Patients need to be informed about potential risks of taking part in clinical trials. A problem is that there are no standards telling researchers or ethics committees how these risks need to be communicated. Our background research suggests that the way harms are communicated can actually increase the risks of the harms occurring. Over half of our sample of 250,000 patients who took placebo pills (like sugar pills) in clinical trials reported some negative side effect (like pain or nausea but also more serious things). The aim of this study is to understand what information about trial harms and benefits stakeholders consider to be important for ‘principled participant information leaflets’ or ‘PrinciPILs’ to contain. The stakeholders will include patients, ethics committee members, industry representatives, medico-legal experts, psychologists, and trial managers.