Methadone treatment as a determinant of HIV risk reduction among injecting drug users: a nested case-control study.

In order to evaluate the protective effect of methadone treatment on HIV infection, we performed a nested case-control study on seroconverters (cases) who were part of a cohort of HIV-negative injecting drug users (IDUs) (controls). Controls were matched with cases by sex, age, duration of drug use and follow-up time. Information on methadone treatment in the year prior to seroconversion was collected using clinical registries. Univariate and multivariate conditional logistic regression were used to identify variables related to HIV-seroconversion. The study included 40 cases and 40 controls. Univariate analysis showed the following variables to be associated to HIV seroconversion: number of cycles of treatment, daily dose and time out of treatment. After performing multivariate analysis, daily dose remained protective with a linear effect noted even at low doses and time out of treatment was the most important risk factor. The risk increased 1.5 times for every 3 months spent out of treatment. Long-term methadone treatment protects against HIV infection. Its effect may be attributable to a reduction in the frequency of injecting drug use or to an increased knowledge of risk factors following counselling in drug centres.