Drugs may be Induced Methemoglobinemia

Methemoglobinemia is a rare disorder of the blood in which there is an increase in the proportion of hemoglobin present in the oxidized form (methemoglobin). It may be inherited, due either to a deficiency of methemoglobin reductase or to a structural abnormality of hemoglobin, or it may be acquired, usually secondary to exposure to drugs or chemicals that oxidize hemoglobin Nearly 90 compounds have been implicated in the production of Methemoglobinemia, nitrates and aniline derivatives are among the most common agents. Drugs rarely produce clinically significant Methemoglobinemia when given to a normal adult in therapeutic doses, while individuals with methemoglobin reductase deficiency or abnormal hemoglobin may exhibit severe effects as well as overdose. Drugs that may cause Methemoglobinemia include nitrates derivatives (nitrates salt, nitroglycerin), nitrites derivatives (nitroprusside, amyl nitrite, nitric oxide), sulfonamides, dapsone, phenacetin, phenazopyridine, some local anesthetics such as prilocaine, topical anesthetics such as emla cream, benzocaine, antimalarial. Administration of low doses over prolonged periods may lead to chronic Methemoglobinemia whereas large doses may lead to an acute effect The risk Methemoglobinemia associated with oxidizing drugs use is increased in Persons with health problems (A genetic deficiency of G-6-PD or methemogloin reductase, hemoglobin "m", renal failure, anemia, HIV infection), infants (less than three months) due to limited enzyme capacity, elderly, large dose, prolonged therapy, concomitant administration of more than one oxidants drugs, potency of the drugs, route of administration of drugs. Methemoglobinemia may not develop for several hours (e.g, 1 to 10) after exposure to some oxidants agents. Observation of asymptomatic individuals for 24 hours may be advisable with exposure to some oxidants drugs which require biochemical transformation before causing Methemoglobinemia.