Objectives To evaluate prevalence and severity of constipation and quality of life (QoL) in a cohort of opioid-addicted patients treated with opioid substitution treatments (OST). METHODS: A total of 1057 heroin-dependent patients treated with methadone or buprenorphine were enrolled in a multicenter observational study. Constipation was assessed by Wexner Constipation Scoring System (Wexner CSS), QoL by General Health Questionnaire (GHQ-12). RESULTS: 38.5% patients reported mild constipation, 33.3% reported moderate constipation, 14.8% severe constipation and 5.1% very severe constipation. Mean Wexner CSS score was 6.6 ± 4.8. 44.9% patients showed a GHQ-12 score ≥14; of these 18.3% patients showed a GHQ-12 score ≥20. Mean GHQ score was 13.8 ± 6.5. Mean Wexner CSS score was significantly higher in methadone patients (p = 0.004), in those taking psychoactive drugs (p = 0.0001) and in female (p < 0.0001) with respect to counterparts. Similarly, GHQ-12 mean scores were higher methadone group (p = 0.003), in those taking psychoactive drugs (p < 0.0001), and in female (p = 0.039) with respect to counterparts. ANOVA and ANCOVA showed a significant influence of methadone and female gender on Wexner CSS score while psychoactive drugs significantly influenced both tests. CONCLUSIONS: The present study shows that patients affected by opioid-dependence in OST with methadone and buprenorphine have a high prevalence of constipation and reduced QoL.