Background: Little is known about the characteristics of people with intellectual disabilities (IDs) who misuse substances and how such problems impinge upon their well-being. The aim of this paper is to describe how alcohol and drugs affect the health of people with IDs. Methods: A questionnaire was forwarded to all the community ID teams and mainstream addiction teams across Northern Ireland: 67 substance users with IDs and substantial substance-related problems were identified. Results: Alcohol was found to be the main substance to be misused, with one-fifth of the substance users also found to be using a combination of illicit drugs and/or prescribed medication. Nearly three-quarters of the sample were found to be hazardously using alcohol for more than 5 years. Being male and young, having a borderline/mild ID, living independently and having a mental health problem were found to be risk factors for developing a "substance related problem." Various problematic behaviours were identified, including aggression, erratic mood changes, sexual exploitation, difficulties in maintaining relationships and loss of daily routine. Conclusions: The findings of this study suggest that greater emphasis needs to be placed upon the early identification of this hidden population by primary and secondary healthcare personnel, and also ID personnel. Such early identification may also diminish the long-established patterns of use and associated related-behaviours that have been reported within this paper.