Background. Interventions and services for people with mental health problems can have broad remits: they are often designed to treat people with a variety of diagnoses. Furthermore, addressing mental health problems can have long term implications for economic, social and health outcomes. This represents a challenge for economic evaluation, where long term trial data can be lacking. In this review we will seek to identify how analysts have sought to tackle this problem. We will review the methods used to extrapolate costs and outcomes for the purpose of economic evaluation, where long term trial data are not available. Methods/design. We will carry out a review of the medical and economic literature evaluating long-term costs and outcomes for mental health interventions and services designed to treat or prevent more than two mental health conditions. We will search the key health economic databases, including: OVID Medline, Embase, Psycinfo, CINAHL, and EconLit. The two authors will independently screen the returned results. Any discrepancies will be resolved by deliberation between the two authors. Key information will be extracted from the papers which successfully pass through the screening process. The findings will be highlighted through a narrative analysis and tabulated data. Discussion. This review will shed light on the existing methods used to model into the future when multiple mental health conditions are considered. The review will discuss the strengths and weakness within current methodologies, highlight existing flaws, and provide guidance for future economic evaluations of interventions targeting multiple mental health conditions.