Methadone maintenance treatment: a protective factor for cocaine injection in a street-recruited cohort of heroin users.

Objective: To evaluate the effect of methadone maintenance treatment (MMT) on cocaine use and cocaine injection among heroin users.

Methods: Generalized estimating equations (GEE) were used to examine the association between MMT enrollment (0, <12, ≥12 months) and changes in frequency of cocaine use or injection in two consecutive follow-up visits among heroin users in the Itinere cohort, and to determine whether these changes were independent of equivalent changes in heroin use or injection. Seven multivariate models were constructed, one for each outcome variable on drug use changes.

Results: Of the 992 participants recruited in 2001-2003, 628 (63.3%) had at least one follow-up visit. Of these, 55.8% were enrolled in MMT at baseline and an additional 23.2% initiated MMT during follow-up. In multivariate GEE, changes significantly and positively associated with MMT enrollment were: less cocaine use [MMT<12 months (OR=1.70, 95% CI=1.17-2.48)] and less cocaine injection [MMT≥12 months (OR=2.98, 95% CI=1.51-5.89)]. Being on MMT≥12 months was negatively associated with more cocaine use (OR=0.62, 95% CI=0.38-0.99) and with more cocaine injection (OR=0.52, 95% CI=0.28-0.98). When equivalent changes in heroin were used as a covariate, the MMT effect on less cocaine use was hardly modified (OR=1.69, 95% CI=1.07-2.65), and the effect on changes in cocaine injection disappeared.

Conclusion: MMT enrollment is a protective factor against both cocaine use and injection among heroin users. The effect of MMT on cocaine injection appears to be mediated by heroin injection, whereas its effect on cocaine use is more direct.