Metformin as an environmental substance transferring to horses – a case report and analysis

Metformin is a widely prescribed oral antihyperglycemic agent and currently a first-line medication in the treatment of human type 2 diabetes, with a total of 92 million US prescriptions in 2022. The daily dose per human can be as much as 2.5 grams/day which is excreted largely unchanged into the environment. Metformin is chemically stable and a widely distributed environmental substance. Metformin therefore has the potential to be identified at trace levels in equine blood and urine samples as a result of random exposure to environmental metformin. Given these circumstances we have reviewed the scientific literature and calculated an irrelevant blood/plasma/serum concentration of metformin of 5 nanograms/ml. We now therefore propose this plasma concentration of metformin as an interim Screening Limit of Detection (SLOD) for metformin, below which concentration a blood/plasma/serum identification of metformin should not be considered appropriate for regulatory action.