Objectives.  We report on the development and psychometric properties of a scale to measure perceived sensitivity to medicines (PSM).Design.  The internal consistency, test–retest reliability, criterion‐related, and predictive validity of the PSM Scale were evaluated using data collected as part of four previously published studies and one unpublished data set.Methods.  Participants (n= 1,166) included patients receiving treatment for HIV infection and hypertension, individuals receiving a travel vaccination, and undergraduate students. Criterion‐related validity was assessed by examining associations between the PSM and beliefs about medicines (Beliefs about Medicines Questionnaire), anxiety and depression (Hospital Anxiety and Depression Scale). Predictive validity was assessed by examining associations between the PSM and medication adherence and with symptom reports following vaccination. Test–retest reliability was assessed in an undergraduate sample who completed the PSM on two occasions, 2 weeks apart.Results.  Test–retest reliability was high (r= .89,p<.001). Cronbach's alpha ranged from 0.79–0.94. Consistent with expectations, high PSM scores were associated with negative beliefs about medicines in general, strong concerns about potential adverse effects of prescribed medicines, and doubts about the necessity for treatment. High PSM scores predicted non‐adherence to anti‐retroviral therapy and a higher incidence of symptoms following vaccination.Conclusion.  The findings present preliminary evidence that the PSM is a valid and reliable measure of perceived sensitivity to medication. While further work is needed to develop and evaluate the scale, the findings support its use as a research tool in studies of the use and effects of medicines.Statement of contributionWhat is already known on this subject?It is now well understood that beliefs about medicines have an important influence on whether patients start and continue with treatment. Researchspanning a range of long‐term conditions and across different countries has shown that treatment uptake and adherence are consistently related to specific beliefs about prescribed medicines, suchas how patients judge their personal need relative to concerns about potential adverse effects as well as more general beliefs about medicines as a class of treatment.What does this study add?The paper reports on the development and psychometric properties ofa new scale to measure patients' perceptions of their sensitivity to medicines. In five studies involving different groups of individuals we found the Perceived Sensitivity to Medicines (PSM) Scaleto be a reliable and valid measure. The PSM may be useful for researchers and clinicians in explaining treatment decisions, adherenc