Results of the unadjusted models indicate that individuals with an early use onset had an increased risk of transition to dependence, which is consistent with the result of previous epidemiological studies (Behrendt et al., 2009; Dawson et al., 2008; Grant et al., 2001b). However, after controlling for the effect of socio-economic, psychiatric comorbidity and drug-use covariates, the direction of this association changed for cannabis and cocaine, indicating that the higher risk of developing dependence on these substances among those who reported an early use onset was mostly explained by those factors. Therefore, the risk of transitioning to dependence may not be a consequence of early substance use onset per se, but rather of the existence of a concomitant psychiatric and SUD that increase the risk of developing a SUD (Behrendt et al., 2009; Degenhardt et al., 2009; Wittchen et al., 2007). The higher risk of transitioning to cannabis and cocaine dependence among those individuals with later use onset, may also be due to the emergence of more intense patterns of use, greater availability and affordability of these drugs in late
