This study attempts to analyse potential gender differences among a group of heroin addicts seeking treatment at a university-based medical centre. The central modality of treatment at this centre is the use of methadone maintenance. Among those patients entering this program there seems to be an emerging pattern of males who tend to use heroin as their opiate of choice, and are more likely to combine it with cannabis, while females are more likely to use to street methadone, with adjunctive use of ketamine, benzodiazepines, hypnotic drugs and/or amphetamines. Women are at higher risk of abusing opioids through a pathway of initial prescription painkiller use, and later to resort to street methadone to cope with prescription pain killer addiction. This latter pattern seems to result in an increased risk for fatal accidental overdoses. The use of these longer-acting agents in women may be influenced by psychosocial and hormonal factors.