This article discusses cases of drug-poisoning death in which 6-acetylmorphine (6-AM) was identified in blood as evidence for recent use of heroin. We report the demographics of the victims, previous drug-related offences, polydrug use, and the concentrations of free morphine in peripheral blood. After solid-phase extraction, morphine, codeine, and 6-AM were determined in blood samples by isotope-dilution gas chromatography-mass spectrometry (GC-MS) using limits of quantitation of 0.005 mg/l for each opiate. The victims of heroin poisoning were mainly men (88%), with a mean age of 35.4 ± 8.4 years (±SD) and no significant gender difference in age (men 35 ± 8.4 years; women 35 ± 8.6 years). The median concentration of free morphine in blood (n = 671) was 0.25 mg/l (66% [ 0.20 mg/l) and women had a higher concentration (0.30 mg/l) than men (0.24 mg/l) (P \ 0.05). No significant difference (P [ 0.05) was found for the concentration of free morphine in blood when heroin was the only drug taken (median 0.26 mg/l, n = 53) compared with multidrug deaths (median 0.24 mg/l, n = 618) (P [ 0.05). The coingested drugs most commonly identified in heroin-related deaths were ethanol (44%), diazepam (27%), cannabis (20%), and flunitrazepam (19%). We found that 61% of victims had previous drugrelated offences ranging from 1 to 48 times. The close agreement between the concentrations of free morphine in blood when heroin was the only drug taken and multidrug deaths suggests that differences in tolerance to opiates is more important in causing death than adverse drug-drug interactions.