Two large studies have been published stating that beta- carotene may actually increase the risk of long cancers for smokers. One Finnish study published in the Journal of the American Medical Association became very famous as it demonstrated that male smokers taking antioxidant pills increased their risk of lung cancer by 18%. A total of 29,133 male smokers were randomly divided in four groups: only vitamin E (50 mg per day), only beta carotene (20 mg per day), both vitamin E and beta carotene and placebo. The smokers were followed up to a period of eight year. The researchers found that supplementation with only vitamin E had no influence on the incidence of lung cancer, but to their surprise they found that the intake of beta-carotene supplements increased the incidence of lung cancer by 18 percent. Only the intake of vitamin E supplement reduced the incidence of prostate cancer. They concluded that these antioxidant supplements may actually have harmful as well as beneficial effects . Another study, The Beta-Carotene and Retinol Efficacy Trial (CARET), looked at more than 18,000 men and women who smoked heavily or were exposed to asbestos. Also in this study the researchers found an increased risk of lung cancer in those taking combined beta-carotene and vitamin E supplements. They found a 28 percent increase in incidence of lung cancer and a 17 percent increase in incidence of death with participants taking the antioxidant supplements, compared with participants in the placebo group. They also found that women were more affected by the adverse effects of the supplements .
Tanvetyanon T and Bepler G conducted a recent meta-analysis of the possible risk of lung cancer by beta-carotene in multivitamin formulas among smokers. They systematically reviewed scientific literature and also looked at the beta-carotene levels in multivitamins. The found that beta-carotene supplementation was significantly associated with an increased risk of lung cancer. The average daily intake in these studies was 20 to 30 mg beta-carotene whereas the median dosage of beta-carotene from supplements is only 0.3 mg. However, some supplements used visual health improvement contained up to 24 mg beta-carotene . An experiments with ferrets was not able to demonstrate the possible pro-oxidant effect of beta-carotene, in combination with vitamin E and ascorbyl palmitate, on ferrets exposed to smoke-derived carcinogenic agent benzo[a]pyrene. The researchers found no effect of the antioxidants on cell proliferation markers or formation of squamous metaplasia lesions. They also found that beta-carotene may prevent against excess cell proliferation.
 The Alpha-Tocopherol, Beta Carotene Cancer Prevention Study Group. "The effect of vitamin E and beta carotene on the incidence of lung cancer and other cancers in male smokers." N Engl J Med. 1994 Apr 14;330(15):1029-35.
 Goodman GE, Thornquist MD, Balmes J, Cullen MR, Meyskens FL Jr, Omenn GS, Valanis B, Williams JH Jr. "The Beta-Carotene and Retinol Efficacy Trial: incidence of lung cancer and cardiovascular disease mortality during 6-year follow-up after stopping beta-carotene and retinol supplements." J Natl Cancer Inst. 2004 Dec 1;96(23):1743-50.
 Tanvetyanon T, Bepler G. "Beta-carotene in multivitamins and the possible risk of lung cancer among smokers versus former smokers: a meta-analysis and evaluation of national brands." Cancer. 2008 Apr 21.
 Fuster A, Picó C, Sánchez J, Oliver P, Zingaretti MC, Murano I, Morroni M, Hoeller U, Goralczyk R, Cinti S, Palou A. " Effects of 6-month daily supplementation with oral beta-carotene in combination or not with benzo[a]pyrene on cell-cycle markers in the lung of ferrets." J Nutr Biochem. 2008 May;19(5):295-304.