Incidence and prevalence of illegal drug use in Switzerland in the 1980s and early 1990s: an analytical study.

The Swiss registry of charges concerning illegal trade and consumption of narcotics offers the possibility of monitoring the incidence and prevalence of illegal drug use in Switzerland. Of course, the relation between the number of charges against users of illegal drugs and the number of users is unknown and may vary in time and space. This makes it difficult to estimate the prevalence of consumption from law enforcement data, but it does not hinder the estimation of incidence if the numbers of first and subsequent charges are known. This is demonstrated by employing a dynamic model with two compartments which represent the not yet charged and the charged users. In contrast to methods which rely on treatment admission data, the new method can detect changes of incidence without time lag. Prevalence estimates can be obtained if the charge frequency distribution is known, as it is for Switzerland from 1990 to 1994. A theoretical distribution is fitted to the data which are available for all illegal drugs as well as for heroin and/or cocaine. This leads to estimates of the populations of users of illegal drugs in general and of heroin and/or cocaine.