New psychopharmacological studies performed these last few years should allow, in future, to improve methadone prescription. In particular, thanks to the identification of enzymes involved in the metabolism of this molecule, it is now possible to better understand the problems of metabolic interactions, to foresee and to avoid them. Studies showing the influence of genetic and environmental factors on blood concentrations of methadone, as well as a recent clinical study performed with methadone doses largely in excess of 100 mg/day, stress the importance of adapting methadone treatment individually, particularly as regards to the choice of the dose. This choice must not result from an administrative standardization, as it is essential for patients that, with an adequate social and psychological treatment, an adequate pharmacological treatment should be associated.