One key hypothesis described in our initial grant (and based on previous work of Anthony and colleagues; Wagner & Anthony, 2002) was that the links between early onset cannabis use and later drug use may potentially be explained by use of cannabis being associated with increased availability and opportunity to use other drugs. To examine this, we conducted a series of survival analyses in which age of onset of opportunity to use cocaine was modeled as a function of age of onset of cannabis use, divided into the five categories described above: onset before age 16, onset at ages 16, 17, onset at ages 18, 19, onset at age 20 or older, and no lifetime cannabis use. These results are summarized graphically in Figure 2 and show a strong association between the age of onset of cannabis use and the age at which subjects reported first having the opportunity to use cocaine (whether or not they did, in fact use cocaine then -– or indeed ever) (HR = 0.74, 95% CI = 0.69–0.79). Thus, the earlier an individual used cannabis for the first time, the lower the age at which they first had the opportunity to use cocaine.
