The effect of joint exposure to smoking and alcohol on risk of development of oral cancer was evaluated from a case-control study. Although a simple 2 × 2 relative risk table suggests almost complete synergy of the two exposures, regression analysis and a more detailed relative risk table which reduces confounding demonstrate the definite individual effects of the two exposures in addition to the synergistic effect. If smoking and drinking both are causes of oral cancer—as the data suggest—then the removal of both will be necessary to prevent a large proportion of the disease in males.
