Although methadone maintenance treatment (MMT) has been shown to be effective, poor compliance is always a threat. It has often been pointed out that one variable that inevitably reduces adherence to treatment regimes is the negative side-effects of the treatment. This study examines seventeen known side-effects in a sample of 110 former opiate addicts consecutively admitted to methadone maintenance treatment. Self-reported side-effects were collected through a questionnaire. Despite the considerable increase, from 23 to 74% of the sample, in the proportion that reported individual side-effects between the period of opiate use and that of methadone treatment, the overall result was a significant decrease in eleven symptoms, no change in four and a substantial increase in only two. In some individuals a symptom that is liable to be problematic actually does become problematic, while the same symptom becomes less frequent in the group as a whole. Weight gain is the symptom that increases most in the whole group and needs to be constantly monitored. Other side-effects that remain high and need to be kept under review in clinical practice are nervousness, decreased libido, daytime drowsiness, constipation and perspiration.