Background Multimorbidity, the co-existence of two or more (2+) long-term conditions in an individual, is common among problem drug abusers. Objective To delineate the patterns, multimorbidity prevalence, and disease severity in patients enrolled in a community-based primary care methadone maintenance treatment (MMT) programme. Design This was a retrospective cohort study ( n=274). The comparator group consisted of mainstream primary care patients. Electronic medical record assessment was performed using the Cumulative Illness Rating Scale. Results Prevalence of multimorbidity across 2+ domains was significantly higher within the MMT sample at 88.7% (243/274) than the comparator sample at 51.8% (142/274), p<0.001. MMT patients were seven times more likely to have multimorbidity across 2+ domains compared with mainstream patients (OR 7.29, 95% confidence interval 4.68–11.34; p<0.001). Prevalence of multimorbidity was consistently high across all age groups in the MMT cohort (range 87.8–100%), while there was a positive correlation with age in the comparator cohort ( r=0.29, p<0.001). Respiratory, psychiatric, and hepatic–pancreatic domains were the three most common domains with multimorbidity. Overall, MMT patients (mean±SD, 1.97±0.43) demonstrated significantly higher disease severity than mainstream patients (mean±SD, 1.18±0.78), p<0.001. Prevalence of moderate disease severity observed in the <45-year MMT age group was 50% higher than the ≥45-year comparator age group. Conclusions Prevalence of multimorbidity and disease severity in MMT patients was greater than in the age- and sex-matched comparators. Patients with a history of drug abuse require co-ordinated care for treatment of their addiction, and to manage and prevent chronic illnesses. Community-based programmes delivered through primary care help fulfil this need.