Retention rates in two naltrexone programmes for heroin addicts in Vitoria, Spain

The main finding of a former Spanish multicenter study (SMS) on the effectiveness of naltrexone maintenance in heroin addicts, was the high retention rate achieved at 24 weeks of follow-up since naltrexone induction (40%). The authors claimed this rate was one of the highest ever reported in the literature for a non-selected sample of opiate addicts and discussed the possible relevance of a set of variables - like motivations and expectations due to a new treatment - on the findings. To assess the possible effects of these variables, we have compared the retention rates in two similar naltrexone programmes. The first programme (hospital sample) included 56 individuals who were also included in the SMS where they accounted for 37% of the total sample. That programme was developed formerly to the naltrexone marketing. The second sample (ambulatory sample) included 67 individuals who were recruited at least a year apart since naltrexone marketing was approved by the Spanish Health Boards. The time-lag between the beginnig of both studies was in the range of 15 to 25 months. The subjects in both programmes had similar distributions regarding age (p = 0.27), sex (p = 0.79), weeks on treatment after naltrexone induction (p = 0.20), and program compliance (p = 0.78). The retention rates evaluated over a period of 24 weeks were also similar (p = 0.45). The only difference appeared at 12 weeks of follow-up, showing in higher retention the hospital sample than the ambulatory sample (+23%; p < 0.05). The results are discussed according to other studies and it is concluded that findings reported in the former SMS and in this study are not unusual but compatible with recent research. Also underlined is the potential importance of naltrexone as a concomitant treatment for extinguishing high risk behaviours and the conditional stimuli associated with treatment relapse in heroin addicts.