Objectives: Non-illicit alternatives to controlled drugs, known as Novel Psychoactive Substances (NPS) have recently risen to prominence. They are readily available with uncertain pharmacology and no widely available assay. Given that psychiatric patients are at risk of comorbid substance abuse, we hypothesized that NPS use would be present in the psychiatric population, and sought to determine its prevalence and investigate the characteristics of those who use these drugs with a retrospective review of discharge letters. Setting: General adult inpatient wards of a psychiatric hospital in a Scottish city. Participants: All adult inpatients (18-65) discharged from general psychiatric wards between 1/7/14 and 31/12/14. 483 admissions identified, 46 were admissions for maintenance ECT and were excluded. Of the remaining 437 admissions, 49 discharge letters were unobtainable, leaving 388 admissions to analyze. Primary outcome measure: The mention or lack thereof of NPS use in discharge letters was our planned primary outcome measure and was also the primary outcome measure we used in our analysis. Results: NPS use was identified in 22.2% of admissions, contributing to psychiatric symptoms in 59.3%. In comparison to non-users, NPS users were younger (p<0.01), male and more likely to have a forensic history ((p<0.001) for both). The diagnosis of drug-induced psychosis was significantly more likely in NPS users (p<0.001, odds ratio (OR) 18.7, 95% confidence intervals (CI) 8.1-43.0) and the diagnosis of depression was significantly less likely (p<0.005, OR 0.133, CI 0.031-0.558). Use of cannabis was significantly more likely in NPS users (p<0.001, OR 4.2, CI 2.5-7.1), as was substitute opiate prescribing (p<0.001, OR 3.7, CI 1.8-7.4). Conclusions: NPS use was prevalent amongst young, male psychiatric inpatients; in particular those with drug induced psychosis and often occurred alongside illicit drug use.