All participants were asked the age at which they had first consumed any form of alcohol “at least once a month for 6 months or more”. Participants who endorsed either this stem question or who indicated they had consumed 5 or more drinks in one day were considered drinkers (91.0% endorsed both items; 6.1% endorsed only the frequency question and 2.9% endorsed only the quantity question). Because alcohol use was endorsed by almost all participants, our base alcohol variable was early alcohol use, which was defined as the onset of alcohol use before age 18. Participants were asked whether they had “ever smoked [tobacco form] daily for a month or more”, but were not asked about age of initiation. Respondents who indicated they had smoked cigarettes, cigars, or pipes daily or used snuff or chewing tobacco daily for a month or more were considered to be nicotine users, which was the base measure of nicotine exposure in our analyses. Both cannabis use (defined as having ever tried cannabis) and early cannabis use (initiation before age 18) were examined. The early
