Antioxidants could have significant impact on risk for cancer but their effects wear off some years after people stop taking the supplements, suggest new findings.

The study, published in this week's JAMA, appears to back the advice that smokers should avoid taking the antioxidant beta-carotene. It also found however that vitamin E supplements may help prevent prostate cancer in men who smoke. But when the team from Finland's National Institute of Public Health followed up the 29,000 male smokers studied in the Alpha-Tocopherol, Beta-Carotene Cancer Prevention (ATBC) Study, they found that the effects of both antioxidants did not last longer than six years.

In the study, 29 133 male smokers aged 50 to 69 years received either 50mg of alpha-tocopherol (vitamin E), 20mg beta-carotene, both supplements, or a placebo daily for five to eight years.

Around 1000 men were diagnosed with lung cancer, with those taking beta-carotene supplements at 1.06 times higher risk. Prostate cancer was however reduced by 22 per cent for participants receiving alpha-tocopherol compared with non-recipients. No late preventive effects on other cancers were observed for either supplement.

For all cause mortality, risk associated with taking the antioxidants was 1.01 for the alpha-tocopherol group and 1.07 for those taking beta-carotene. "The excess risk for beta-carotene recipients was no longer evident four to six years after ending the intervention and was primarily due to cardiovascular diseases", reported the researchers.