PurposeThere have been few studies about the prevalence of substance abuse and links to offending behaviour among those with intellectual disabilities. This paper aims to address this issue.Design/methodology/approachThis baseline audit describes: the prevalence of alcohol and substance misuse in patients within a forensic intellectual disability service; and the introduction of a Drug and Alcohol Awareness Course. In total, 74 patients were included in the audit.FindingsRoughly half of the patients audited had co‐morbid harmful use or dependence with the problem being equally prevalent in men and women. Whilst alcohol and cannabis were the commonest drugs of abuse, cocaine, stimulants and opiates were abused by a small but significant number. Of those with harmful use or dependence, 35 per cent had used the drug in the immediate lead up to their index offence. A diagnosis of personality disorder and past history of convictions for violent offences was significantly more likely to be present in the group with harmful use or dependence. There were no differences on major mental illnesses or pervasive developmental disorders.Practical implicationsThe high prevalence of substance abuse makes it an important risk factor that could determine treatment outcomes. Forensic intellectual disability units need replicable substance abuse targeted treatment programmes that can be formally audited and evaluated.Originality/valueFindings are discussed in relation to service planning and treatment outcomes from forensic intellectual disability services.