The purpose of the study ( observational and prospective ) is to evaluate the efficacy and tolerability of the methadone maintenance treatment in patients addicted to heroin. The efficacy and tolerability are assessed by correlating to the adequacy of the dosage of methadone. The patients will be observed for two years.

500 patients will be selected among all patients addicted to heroin according to DSM-IV_TR (Diagnostic and Statistical Manual of Mental Disorders IV_ text revision), in maintenance methadone treatment, observed in the involved sites. The investigators will use treatment schemes with methadon conform to clinical practice and according to SPC (Summary of Product Characteristics).

The enrollment will be competitive. Patients that meet incl/excl and that sign the informed consent, will be observed for 2 years. Follow up visits will be performed according to routine procedures, for patients that belong regularly to site at 3/6/9/12/18/24 months post V1. During the visits, patients will be assessed by validated and used as clinical practice questionnaires . For the evaluations of tolerability and safety the laboratory tests and electrocardiograms routinely performed will be collected.

Inclusion Criteria:

• Both sexes
• Over 18 years of age
• Patients with a diagnosis of heroin addiction according to ICD-10 (International Classification of Diseases)
• Patients who have undertaken a methadone maintenance treatment
• Adhesion to observational study by signing informed consent.

Exclusion Criteria:

• Inability to follow the observation
• Diagnosis of decompensated psychotic disorders

patients addicted to heroin and who have undertaken a methadone maintenance treatment, observed in involved centres.