Although prescription charges in New Zealand are low compared with many other countries, many people report that they cannot afford the medicines they need. We plan to conduct a randomised controlled trial of prescription charges to see whether removing charges would improve people’s health. We will recruit a group of people who have diabetes and/or ongoing mental health conditions requiring medication, and live in deprived neighbourhoods. We will divide the group in half and pay prescription charges for one group for twelve months. We will then compare how many days people from each group spend in hospital to see whether free prescriptions make a difference. Additional differences in health services, quality of life, and medicines use between the groups will also be investigated.