"Among 50-year-old high school graduates in 2012, we estimate that about three quarters (74%) have tried marijuana, and that about two thirds (64%) have tried an illicit drug other than marijuana. (These estimates are adjusted to correct for panel attrition, as described in chapter 4 of Volume II.)
"Their current behavior is far less extreme than those statistics might suggest, but it is not by any means negligible. One in eight (12%) indicates using marijuana in the last 12 months, and one in ten (10%) indicates using any other illicit drug in the same period. Their past-month prevalence rates are lower—7.3% and 6.2%, respectively, for marijuana and any other illicit drug. About 1 in 43 (2.3%) is a current daily marijuana user, though substantially more indicate that they have used marijuana daily at some time in the past."
Johnston, L. D., O’Malley, P. M., Bachman, J. G., and Schulenberg, J. E., (2013). Monitoring the Future national survey results on drug use, 1975–2012: Volume 2, College students and adults ages 19–50. Ann Arbor: Institute for Social Research, The University of Michigan.