In the preceding analyses, some of the “control” twins used cannabis later whereas other “control” twins were abstainers. Table 3 compares the four subgroups of individuals from the two types of discordant pairs: early users from early-later discordant pairs, the later-using cotwin, early users from early-never discordant pairs, and the never-using cotwin. In general, there are few significant differences between the three groups of cannabis users, but all three have substantially higher rates of other drug use/abuse/dependence compared to the never-using cotwins. Analyses of the 190 pairs in which one twin used cannabis early and the other later indicated that, although early-using twins were at significantly greater risk of using sedatives, opiates, and hallucinogens, rates of illegal drug abuse/dependence and alcohol dependence were similar for the early- and later-using discordant twins (see Table 4). Although power to detect differences between early users and their discordant cotwins is reduced with the smaller sample size, the reduction in significance does not seem to be a function of power: the odds ratios in Table 4 are consistently considerably smaller than those in Table 2.
