An antioxidant is a substance that protects cells from damage by inhibiting oxidation, a chemical reaction that produces free radicals (highly reactive chemicals) that damage the DNA in cells. These free radicals are produced regularly in our bodies as by-products of natural metabolic processes.
Since damage to cells by free radicals can cause the damage that leads to cancer, antioxidants in our diet may reduce the risk of developing cancer. Examples of antioxidants include vitamin C, vitamin E and flavonoids.
While antioxidant supplements may have benefits for some people, they can be harmful when taken in large quantities. As a rule of thumb, it may help to keep in mind that any medication -- whether a prescription drug or a nutritional supplement such as an antioxidant -- that has the potential to be helpful, also has the potential to be harmful.
For those with cancer, it is very important to discuss any antioxidant supplements you consider using with your oncologist. In theory, since antioxidants protect cells from damage, there is the potential that these supplements could protect cancer cells from "damage" and reduce the effectiveness of chemotherapy.