Abstract: Only one randomised trial met our inclusion criteria, despite our initial search yielding 141 citations. This was a small study, with 17 people completing a trial comparing CBT to an attention placebo (supportive psychotherapy) for people with delusional disorder. Most participants were already taking medication and this was continued during the trial. We were not able to include any randomised trials on medications of any type due to poor data reporting, which left us with no usable data for these trials. For the included study, usable data were limited, risk of bias varied and the numbers involved were small, making interpretation of data difficult. In particular there were no data on outcomes such as global state and behaviour, nor any information on possible adverse effects. A positive effect for CBT was found for social self esteem using the Social Self-Esteem Inventory (1 RCT, n = 17, MD 30.5, CI 7.51 to 53.49, very low quality evidence), however this is only a measure of self worth in social situations and may thus not be well correlated to social function. More people left the study early if they were in the supportive psychotherapy group with 6/12 leaving early compared to 1/6 from the CBT group, but the difference was not significant (1 RCT, n = 17, RR 0.17, CI 0.02 to 1.18, moderate quality evidence). For mental state outcomes the results were skewed making interpretation difficult, especially given the small sample. Despite international recognition of this disorder in psychiatric classification systems such as ICD-10 and DSM-5, there is a paucity of high quality randomised trials on delusional disorder. There is currently insufficient evidence to make evidence-based recommendations for treatments of any type for people with delusional disorder. The limited evidence that we found is not generalisable to the population of people with delusional disorder. Until further evidence is found, it seems reasonable to offer treatments which have efficacy in other psychotic disorders. Further research is needed in this area and could be enhanced in two ways: firstly, by conducting randomised trials specifically for people with delusional disorder and, secondly, by high quality reporting of results for people with delusional disorder who are often recruited into larger studies for people with a variety of psychoses.

Plain-language summary: This review aimed to assess the effectiveness of all current treatments for people with delusional disorder. A search for randomised controlled trials was run in 2012. Authors found 141 citations in the search but only one trial, randomising 17 people, could be included in the review. The study compared the effectiveness of CBT with supportive psychotherapy for people with delusional disorder. Participants were already taking medication and this was continued during the trial. The review was not able to include any studies or trials involving medications of any type used to treat delusional disorder. For the study that was included, there was limited information presented that we could use. Firm conclusions were difficult to make and no evidence on improving people's behaviour and overall mental health was available. More people left the study early from the supportive psychotherapy group, but number of participants was small and the overall difference between the groups was not enough to conclude one treatment was better than the other. A positive effect for CBT was found for people's social self esteem, although again, this finding is limited by the low quantity and quality of the data and does not relate to people's social or everyday functioning. Currently there is an overall lack of high quality evidence-based information about the treatment of delusional disorders and insufficient evidence to make recommendations for treatments of any type. Until such evidence is found, the treatment of delusional disorders will most likely include those that are considered effective for other psychotic disorders and mental health problems. Further large-scale and high quality research is needed in this area. Research could be improved by conducting trials specifically for people with delusional disorder. Ben Gray, Senior Peer Researcher, McPin Foundation: http://mcpin.org/.