The purpose of this study is to describe clinical, pharmacokinetic and genetic factors associated with the variance of oral methadone dosage for patients at the steady state of heroin dependence maintenance treatment. The hypothesis is that the investigators can predict 70% of the variance with few factors, including CYP 3A4 function measured with oral midazolam challenge.

Patients at the steady state of methadone maintenance treatment may receive oral dosage ranging from 5 to 130 mg per day in our clinical practice. This study is aimed at providing a comprehensive cross-sectional description of factors involved in this variance:

• comorbidity with addictive and psychiatric disorders
• severity of pre-existing heroin dependence
• function of CYP 3A4 enzyme assessed with oral midazolam challenge
• genetic polymorphisms of enzymes implicated in methadone pharmacokinetic and pharmacodynamic (CYPs, MDR1, OPRM1, COMT)

The expected result is a predictive equation of oral methadone dosage at steady state.

Inclusion Criteria:

• heroin dependence
• under maintenance treatment with methadone
• at steady state: stable oral methadone dosage since 3 months at least

Exclusion Criteria:

• current heroin dependence or abuse
• current cocaine and/or alcohol and/or sedatives dependence
• pregnancy

Defined population: participants or population are selected based on predefined criteria