RESUMO
This study evaluated the relationship between the plasma concentration of l-methadone and response to methadone in real-world patients, in order to identify a minimum plasma concentration above which methadone treatment is effective. Ninety-four patients with opioid dependence under maintenance methadone treatment were consecutively recruited. Response was defined as negative urine analyses in the three weeks prior to the blood sampling. The percentage of participants with a plasma l-methadone concentration between 100 and 250 ng/ml was 54.2% among those with a methadone dosage ≥60 mg/day. Plasma l-methadone concentrations were significantly higher in patients with negative urine analyses compared with those with positive urine analyses (median 93 vs. 77 ng/ml, Mann-Whitney test, P<0.05). Above plasma l-methadone concentrations of 200 ng/ml no heroin use was reported and urine analyses were negative. Moreover, above concentrations of 250 ng/ml craving was absent. Examination of demographic correlates of treatment outcome indicated that older age, a stable job and being married were protective against the use of heroin. Mean plasma l-methadone concentration was significantly lower in patients who used cannabis compared with those who did not use cannabis, after adjusting for methadone dosage. In conclusion our results identify specific cut-offs for plasma l-methadone concentrations about which therapeutic response is observed and provide new evidence that therapeutic response is associated with patient׳s demographic characteristics. This underscores the need to monitor plasma methadone concentrations as part of Drug Addiction Services routine practice, in order to provide an objective framework for changing the methadone dosage.