The article shows the methodological weaknesses of a research published previously in Déviance et Société (1999,23,4,437-445). Instead of measuring the impact of methadone prescription on the delinquency of drug-addicts under treatment, that research tried to measure its impact on the overall drug related delinquency in Geneva. It is shown that, in that context, it is essential to take into account, among other factors, the percentage of drug-addicts under treatment and the evolution of drug-addiction in the general population. When these factors are taken into account, it can be seen that – contrary to what is said in the criticized research – the increase in the number of methadone prescription treatments was accompanied by a decrease in the number of imprisoned drug-addicts.