In the current paper we use a cotwin-control design to explore whether early-onset cannabis use is associated with increased risk of other illegal drug use, alcohol dependence, and illegal drug abuse/dependence in a sample of U.S. Vietnam-era veteran male twins after controlling for latent familial vulnerability. Two separate cotwin-control analyses were conducted: 1) using pairs in which one twin used cannabis early and the other either used cannabis later or never used cannabis, and 2) restricting the data to the subset of discordant pairs in which the cotwin later initiated cannabis use. The first analysis addresses discordance at the population-level (in which some individuals use later and others never initiate), and the second addresses discordance for age of initiation among users. Previous cotwin-control analyses have indicated that there are individual-specific influences that put early cannabis users at greater risk of other drug use, abuse, and dependence relative to those who do not use cannabis early (Agrawal, Neale, Prescott, & Kendler, 2004; Lessem et al., 2006; Lynskey et al., 2003; Lynskey, Vink, & Boomsma, 2006). However, only Lynskey et al. (2003)
