"As shown in Table 4-3.5, drug court offenders continued to report less drug use than the comparison group in the year prior to the 18-month survey. Drug court offenders had significantly fewer occurrences of any use (56 percent vs. 76 percent, p < .01), serious use (41 percent vs. 58 percent, p < .01), days of use per month (2.1 vs. 4.8, p <.001), and days of serious use per month (1.1 vs. 2.3; p < .001). Regarding specific substances, drug court offenders were significantly less likely than the comparison group to report use of marijuana (23 percent vs. 36 percent), alcohol (47 percent vs. 67 percent), “heavy” use of alcohol (29 percent vs. 42 percent), illegal use of prescription drugs (6 percent vs. 15 percent) and illegal use of methadone (2 percent vs. 4 percent). Although the sample differences for cocaine, heroin, amphetamines, and hallucinogens were all non-significant, every one of these latter differences also trended towards less use in the drug court sample. Once again, in the most recent month prior to the 18-month follow up, the differences between the two samples generally mirrored those detected for the entire previous year."

Source

Rossman, Shelli B., et al., "Final Report, Volume 4: The Multi-Site Adult Drug Court Evaluation: The Impact of Drug Courts" (Washington, DC: Urban Institute, June 2011), p. 35.
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