Wikipedia
A Co-carcinogen is a chemical that promotes the effects of a carcinogen in the production of cancer. Usually, the term is used to refer to chemicals that are not carcinogenic on their own, such that an equivalent amount of the chemical is insufficient to initiate carcinogenesis. A chemical can be co-carcinogenic with other chemicals or with nonchemical carcinogens, such as UV radiation.
For example, sodium arsenite can be administered to mice at a low enough concentration that it does not cause tumors on its own, but it increases the rate of formation and size of tumors formed after UV exposure.
A chemical may act as a co-carcinogen even if it does not cause direct DNA damage such as mutation, as long as it can affect a cancer-related pathway. An example of this category includes chemicals within the phorbol ester family, which mimic a native signalling molecule. This ester is not mutagenic, but can increase the rate of cancer by promoting cell growth, a traditional hallmark of cancer.
A chemical may both have anti-carcinogenic properties and yet still be a co-carcinogen in combination with some carcinogens. Additionally, the carcinogenic modifying ability of a chemical can often be dose dependent, where low doses of the compound produce beneficial (or at least non-harmful) results (as in hormesis) while higher doses can lead to a toxic effect.
Evidence points to beta carotene being one example of such a compound, which has led researchers to caution against the emphasis on isolated dietary supplements and instead recommend a focus on promoting a diverse diet rich in fruits and vegetables.