| Abstract: | Although there is strong epidemiological evidence to suggest the preventability of many cancers based on non-experimental studies at both ecological and individual levels, there have been few applications of that knowledge to randomized trials. The randomized trials were mainly designed to demonstrate the effect of chemoprevention on risk of cancer. The results of such preventive trials have not been convincing and the effects remained small or non-existent. Before any negative conclusions, the inherent deficiencies of such studies should be considered. The number of cancers in many of the studies is limited, allowing large random variation in the results. The follow-up is short compared with the long period of carcinogenesis from the first exposure to the occurrence of an invasive cancer. The negative results do not prove that cancer occurrence is independent of chemical substances. Environmental factors, individual habits and lifestyle play a role in the aetiology of cancer. A healthy lifestyle and a healthy environment from early childhood are likely to affect the cancer risk, whereas the consumption of pharmaceutical products starting in middle age may remain relatively useless and is likely to be of low cost-effectiveness. However, for the time being very little is known about the effects of chemoprevention and the research is likely to become more intense. |
