Methemoglobinemia is the presence of excessive methemoglobin in the blood. Methemoglobinemia is a rarebut clinically important condition that should be considered among the differential diagnoses of cyanosis. It isa fast and relatively easy-to-diagnose procedure. In the treatment of emergency patients; high current O2(HFO2) is important to ensure saturation of available hemoglobin. Methylene blue is the first-line treatment inasymptomatic patients with symptomatic methemoglobinemia (signs of hypoxia such as chest pain, shortnessof breath, or confusion) or with MetHb levels> 20%. A 19-year-old female patient was brought to ouremergency department after taking 2.5 grams of dapsone for suicide purposes from another center when thelevel of methemoglobinemia measured in blood gas was measured as 34%. Hyperbaric oxygen therapy wasapplied for 90 minutes to the patient who did not respond to high flow oxygen therapy. In this case report, wewanted to describe a patient with hypoxic and cyanotic methemoglobinemia who did not respond to standardmethylene blue therapy.