The aim of this study is to shed light upon an observed association between the use of marijuana and hallucinogens (e.g. LSD), with a specific focus on the idea that two separate mechanisms might link marijuana use to hallucinogen use: (1) greater hallucinogen exposure opportunity for marijuana users versus nonusers; (2) increased probability of hallucinogen use for marijuana users versus nonusers, once the opportunity to use hallucinogens has occurred. This work is based on a novel analysis of retrospective, self-report data from more than 40000 young participants in the 1991-1994 National Household Surveys on Drug Abuse (NHSDA), with discrete time survival analysis models. Youths who had used marijuana were substantially more likely than nonusers to have the opportunity to use hallucinogens (estimated unadjusted relative risk, uRR=16.3; 95%. Confidence interval (CI)=14.3-18.6). Once an opportunity to use hallucinogens occurred, marijuana users were more likely than nonusers to initiate hallucinogen use (uRR=12.6; 95% CI=9.0-17.6). This evidence provides a more complete view of interconnections between marijuana use and hallucinogen use, and helps to clarify the pivotal role for drug exposure opportunities. Important next steps will be to understand what accounts for variation in the exposure opportunities experienced by marijuana users, and to understand why some marijuana users do not progress even when they have a chance to do so.