Since Beta Carotene breaks down into Vitamin A in the body, it has basically the same antioxidant qualities of Vitamin A and may affect the chances of getting lung cancer. For a long time it was thought that mega doses of Beta Carotene would help prevent cancer and even fight cancer because it's an antioxidant.
It is shown that low to moderate doses of Beta Carotene reduce the risk of cancer in people. Recently, scientists carried out studies to test the theory that regular higher doses of Beta Carotene and Vitamin A can be even more effective. The studies were the Alpha-Tocopheral, Beta Carotene Cancer Prevention Trial and the Beta Carotene and Retinol Efficacy Trial (CARET) and the Physician's Health Study.

The Cancer Prevention Trial invited male smokers between the ages of fifty and sixty nine to take part in a study to determine if certain vitamin supplements would prevent lung and other types of cancer. They were given a pill with either 50mg of a form of Vitamin E, 20mg of Beta Carotene or a placebo daily for five to eight years.

CARET studied if the combination of Beta Carotene and Vitamin A supplements would actually prevent lung and other cancers in both genders between the ages of fifty to sixty nine who were smokers or former smokers and men between the ages of forty five and sixty nine who had been exposed to asbestos.

The Physician's Health Study tested whether Beta Carotene supplements and low-dose aspirin reduced the risk of cancer and heart disease in American male doctors, only 11 percent of the participants smoked.

Each of these trials showed that Beta Carotene wasn't helping many of its smoking participants and was in fact hurting some of them. The CARET interim results showed that there were twenty eight percent more lung cancers in the group taking Beta Carotene and Vitamin A. While there were seventeen percent more deaths for the Beta Carotene takers. These results were so similar to the Cancer Prevention Trial final results from 1994 that they told the participants to stop taking the supplements twenty one months before the trial was scheduled to end. The Cancer Prevention Trial showed eighteen percent more lung cancer and eight percent more deaths. While the Physician's Health Study showed that there were no significant benefits or problems associated with taking Beta Carotene.

Ultimately these trials showed that while Vitamin A and Beta Carotene are antioxidants in moderate amounts and even helped to lower the risk of cancers in nonsmokers in the case of smokers it gives a higher risk of lung cancer. There are several possible theories for why it does this but the leading thought is that it acts as a pro oxidant in cells exposed to the tar created over time by smokers, and exacerbating the oxidative damage that's already been done and causing changes in the cell which lead to abnormal cell growth.