AbstractAimPrisoners exhibit high rates of substance use and mental health problems. In the present study, we sought to gain a detailed understanding of substance use amongst young prisoners to inform early detection and early intervention strategies in a prison setting.MethodsThis is a cross‐sectional study of 2102 prisoners who were screened by the London Early Detection and Prevention in Prison Team (LEAP). Data on the use of substances were collected including age of first use, recent use, duration of use and poly‐drug use. The Prodromal Questionnaire – Brief Version was used to screen for the at‐risk mental state.ResultsWe found high rates of lifetime and recent use and low age of first use of a number of substances. We also found strong associations between substance use and screening positive for an at‐risk mental state. Logistic regression analysis confirmed that use of any drug in the last year, poly‐drug and early use, as well as heavy alcohol use, were related to an increased risk of screening positive.ConclusionsSubstance use in the prison population is not only widespread and heavy but is also strongly linked with a higher risk of developing mental health problems. The need for early detection and early intervention in prison is discussed.