BackgroundLittle is known about the characteristicsof people with intellectual disabilities (IDs) whomisuse substances and how such problems impingeupon their well-being. The aim of this paper is todescribe how alcohol and drugs affect the health ofpeople with IDs.MethodsA questionnaire was forwarded to all thecommunity ID teams and mainstream addictionteams across Northern Ireland:substance userswith IDs and substantial substance-related problemswere identified.ResultsAlcohol was found to be the main substanceto be misused, with one-fifth of the substance usersalso found to be using a combination of illicit drugsand/or prescribed medication. Nearly three-quartersof the sample were found to be hazardously usingalcohol for more thanyears. Being male and young,having a borderline/mild ID, living independentlyand having a mental health problem were found tobe risk factors for developing a ‘substance relatedproblem’. Various problematic behaviours were identified,including aggression, erratic mood changes,sexual exploitation, difficulties in maintaining relationshipsand loss of daily routine.ConclusionsThe findings of this study suggest thatgreater emphasis needs to be placed upon the earlyidentification of this hidden population by primaryand secondary healthcare personnel, and also ID personnel.Such early identification may also diminishthe long-established patterns of use and associatedrelated-behaviours that have been reported withinthis paper.