decrease in the oxygen-carrying capacity of the red blood cells (erythrocytes (erythrocyte)) due to the presence of methemoglobin in the blood. The severity of the symptoms of methemoglobinemia is related to the quantity of methemoglobin present in the circulation and range from a bluish discoloration of the skin and mucous membrane to weakness, difficulty in breathing, and dizziness in the more severe cases.

The iron component of the hemoglobin of the red blood cells must be in the reduced (deoxidized) state to bind with oxygen; methemoglobin contains the oxidized form of iron and is useless for oxygen transport. Normally, various organic catalysts or enzymes are active in keeping the iron in the reduced form. Hereditary methemoglobinemia occurs when there is an inborn defect in this enzyme system or when the hemoglobin molecule is abnormally structured (hemoglobin M) and is thereby more susceptible to oxidation of the iron component. Acquired methemoglobinemia may arise as a result of contact with certain drugs and chemicals that produce oxidant compounds in the circulation, causing the oxidation of iron to occur faster than the enzyme system can keep it in the reduced state.

Treatment of hereditary methemoglobinemia usually includes the administration of reduction compounds such as methylene blue. Acquired methemoglobinemia usually disappears spontaneously when the cause is removed.