It is necessary to be cautious in the interpretation of the implications of these results for policy, as this study represents analysis from one, albeit relatively large, population-based sample at one time-point. It is also important to view the findings in the context of the known life-course for each of the three substances in the United Kingdom. For alcohol use, the moderate degree of relatedness between initiation and quantity/problem use suggests that research may benefit from looking for risk factors specific to the development of quantity/problem use of alcohol rather than that of initiation. It may be more appropriate to develop interventions based on these risk factors rather than attempting to reduce initiation of alcohol use. This is particularly the case given the high proportion of twins who have tried alcohol (86%), and the much smaller proportion of that group who reported heavy use of alcohol (26%). This suggests it may be difficult and unrealistic to reduce initiation of alcohol use during adolescence, given that it is the period during which the vast majority of the population first tries alcohol.
