Metformin – Postmortem fatal and non-fatal reference concentrations in femoral blood and risk factors associated with fatal intoxications

•Possible fatal metformin intoxication when blood concentration exceeds 10 μg/g.•Elevated metformin concentrations may not always be due to an acute high intake.•Living alone and alcohol abuse increase the risk of fatal metformin intoxication. To improve the interpretation of fatal intoxications by establishing fatal and non-fatal reference concentrations of metformin in postmortem femoral blood and to further evaluate risk factors associated with fatal metformin intoxication. All forensic autopsies in Sweden where metformin was detected in femoral blood 2011–2016 were identified in the National Board of Forensic Medicine databases (NFMD). The cases were classified as single substance intoxications, A (n = 22), multiple substance intoxications, B (N = 7) and postmortem controls, C (N = 13). The control group consisted of cases where metformin was detected, but the cause of death excluded the incapacitation by metformin or other substances. Strict inclusion criteria were used, and all postmortem cases were assessed by two independent reviewers. All other cases where the inclusion criteria of groups A–C where not met formed group O (N = 78). The forensic findings logged in the NFMD where linked to national registers whereby information on comorbidities, dispensed drugs and clinical data could be obtained. The mean age was 66 ± 10 years in the total study population and did not differ between the groups. The proportion of men was 64% in group A, 71% in B, 77% in C and 74% in group O. The median values of metformin in group A (48.5 μg/g; range 13.0–210 μg/g) and B (21.0 μg/g; range 4.40–95.0 μg/g) were significantly (p