characterized by the presence of a higher than normal level of methemoglobin (metHb) in the blood.

Methemoglobin is a form of hemoglobin that does not bind oxygen. When its concentration is elevated in red blood cells, tissue hypoxia can occur.

Normally, methemoglobin levels are <1%, of total blood hemoglobins.



  1. congenital methemoglobinemia – NADH methemoglobin reductase mutation
  2. acquired methemoglobinemia –
  •  exogenous oxidizing drugs and their metabolites
  • benzovaine
  • dapsone
  • nitrates
  •  may accelerate the rate of formation of methemoglobin up to one-thousandfold, overwhelming the protective enzyme systems and acutely increasing methemoglobin levels.