Prescribing of injectable diacetylmorphine (DAM) for heroin dependence has raised concerns about its safety. In light of various reports by heroin-maintained patients of DAM-related adverse events, and previously established unwanted effects of opioids in pain management, we undertook a survey in February 2001 of a random sample of 132 (127 participated) of 1061 patients prescribed DAM in Switzerland at that time. The purpose was to document the prevalence rates of a list of unintended symptoms experienced and attributed to DAM by patients. To assess symptom complaints and other data, staff administered a six-page self-report questionnaire. The patients ascribed numerous symptoms to DAM, with the best-known being the most frequently reported (e.g. skin itching, sweating, constipation). Among potentially more problematic complaints ranged irregular menses, cognitive deficits, muscle twitches, labored breathing, pains in the cardiac region, and temporary paralysis of limbs. In the absence of a control group, however, these may also be due to other factors, such as expectation, co-medication, concomitant substance use or co-morbidity. This pilot study emphasizes the necessity of rigorous assessment of the true rates, types, severity and preventability of such complications, especially given the current efforts to establish heroin maintenance as an optional treatment for heroin dependence.