Underreporting of drug use is commonly found more often than overreporting. Overreporting may, however, occur in particular settings, e.g. in subjects entering a detoxification program. METHODS: Self-reports (standardized semi structured interview) of recent drug use of 554 patients consecutively admitted to a drug detoxification inpatient unit were compared to urine screening results at admission. Overreporters were defined as indicating a consumption of a specific drug during the preceding 7 days (3 days for cocaine) which was not confirmed by the urine screening. Underreproters denied consumption but presented positive urine. RESULTS: Overreporting was especially prevalent for opiates, and relatively more frequent (59.9% heroin, 40% methadone) than underreporting (6.8% heroin, 20.4% methadone). Signs of intoxication at admission, current methadone substitution, and previous institutional detoxification experiences influenced opiate overreporting. CONCLUSIONS: Some of the retained parameters predicting overreporting of recent opiate consumption corroborated the hypothesis of patients trying to receive more consideration from the therapeutic team and to get more intensive pharmacological care.