However, the effects of metformin on mitochondria likely depend on dose, in vivo just as in vitro. Patients enrolled in this present study had an average serum drug level of 32 �� 14 mg/L, much lower than that of severely intoxicated patients (61 �� 25 mg/L in our previous series) [7]. in vitro, metformin was added to plasma to obtain an initial (toxic) concentration of 166 mg/L, or higher. Even if final drug levels were probably lower, due to cellular uptake [13], they likely exceeded 32 �� 14 mg/L. In other words, overdose severity was high (or very high) in vitro, but only moderate in vivo. We cannot exclude that a 24- to 48-hour delay in evaluating platelet mitochondrial function further contributed to diminish our capacity to detect early larger alterations.The constant decrease in platelet complex IV activity was totally unexpected, as it never occurred in vitro, not even at the highest drug dose. Underlying mechanisms were not specifically investigated so that we can only speculate on them. Only some of the patients received sedation, catecholamines and/or mechanical ventilation, so that these factors likely had no major role. Conversely, all patients had undergone renal replacement therapy by the time their platelet mitochondrial function was assessed. Whether renal failure per se or extra-corporeal support can inhibit human platelet complex IV is currently unknown.All patients enrolled in this study had a favourable outcome, despite signs of mitochondrial inhibition in platelets (and possibly other tissues). This may suggest that prognosis does not depend on the effects of metformin on the respiratory chain. However, it may also indicate that the rate of survival will be unexpectedly high if mitochondrial dysfunction is due to a compound that can be easily removed from the body (using renal replacement therapy, for instance) [32].ConclusionsSevere metformin overdose can alter mitochondrial function and increase lactate production of human platelets, in vitro and, possibly, ex vivo. If analogue changes also occur in other organs, they will likely contribute to the pathogenesis of metformin-induced lactic acidosis.Key messages? In pigs, severe metformin intoxication causes mitochondrial dysfunction in platelets as well as in other more vital organs, including the heart, kidney and skeletal muscle.? Human platelets exposed to a toxic dose of metformin, either in vitro or in vivo, have clear signs of mitochondrial dysfunction.? If mitochondrial dysfunction is a generalized phenomenon even in humans, it will likely contribute to the development of lactic acidosis (possibly by augmenting tissue lactate production).