"Indeed, all 11 states that have passed medical marijuana laws ranked above the national average in the percentage of persons 12 or older reporting past-month use of marijuana in 1999, as shown in Table 2. It is at least possible, however, that this analysis confuses cause with effect. It is logical to assume that the states with the highest prevalence of marijuana usage would be more likely to approve medical marijuana programs, because the populations of those states would be more knowledgeable of marijuana’s effects and more tolerant of its use.
"It is also the case that California, the state with the largest and longest-running medical marijuana program, ranked 34th in the percentage of persons age 12-17 reporting marijuana use in the past month during the period 2002-2003, as shown in Table 1. In fact, between 1999 and 2002-2003, of the 10 states with active medical marijuana programs, five states (AK, HI, ME, MT, VT) rose in the state rankings of past-month marijuana use by 12- to 17-year-olds and five states fell (CA, CO, NV, OR, WA).111 Of the five states that had approved medical marijuana laws before 1999 (AK, AZ, CA, OR, WA), only Alaska’s ranking rose between 1999 and 2002-2003, from 7th to 4th, with 11.08% of youth reporting past-month marijuana use in 2002-2003 compared with 10.4% in 1999. No clear patterns are apparent in the state-level data. Clearly, more important factors are at work in determining a state’s prevalence of recreational marijuana use than whether the state has a medical marijuana program."
Eddy, Mark, "Medical Marijuana: Review and Analysis of Federal and State Policies," Congressional Research Service (Washington, DC: March 31, 2009), p. 32.