Abstract Background and Aim The number of patients with ulcerative colitis (UC) has been increasing in Japan. To elucidate the risk factors for developing UC in Japan, a hospital‐based case–control study was conducted. This study examined the association between smoking/drinking habits and UC onset in detail. Methods Cases comprised 132 Japanese patients who had been newly diagnosed with UC between 2008 and 2014 at 38 collaborating hospitals in Japan, and controls comprised 167 patients without UC. Detailed data on smoking and drinking habits were collected using a self‐administered questionnaire. Results Ex‐smokers showed an increasing odds ratio (OR) for UC development compared with never smokers (OR 2.42, 95% confidence interval 1.24–4.72). The ORs of ex‐smokers were particularly high among subjects aged less than 40 years, subjects who had smoked more than 10 pack‐years, and subjects who were within 13 years of quitting smoking. Regarding drinking habits, ex‐drinkers also showed a more than twofold higher OR for UC compared to never drinkers. Ex‐drinkers 40 years or older, ex‐drinkers who had consumed more than 364 drink‐years, and subjects who were less than 6 years after quitting drinking showed increased ORs for UC. Conclusion These findings suggest the need for careful attention for UC onset among heavy smokers who quit smoking before 40 years of age and heavy drinkers who quit drinking at ≥40 years of age.
