Methemoglobinemia must be considered in the differential diagnosis of the cyanotic patient. Methemoglobin cannot carry oxygen or carbon dioxide. Methemoglobinemia can result from exposure to a wide variety of chemicals, including many commonly prescribed drugs, usually in an overdose situation. Most cases require only supportive therapy and assessment for other toxic complications. Severe cases may result in hypoxia and require treatment with methylene blue. Not all cases respond to methylene blue, and methylene blue itself may produce serious side effects.