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Background Opioid replacement treatment (ORT) with methadone is regarded as gold standard in the treatment of opioid addiction. Treatment doses of 60 mg methadone per day and above are associated with better treatment retention and reduction in the use of heroin and cocaine. However, an absolute dose level cannot function as parameter for adequate dosing. This study aims to determine dose adequacy in a sample of patients on stable methadone treatment, and to relate dose adequacy to disease severity. Methods This study was designed as open prospective cohort study over 12 months, with baseline data reported here. Patients on stable substitution treatment with methadone (Eptadone®) were consecutively included. Medical and socio-demographic data were gathered and the instruments Opiate Dosage Adequacy Scale (ODAS), European Addiction Severity Index (EuropASI) and the Derogatis Interview for Sexual Functioning – Self Report (DISF-SR) were applied. Results Five hundred and sixteen subjects, who received on average 60.3 (±30.4) mg methadone per day, were included. According to ODAS, 40.6% suffered from an inadequate dosing, and 59.4% had an adequate dose. Patients with an adequate dose received on average 57.8 (±27.5) mg methadone per day, whilst patients with an inadequate dose received on average 70.6 (±33.0) mg per day. The frequencies of patients with methadone doses of less than 60 mg per were 45.4% in the inadequate and 60.6% in the adequate group. The inadequate group suffered from a statistically significant higher burden of addiction related problems in all EuropASI domains. Sexual functioning did not differ by adequacy group, but women suffered from more pronounced sexual dysfunction as compared to men. Conclusion A high frequency of inadequate dosing was found in this sample of patients on ORT. Higher disease severity should alert for possible need of even higher methadone doses. The tendency to low methadone doses warrants further research in the treatment system. Higher methadone doses are not related to increased sexual dysfunction. Sexual dysfunction, especially in women, should be considered in treatment.