The findings for marijuana use were similar to those for smoking, indicating that the same underlying aetiological factors are responsible for initiation and quantity of use. However, these results and their implications for policy must be interpreted tentatively, given the comparatively small number of individuals who reported marijuana use in our sample. Experimentation with drug use typically peaks at the end of adolescence [1] and the number of individuals who continue to use marijuana during adulthood declines with age. Therefore, our results should be interpreted in the context of other research into marijuana use and take into account the differences in life-course use of marijuana and cigarettes.
