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Efficacy and safety of peppermint oil in IBS 185 7 Exploratory supplementary analyses of effect modification Effect modification gender To explore possible effect modifiers in a supplementary exploratory analysis, we added the interaction of treatment group with the possible effect modifier to the model. This explorative post hoc analysis showed that gender was an effect modifier of treatment group and the primary abdominal pain outcome, likelihood ratio (LR) test for interaction term: P =0.016). For men ( N =42), the small-intestinal release peppermint oil did have a significant treatment effect on the primary outcome with 81.8% of men being a responder (OR9.14, 95%CI 1.36; 61.54, P =0.02), compared with 33.3% in the placebo group. For women ( N =147), however, no significant differences were found in abdominal pain response rate between small-intestinal release peppermint oil, with 39.2% of women being a responder, (OR of 1.20, 95% CI0.53; 2.76, P =0.67), compared with 34.7% in the placebo group. The relatively low number of included men implies that the effect found should be interpreted with appropriate caution. Effect modification primary care versus secondary/tertiary care We explored a potential effect modification of being a primary care patient versus secondary/tertiary care patient in a supplementary exploratory analysis. The proportion of abdominal pain responders according to FDA definition (30% decrease in worst abdominal pain, in at least 50% of treatment weeks) did not differ significantly between primary and secondary/tertiary care patients, i.e. 43/109 (39.4%) primary care patients were responders, compared with 34/80 (42.5%) secondary/tertiary care patients ( P =0.793). To double-check however, we added the interaction of treatment group with the categorical variable of being a primary care patient (or not) to the model that was corrected for minimization variables age, gender, IBS-subtype, and inclusion center. This explorative post hoc analysis showed that being a primary care patient was not a significant effect modifier of treatment group and the primary abdominal pain response outcome (likelihood ratio (LR) test for interaction term: P =0.398). Effect modification baseline abdominal pain scores To assess potential effect modification of baseline abdominal pain on the primary outcome abdominal pain response, we added the interaction of treatment group with baseline mean worst abdominal pain to the model that was corrected for minimization variables age, gender, IBS-subtype, and inclusion center. This explorative post hoc