Average Cannabis Potency in Europe and the US
"Statements in the popular media that the potency of cannabis has increased by ten times or more in recent decades are not support by the data from either the USA or Europe. As discussed in the body of this report, systematic data are not available in Europe on long-term trends and analytical and methodological issues complicate the interpretation of the information that is available. Data are stronger for medium and short-term trends where no major differences are apparent in Europe, although some modest increases are found in some countries. The greatest long-term changes in potency appear to have occurred in the USA. It should be noted here that before 1980 herbal cannabis potency in the USA was, according to the available data, very low by European standards."
King, Leslie A., European Monitoring Centre for Drugs and Drug Addiction, "EMCDDA Insights - An Overview of Cannabis Potency in Europe" (Luxembourg: Office for Official Publications of the European Communities, 2004), p. 14.