Cannabis use is increasing and continues to be high in West and Central Africa, Western and Central Europe and Oceania, as well as in North America, where the most recent data available indicate an increase in the prevalence of cannabis use in the United States. Europe is still one of the world's largest markets for cannabis resin, but its use is concentrated in a few countries. The use of cannabis herb is more evenly spread across European countries, and the market in Western and Central Europe is shifting from cannabis resin to cannabis herb.
Advances in cannabis plant cultivation techniques and the use of genetically selected strains have led to an increase in the number of cannabis harvests, as well as in the yield and potency of cannabis. The potency of cannabis, commonly measured in terms of the concentration of THC (Δ9-tetrahydrocannabinol, the main psychoactive ingredient in cannabis), has been increasing in many markets over the past decade, leading to growing concern about the potential of cannabis to cause serious health problems. Despite the fact that increasing professionalism and sophistication have enhanced the capacity of cannabis plant growers to avoid detection by law enforcement authorities, data for 2013 show an increase in the quantities of cannabis herb and cannabis resin seized worldwide.