The aim of this study was to ascertain the prevalence of oral lesions among 500 psychoactive substance users in a hospital-based population. The study group consisted of 500 consecutive patients attending TTK Hospital, a non-governmental organisation involved in rehabilitation of substance users. Patient history was recorded in a pre-determined format and clinical findings were recorded by a trained physician and dental surgeons. Psychoactive substances used by the patients were alcohol (97%), tobacco (72%), arecanut (57.2%), narcotics (6.8%), cannabis (3.2%) and benzodiazipines (1.8%). Ninety-one percent of patients had one or more oral lesions: dental caries (39%), gingivitis (37.6%), extrinsic stains (24%), oral submucous fibrosis (OSF) (8%), periodontitis (7.4%), leukoplakia (6.6%), melanosis (5.2%), nicotina palatini (2.2%) and erythroplakia (0.6%). For OSF, those using arecanut and alcohol had an odds ratio (OR) of 2.4 [95% confidence intervals (CI) 1.23-4.69, P=0.009], smokers using arecanut products and alcohol had an OR of 3.07 (95% CI 1.59-5.91, P=0.000), and smokers who chewed arecanut products and used drugs had an OR of 23.1 (95% CI 2.05-260, P=0.001) compared with the general population. Those who smoked and used alcohol, arecanut and drugs had a 20.67-fold higher risk of developing leukoplakia compared with those who did not engage in these habits. In conclusion, 91% of patients had one or more oral lesions that needed dental treatment, and most patients were not aware of their oral lesions. The high prevalence of OSF and leukoplakia in substance abusers compared with the general population emphasises the need for regular dental assessments in these patients.