In clinical or forensic practice there are few studies assessing which risk factors are associated with heroin overdoses. A series of 76 consecutive non-fatal heroin overdoses were compared to 22 consecutive subjects who self-injected heroin within 1 h before admission to the emergency room. Whereas blood levels of alcohol and IgE and urinary cocaine metabolite levels were similar in both groups, higher benzodiazepine plasma levels were detected in the heroin overdose group. The assessment of methadone, dextropropoxyphene, amphetamines and cannabis in urine analysis did not show differences between both groups. The interview revealed that only 48% of subjects in the heroin overdose group self-administered the last dose of heroin before admission in the usual setting as compared to 100% of subjects in the non-overdose group. The application of a log-linear regression model identified self-injection of heroin in an unusual place and plasma concentrations of total morphine and benzodiazepines as risk factors for heroin overdose.