Background Researchers often overlook potential adverse effects of educational and public health interventions. Adverse effects of an intervention are increases in adverse outcomes, or decreases in beneficial outcomes, attributed to the intervention in a randomised trial. In a previous study, we developed a framework of potential adverse effects of an intervention intended to improve critical thinking about health choices: the IHC secondary school intervention. The framework includes potential effects on both students and teachers. In this study, building on the framework, we will develop and evaluate (validate) questionnaire items (questions), for measuring outcomes identified in the development of the framework. In a separate, subsequent study, we will conduct a quantitative evaluation of the items. Objectives Prioritise potential adverse effects of the IHC secondary school intervention, for the 1-year-follow-up assessments of the trials of the intervention Develop and evaluate questionnaire items for measuring potential adverse effects, focusing on those that we have prioritised Methods The overarching steps of this study are 1) prioritising outcomes included in the framework developed previously; 3) brainstorming and drafting questionnaire items; and 3) conducting a qualitative evaluation of the items. Results Results of the study will include: a set of potential adverse effects (increases in adverse outcomes) prioritised for the trials of the IHC secondary school intervention, and measures of potential adverse outcomes that have been qualitatively evaluated.