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Co-administering cannabis with tobacco (i.e. co-administration of the substances mixed together) is a common practice among cannabis users, but the consequences of this practice are not well understood. This study examines the relationship between co-administering cannabis with tobacco and the long-term frequency of cannabis use in a young adult population group with high rates of cannabis and tobacco use. The data are from an Australian prospective population-based study of young adults who recurrently used amphetamine-type stimulants (ATS). The mean age of participants was 20.8 years at baseline, sample size (n = 277), and 47% were female. We examined the frequency and quantity of cannabis consumption over 4 ½ years. Negative binomial regression analysis was conducted to examine the frequency of cannabis use at 12-month follow-up and at 4 ½ years, with co-administering practices as the predictor. At every time interval, participants who always co-administered their cannabis with tobacco used cannabis on more days in the last month than those who only sometimes co-administered, rarely co-administered, or never co-administered these substances (p < 0.001). Sometimes co-administering cannabis with tobacco at baseline predicted more frequent cannabis use at 12-month follow-up (adjusted IRR: 2.25, 95% CI: 1.05, 4.78), independently of the baseline frequency of cannabis use. However, levels of co-administering cannabis with tobacco at 12-month follow-up (rarely, sometimes, and always) did not predict high levels of cannabis use at 4 ½ years follow-up after adjusting for cannabis use at 12-month follow-up. Among people who use ATS and cannabis, frequent cannabis use may be a marker of the practice of co-administering cannabis with tobacco, and can be used to target tobacco cessation interventions in these populations. [ABSTRACT FROM AUTHOR]