Introduction

Consumption patterns may depend on factors such as the price and purity of different drugs, and on the availability of - locally specific - licit and illicit substances. A crucial issue has been how many people transgress social and legal norms. The general public, policy-makers, politicians and drug professionals alike demand, first of all, information on the prevalence of drug consumption. In most countries, even where quantitative figures on consumption are available, these are gathered using various methodologies in different countries and are usually limited to "heavy drug users". Drug consumers are minorities involved in illegal activity. They are therefore, "hidden populations" underrepresented in general population surveys but also under- or misrepresented in studies of subgroups of "heavy drug users" contacted for instance via the drug services or via penal organizations. With drugs sold in "recreational settings", pill-testing programmes provide purity data at the retail level, but generally only for selected substances.