Glucose-6-phosphate dehydrogenase (G-6-PD) catalyzes the first step in the pentose phosphate pathway, oxidizing glucose-6-phosphate to 6-phosphogluconate and reducing NADP to NADPH. G-6-PD deficiency in erythrocytes is a major cause of congenital non-spherocytic hemolytic anemia.

This deficiency affects approximately 200 million people worldwide, especially those individuals originating from Southeast Asia, Africa, the Mediterranean and India. In this disorder of RBC metabolism, a decreased enzyme level is found in older erythrocytes, resulting in RBC lysis in response to certain drugs (primaquine, chloramphenicol, acetanilide, phenacetin, vitamin K, sulfonamides and nitrofurans). Variants of G-6-PD-deficiency differ in the degree of residual enzyme activity (severe to mild). Severe deficiency decreases the RBC life span to such an extent that a chronic hemolytic anemia with jaundice is found even in the absence of a drug challenge, Females with G-6-PD deficiency may have difficulty in carrying pregnancy to term, possibly because of the effect of changing hormones on the sensitive RBCs.