. These nutrients stop the formation of free radicals and may reduce the damage they would cause in the body.

The power of antioxidants to fight free radicals is one reason why a diet rich in vegetables and fruits has been linked with a lower risk of many diseases.

Examples of antioxidants that may help combat free radicals and oxidative stress include:

Many foods and drinks are good sources of different antioxidants, like berries and green tea.

Do Antioxidant Supplements Fight Free Radicals?

Studies have shown that a diet rich in antioxidants is associated with a lower risk of many chronic diseases, including cancer. However, using antioxidant supplements does not appear to have the same effect.

For example, research had shown that people who had a higher intake of foods rich in beta-carotene and vitamin E had a lower risk of developing lung cancer.

To find out why this might be the case, researchers did a study where one group of people took a daily supplement of beta-carotene, and the other did not to see if their risk of lung cancer would be affected.

The results were a bit surprising: the men in the study who smoked and took beta-carotene had a higher risk of developing lung cancer, not a lower risk.

Can People With Cancer Take Antioxidant Supplements?

If you're having treatment for cancer, you might be worried about free radicals and wonder if you should up your antioxidant intake to fight off more damage.

Always talk to your oncologist about any supplement you're thinking about trying. They will guide you on what is safe to take (or not) while you are having treatment.

However, taking antioxidant supplements may worsen the prognosis for some cancers, and certain vitamin supplements may make cancer treatments less effective.

In one study, postmenopausal women with breast cancer who used antioxidant supplements during chemotherapy and radiation had a poorer prognosis. Two other, separate studies found that antioxidant supplements (such as vitamin E) may promote the growth and spread of lung cancer.

While antioxidant supplements are often not recommended, your oncologist will likely encourage you to eat a balanced, nutritious diet that naturally contains antioxidants.

How to Reduce Free Radicals in Your Body

You can't completely avoid free radicals because they're part of a natural process in your body that you don't control. You also can't always avoid being exposed to toxins—for example, you might run into them at your job.

That said, you can do your best to avoid exposures and consider safety when you can't avoid them.

You can also arm your body with antioxidants to fight free radicals. While your body does make antioxidants, it doesn't make enough. For example, eating a "rainbow of foods" that will supply you with them is key.

That said, even when people "do everything right"—like avoiding carcinogens and eating an antioxidant-rich diet—they can still get cancer or other diseases.


Free radicals are unstable molecules in the body that can damage DNA in cells. In turn, this can increase your risk for disease, including cancer.

The body naturally makes some free radicals as a byproduct of the processes it normally does, but you can also get more free radicals by exposure to certain toxic substances.

Antioxidants, like those found naturally in fruits and vegetables, are a key way to "fight" free radicals and the oxidative stress they cause in your body. However, antioxidant supplements are less likely to help and may even do more harm than good.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Lynne Eldridge, MD
 Lynne Eldrige, MD, is a lung cancer physician, patient advocate, and award-winning author of "Avoiding Cancer One Day at a Time."