Glucose-6-phosphate dehydrogenase (G6PD) is an enzyme that is present in all human cells but is particularly important to the function of red blood cells. If there is not enough G6PD (G6PD deficiency), a process called hemolysis weakens the red blood cells. This causes them to break apart easily.

G6PD is an inherited condition that is most common in people from tropical Africa, the Mediterranean, and certain parts of Asia. It also occurs in African Americans.

When hemolysis happens, it may result in the person having too few red blood cells (hemolytic anemia). Attacks of hemolytic anemia may occur in people with G6PD deficiency if they take certain medicines or have infections. For some people, eating fava beans (also called broad beans) can cause an attack.