Background There is an increasing international interest in patient and public involvement (PPI) in research, yet relatively little robust evidence exists about its impact on health and social care research.

This is the first international systematic review to focus on the impact of PPI on the people involved in the process. The beneficial and challenging impacts reported highlight the importance of optimising the context and processes of involvement, so creating the potential for PPI to impact positively on the research itself.

There is a need for significant development of the PPI evidence base particularly around guidance for the reporting of user activity and impact. The evidence base needs to be significantly strengthened to ensure the full impact of involving service users in NHS healthcare services is fully understood.