My effort to determine how many individuals in the United States are being "supervised" (incarceration, parole, probation) by federal, local, and state correctional authorities due to crimes involving the use, distribution, possession, or sale of illicit drugs led me to various Bureau of Justice Statistics reports in the Adobe pdf format. Each may be reviewed by clicking on the links below. Adobe Reader is needed to view the studies; if you don't have Reader, click the Adobe icon at the bottom of the page to add it to your system at no cost.
The first paragraph of Correctional Populations in the United States, 2009 lists the number of individuals (7,225,800 ) under supervision in 2009. Tables in three additional reports provide a rough estimate of how many were in trouble due to drug-related offenses.
Appendix Table 5 of Probation and Parole in the United States, 2009 indicates that 26% of adults on probation in 2009 committed drug-related crimes; Appendix Table 15 of the same report shows 36% were on parole in 2009 due to drug violations.
Drug-related offenders were 18% of the "sentenced prisoners under state jurisdiction" by yearend 2008 as listed in Appendix Table 16c of Prisoners in 2009.
About 25% of the inmates listed in Table 10 of Sexual Victimization in Prisons and Jails Reported by Inmates, 2008-09 were
incarcerated due to drug-related crimes.
The average of the four percentages referenced is 26%, and it is in the ballpark of reality--though not remotely precise--to extrapolate that approximately 26% or 1,878,708 of the "7,225,800 offenders at year end 2009" listed in the Correctional Populations in the United States, 2009 were being supervised due to infractions involving illicit drugs.