Background Little is known about patients’ Perceived Sensitivity to Medicines (PSM), “the belief that one is especially sensitive to the actions and side effects of medicines”. Objective (i) To explore the extent of and factors associated with high Perceived Sensitivity to Medicines in a Norwegian population of chronic medicine users; (ii) to assess the psychometric characteristics of the tool to measure PSM. Setting Community pharmacies in the Oslo area, Norway. Method A cross-sectional, questionnaire-based study was conducted between October 2015 and January 2016. Patients filling prescriptions for chronic disorders were recruited. Main outcome measure Perceived sensitivity to medicines. Results The study population included 214 patients (response rate 36.7%). In total 20.1% of the patients reported low, 61.7% moderate and 18.2% high perceived sensitivity to medicines. Factors positively associated with high perceived sensitivity were female gender (Adjusted Odds Ratio (aOR) 5.33, 95% CI 1.52 to 18.72, p < 0.001) and having a non-native language (aOR 4.76, 95% CI 1.48 to 15.30, p < 0.001); lower educational level (aOR 0.43, 95% CI 0.17 to 1.07, p < 0.001) and using generic medicines (aOR 0.12, 95% CI 0.03 to 0.57, p < 0.001) were negatively associated with high perceived sensitivity to medicines. There was no association between the perceived sensitivity and the number of prescription medicines taken. The Norwegian version of the Perceived Sensitivity to Medicines tool demonstrated good psychometric characteristics. Conclusion Almost one out of five patients in this study reported high sensitivity to medicines. Female gender, having a non-native language, lower educational level and using generic medicines were important factors related to the perceived sensitivity. Health care providers should be aware of the impact negative expectations about medicines can have on health behaviors and treatment outcomes, and seek to elicit and address patients’ beliefs about their personal sensitivity to medicines.