This report presents the different strategies for identifying heroin users. The criteria allowing a clear distinction between an abuse of heroin and a lawful consumption of opiates are deeply discussed. Reliable and sensitive analytical methods are now available for forensic opiate testing. The detection of 6-mono-acetylmorphine (MAM) indicates that heroin was administered within 24 hours or less of specimen collection. In the absence of MAM or after consumption of several opiates, the relative ratios of morphine, codeine and eventually ethylmorphine must be known in order to determine which opiate(s) was (were) taken. A total amount of opiates of less than 0.3 mg/l very often precludes any characterization of the ingested drug(s). Here we have to point out that forensic opiate testing should be done carefully. Interpretation of results requires more than detection of opiates or morphine alone, irrespective of the number of techniques used.