Visit Baseline Week 6 Week 12 Week 18 Week 24 Visit Baseline Week 6 Week 12 Week 18 Week 24 Visit Baseline Week 6 Week 12 Week 18 Week 24 Total 44/127 (35) 27/92 (29) 33/100 (33) 39/97 (40) 39/105 (37) Total 83/127 (65) 32/92 (35) 39/102 (38) 37/99 (37) 40/107 (37) Total 59/127 (46) 22/91 (24) 28/99 (28) 23/97 (24) 30/105 (29) Positive methamphetamine urinalysis, n (%) Contingency Management Self-reported weekly or more frequent methamphetamine use, n (%) Contingency Management Self-reported use of more than eight quarters of methamphetamine, n (%) Contingency Management 28/70 (40) 17/48 (35) 20/55 (36) 24/52 (46) 29/58 (50) 51/70 (73) 20/48 (42) 25/56 (45) 26/53 (49) 29/60 (48) 37/70 (53) 14/47 (29) 21/55 (37) 19/52 (36) 24/59 (40) Control 16/57 (28) 10/44 (23) 13/45 (29) 15/45 (33) 10/47 (21) Control 32/57 (56) 12/44 (27) 14/46 (30) 11/46 (24) 11/47 (23) Control 22/57 (39) 8/44 (18) 7/44 (16) 4/45 (9) 6/46 (13) Unadjusted RR (95% CI) Adjusted RRa (95% CI) 1.39 (0.81, 2.38) 1.77 (1.13, 2.78) 1.09 (0.76, 1.57) 1.21 (0.95, 1.54) Unadjusted RR (95% CI) Adjusted RRb (95% CI) 1.50 (0.93, 2.42) 1.29 (0.82, 2.04) 2.06 (1.29, 3.29) 1.76 (1.13, 2.73) Unadjusted RR (95% CI) Adjusted RRc (95% CI) 2.01 (1.09, 3.73) 1.80 (0.95, 3.40) 3.52 (1.70, 7.30) 3.02 (1.47, 6.23) Menza et al. BMC Public Health 2010, 10:774 http://www.biomedcentral.com/1471-2458/10/774 Page 10 of 13 Table 4 Effect of a contingency management intervention on detection of methamphetamine use by rapid urine screen and on self-reported frequency and quantity of methamphetamine use in the prior 6 weeks CI, confidence interval; RR, relative risk; one quarter of methamphetamine is equivalent to 0.25 grams. Statistically significant results (P < 0.05) are bolded. aAdjusted for methamphetamine urinalysis result at baseline and stage of change for methamphetamine use at baseline (maintenance, action, preparation, contemplation, pre-contemplation, missing). bAdjusted for baseline self-reported weekly or more frequent use of methamphetamine in the prior 6 weeks. cAdjusted for baseline self-reported use of more than eight quarters of methamphetamine in the prior 6 weeks. receptive UAI as other psychosocial interventions [15]. However, none of the previously published randomized trials included a no-treatment/minimal intervention con- trol arm making it difficult to determine whether the absence of differences between CM and other interven- tions reflects comparable levels of efficacy or an absence of effect with all interventions. Our study differs in four important ways from pre- vious studies of CM. First, while we did not exclude par- ticipants who were receiving other substance use interventions, participants were not enrolled because they were seeking drug treatment. Second, we compared CM to a minimal intervention control arm. Third, CM was employed as a stand-alone intervention. Finally, CM participants were only expected to submit urines sam- ples two times per week rather than three times, as has been done in some, but not all studies of CM [21]. We cannot say whether CM’s failure to reduce methamphe- tamine use in our study reflects a lack of effect among persons who are not expressly seeking drug treatment, CM’s ineffectiveness as a stand-alone intervention, the ineffectiveness of our CM schedule, or a more general lack of efficacy of CM that would have been consistently observed in other trials had those studies included no- treatment/minimal intervention control groups. We found that self-reported methamphetamine use declined among control participants but remained rela- tively stable among CM participants. We cannot readily explain this effect. We do not believe that CM partici- pants used vouchers to obtain methamphetamine, since the difference in drug use by study arm was only evident after CM recipients ceased to receive vouchers. Likewise, we do not believe our findings simply reflect more accu- rate reporting of methamphetamine use by CM partici- pants since we found that CM recipients were also more likely to test positive for methamphetamine. Participants in the two study arms reported similar use of outside treatment and support services, making it unlikely that differential use of other treatments affected our results. It is possible that participation in the CM arm may have put participants in contact with other participants and CBO clients in ways that facilitated access to and use of methamphetamine. Similar phenomena have been reported in other studies [42,43]. Also, discontinuation of twice-weekly CM visits, which may have provided a source of support for CM participants, may have led to heavier methamphetamine use among CM participants compared to controls. Similar to a study of CM and other psychosocial inter- ventions in the substance abuse treatment setting [15],