The present invention provides a method capable of detecting a cancer and/or a carcinogenic risk, and a reagent for the detection thereof. The present invention also provides a method for screening for a pharmaceutical drug for reducing a carcinogenic risk and a pharmaceutical composition. Since cancer development closely correlates with alteration in gut microbiota, cancer development and/or a risk of cancer development are detected by detecting alteration in gut microbiota and a secondary bile acid produced by an intestinal bacterium. The present invention further provides a method for screening for a pharmaceutical drug with a gut microbiota as an index, and a pharmaceutical composition.