"In 2004, about 642,000 State prisoners were drug dependent or abusing in the year before their admission to prison. An estimated 258,900 of these inmates (or 40%) had taken part in some type of drug abuse program (table 10). These inmates were more than twice as likely to report participation in selfhelp or peer counseling groups and education programs (35%) than to receive drug treatment from a trained professional (15%).
"In Federal prison, a higher percentage of drug dependent or abusing inmates (49%) reported taking part in some type of drug abuse programs. Nearly 1 in 3 took part in drug abuse education classes, and 1 in 5 had participated in self-help or peer counseling groups. Overall, 17% took part in drug treatment programs with a trained professional, and 41% had participated in other drug abuse programs."
Mumola, Christopher J., and Karberg, Jennifer C., "Drug Use and Dependence, State and Federal Prisoners, 2004," (Washington, DC: Bureau of Justice Statistics, Dept. of Justice, Oct. 2006) NCJ-213530, p. 9.