Methemoglobinemia is a blood disorder in which an abnormal amount of hemoglobin builds up in the blood. Hemoglobin is the oxygen-carrying molecule found in red blood cells. In some cases of methemoglobinemia, the hemoglobin is unable to carry oxygen effectively to body tissues.

Methemoglobinemia may be passed down through families (inherited). Or, it may result from exposure to certain drugs, chemicals, or foods (acquired).

There are two forms of inherited methemoglobinemia. The first form is passed on by both parents. The parents usually do not have the condition themselves, but they carry the gene that causes the condition. It occurs when there is a problem with an enzyme called cytochrome b5 reductase.

There are two types of this form of methemoglobinemia:

• Type 1 (also called erythrocyte reductase deficiency) occurs when red blood cells lack the enzyme.
• Type 2 (also called generalized reductase deficiency) occurs when the enzyme doesn't work anywhere in the body.

The second form of inherited methemoglobinemia, called hemoglobin M disease, is caused by defects in the hemoglobin molecule itself. Only one parent needs to pass on the abnormal gene in order for the child to inherit the disease.

Acquired methemoglobinemia is more common than the inherited forms. It occurs after exposure to certain chemicals and drugs, including:

• Anesthetics such as benzocaine and Xylocaine
• Benzene
• Certain antibiotics (including dapsone and chloroquine)
• Nitrites (used as additives to prevent meat from spoiling)

The condition may also occur in infants who are very ill or who are fed too many vegetables containing nitrates (such as beets).