Methylene blue may cause serious or fatal serotonergic syndrome when used in combination with serotonergic drugs.

https://www.ncbi.nlm.nih.gov/books/NBK557593/ Methylene blue is a medication used in the management and treatment of methemoglobinemia, a condition in which hemoglobin decreases its ability to carry oxygen. This activity reviews the indications, action, adverse effects, and contraindications for methylene blue as a valuable agent in the therapy of methemoglobinemia, vasoplegic syndrome, Plasmodium falciparum, and ifosfamide-induced encephalopathy, as well as for diagnostic purposes such as sentinel lymph node mapping during breast procedures and parathyroid gland mapping during parathyroidectomies. This activity will highlight the indications, mechanism of action, adverse event profile, and other key factors pertinent for members of a healthcare team managing patients with methemoglobinemia and related conditions.

Methylene blue has FDA approval for the treatment of methemoglobinemia, a condition when Fe2+ of hemoglobin gets oxidized to Fe3+, reducing the oxygen-carrying capacity of hemoglobin, and typically presents with cyanosis of the lips and extremities, characteristic “chocolate-colored urine,” and hypoxia.[1] Methemoglobinemia results from exposure to certain drugs such as dapsone, a drug indicated for the treatment of Mycoplasma leprae and Pneumocystis jirovecii prophylaxis, benzocaine (a local anesthetic), high altitude water sources, and nitrites such as nitroglycerin or amyl nitrite used for treating coronary artery disease.[