Recent progress in the long-term pharmacological research on cannabis.

Rapid increase in our knowledge of acute biological effects of cannabis and its biologically active components is contrasted with the slow rate of progress achieved in the area of effects of long-term use of cannabis in man. Such studies belong to the difficult category of long-term clinical pharmacological studies, that are time-consuming, require large population samples, and considerable expertise of staff. Recently published epidemiological studies on effects of long-term cannabis use, although still not yet conclusive, supply increasing evidence of harmful effects of such use. Recently, the World Health Organization has sponsored several research projects of this nature in countries with easy access to long-term heavy cannabis users.