4-Methoxybutyrfentanyl: 4-MeO-BF is a highly potent fentanyl analogue first identified in Europe in 2015. Fatal blood concentrations have ranged 2-18 ng/mL, only modestly lower than fentanyl itself. Street material varies wildly in purity; volumetric dosing with a 1 mg/mL (or weaker) solution and a
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