Rare cause of dyspnoea: phenazopyridine-induced methemoglobinemia

Methemoglobinemia, due to the oxidation of iron from ferrous to ferric form in the haemoglobin molecule, results in decreased oxygen delivery to the tissues. Phenazopyridine, a commonly used medication for urinary symptoms, is a rare cause of methemoglobinemia. We report an elderly woman with advanced vaginal cancer, who developed methemoglobinemia following the use of phenazopyridine at a dose of 200 mg three times a day for 14 days. She presented with dyspnoea without cyanosis, oxygen saturation of 70%, methemoglobin concentration of 32.2% in arterial blood gas analysis and haemoglobin of 5 g/dL. This condition was identified and treated with methylene blue and oxygen support, which resulted in complete recovery in two days. Though rare, this highlights the importance of ascertaining reversible causes of symptoms, even in palliative care settings. Medications as an aetiology of symptoms should be considered even if they were used on a long-term basis.