A few details are unclear to me. Metabolism of SNP involves a few steps, including . SNP + oxyHb --> 5 x CN- + 1 x metHb . CN- + metHb --> cyanmetHb (Skipping the other details concerning thiosulfate, amyl nitrate, etc etc.) Question # 1 - Can you get a net production of methemoglobin from a SNP infusion? My understanding of this is that there is NO net production of methemoglobin because there's an excess of cyanide produced simultaneously, and any available metHb is sucked up and converted to cyanmetHb. Does cyanmethemoglobin also produce a pulse ox reading of 85? There's a question in Hall concerning a patient with a prolonged infusion of SNP presenting with a SpO2 of 85% and the correct answer per the key is methemoglobinemia. There's also an old ITE question from 1994 in which the key states that methemoglobin is specifically NOT a consequence of SNP infusion (though the scenario implies a short-term infusion of SNP). Where does the metHb come from after prolonged SNP infusions? The best answer I can come up with is that over time, the cyanmetHb dissociates to CN + metHb and that CN gets sucked up by thiosulfate that is produced in the interim. This isn't spelled out in any reference I can find though. Question # 2 - Is it safe or appropriate to treat methemoglobinemia with methylene blue in the setting of a prolonged SNP infusion? Morgan & Mikhail says yes. My understanding is that the answer here is a definite no, because methylene blue will also "fix" cyanmethemoglobin ... potentially dumping a lot of newly freed CN in the process.