contains adherence data across the 6-month study period for both the drug-positive and drug-negative groups. Results of repeated measures ANOVA revealed a main effect for drug use with the drug-positive group demonstrating significantly poorer medication adherence than did the drug-negative group, F(1,148) = 13.9, P < .01, η2 = .09. Averaged across the 6-month study, the drug-negative group's adherence rate was 79% as compared to 63% for the drug-positive group. A main effect for time was also present, F(2,147) = 18.4, P < .01, η2 = .11. Over time, adherence rates for the entire sample dropped from 74.4% for the first 2 months, to 68.5% for months 3–4, down to 62.6% for months 5 and 6 of the study. As can be seen in , comparison of the rate of decline in adherence between the drug-positive versus drug-negative groups revealed a trend towards a steeper rate of decline in adherence amongst the drug-positive group, F(2,147) = 2.89, P = .06. While adherence rates among the drug-negative group only dropped 6% points (from 83% to 77% across the 6 month sampling period), adherence rates for the drug-positive group dropped more than twice as much (from 70% to 56%).
Medication adherence rates for drug-positive and drug-negative participants across the 6-month study period
Medication adherence rates among drug-positive and drug-negative participants
Given that the majority of participants within the drug-positive group were stimulant users (i.e., cocaine and/or methamphetamine), the next series of analyses compared stimulant users versus other-drug users who were toxicology negative for recent stimulant use versus drug free participants. The adherence rates for these three groups are depicted in . Results of mixed model ANOVA again revealed a main effect for time, F(2,146) = 21.8, P < .01, η2 = .13 with adherence rates for all three groups declining over time. A main effect for group was also found, F(2,147) = 12.9, P < .01, η2 = .15. Follow-up pairwise comparisons revealed that the stimulant positive group's adherence rate was significantly lower than both the other-drug-positive group (P = .001) as well as the non-drug group (P < .001). Of note, adherence rates for the drug-negative and the stimulant negative/other drug-positive group did not statistically differ.
Medication adherence rates among stimulant users, non-stimulant drug users, and drug-negative participants
Having determined that stimulant use in particular, rather than drug use in general, was associated with poorer medication adherence, the next series of analyses explored whether adherence rates differed as a function of whether participants only used cocaine versus using cocaine and methamphetamine. Of the 71 stimulant users, 19(27%) tested positive for cocaine, 50(70%) tested positive for both cocaine and meth-amphetamine, and 2(3%) tested positive for amphetamines. For the purpose of this analysis we compared the 19 cocaine only participants with the 50 participants who tested positive for both cocaine and methamphetamine, omitting the 2 methamphetamine only participants. Between group comparisons revealed a trend toward the cocaine + methamphetamine group evidencing poorer adherence than did the cocaine only group, F(1,67) = 3.6, P = .06, η2 = .05. The mean adherence rate for the cocaine only group was 68.1% vs. 54.5% for the cocaine + methamphetamine group.
To determine the relative risk of adherence failure associated with drug use, a logistic regression analysis was performed. For the purpose of this analysis, adherence failure was defined as an overall adherence rate of less than 90% of doses taken. Results of logistic regression revealed that drug use was associated with a 4.1 times greater risk of being a poor adherer, Wald = 11.3, P < .01, OR = 4.1; 95% CI = 1.8–9.3. Only 18.6% of the drug-positive group were classified as good adherers versus 50% of the drug-negative group. Follow-up logistic regression analyses comparing stimulant users to drug free participants found stimulant use to be associated with a 7.0 greater risk of poor adherence relative to a non-drug using contrast group, Wald = 13.8, P < .01; OR = 7.0, 95% CI = 2.5–19.3.
The next analysis was conducted to explore whether lower levels of adherence among the stimulant using group was attributable to the adverse effects of drug use itself or instead was a marker of a trait that is characteristic of individuals who use stimulants, even when they are clean. This analysis was therefore confined to participants who tested positive for stimulant use on at least one study visit and who also tested clean on at least one other occasion (N = 41). Three-day adherence rates for visits at which participants tested stimulant positive were grouped and averaged, as were adherence rates for visits at which the same participants were stimulant negative on urine toxicology assays. Three-day adherence rates were employed because that corresponds to the duration that stimulant use can be detected after ingestion. Hence, for each participant this yielded two adherence rates corresponding to when they were and were not using stimulant drugs. The three-day mean adherence rate for participants who tested positive for recent stimulant use was 51.3% compared to a three-day mean adherence rate of 71.7% for the same participants when they had not recently used stimulants. Paired t-tests revealed this to be a statistically significant difference, t(40) = –3.8, P < .01.
The above analyses examined the relationship between medication adherence and recent drug use without regard to whether participants’ drug use rose to the level of abuse or dependence. Based upon a structured psychiatric diagnostic interview (SCID-IV), 31% of participants were found to meet DSM-IV-TR diagnostic criteria for drug abuse/dependence (hereafter termed the drug abuse group). Results of mixed model ANOVA comparing current substance abusers to non-abusers revealed a main effect for group, F(1,140) = 7.1, P < .01, η2 = .05. As anticipated, the abuse/dependence group evidenced poorer medication adherence than did the non-abuse group (61.0% vs. 72.7%, respectively). While adherence rates declined for both groups, a significant Group by Time interaction effect was found, F(2,139) = 4.2, P < .05, η2 = .03. Although the non-abuse group's mean adherence rates dropped 10% points, from 77.4% to 68.4%, the drug abuse group experienced a more precipitous decline with their mean adherence rate dropping over 18% points (from 70.1% to 51.3%). Logistic regression analyses revealed that participants who met DSM-IV-TR diagnostic criteria for drug abuse/dependence had a 3.3 times greater risk of poor adherence relative to participants without a current drug abuse diagnosis, (Wald = 4.7, P = .03; OR = 3.3, 95% CI = 1.1–9.8).