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• Metformin intoxication can be fatal. • Hair analysis is important to analyze the historical consumption of metformin in fatalities. • Metformin hair concentration could be altered by sweat contamination due to fatal hypoglycaemia during the agony period. • Metformin hair concentration after acute poisoning are 10 times more elevated when compared to therapeutic users. Hair analysis is very useful for toxicological investigations since, by providing a wider detection window, it gives the possibility to perform a retrospective study on the historical consumption of a substance. Unfortunately, there are no data available for hair concentrations in metformin-related deaths. In this study, the authors present 2 cases of fatal metformin intoxication in which, for the first time, hair analysis was performed using a specific GC–MS/MS method. Metformin was tested positive in femoral blood (112.3 mg/L and 64.7 mg/L respectively) and cardiac blood (226.9 and 203.2 mg/L) of the two subjects. For case 1, other samples were also tested positive, including vitreous humor (31.1 mg/L) and gastric contents (773.5 mg/L). In case 2, metformin was measured at 844.9 mg/L in urine. Metformin hair concentrations were 28.3–44.8 and 22.5 ng/mg for both cases, respectively. The concentrations found in the 2 fatal cases are clearly higher than those obtained in a previous study with subjects under treatment (0.3–3.8 ng/mg) or those found in 3 post-mortem cases where metformin death was excluded (0.6–1.4 ng/mg). Excessive sweating during the agonal phase due to fatal hypoglycemia could explain these elevated concentrations as sweat can have contaminated the hair. [ABSTRACT FROM AUTHOR]