Glucose-6-phosphate dehydrogenase (G6PD) is a housekeeping enzyme that protects erythrocytes against oxidative injury by providing reducing power in the form of nicotinamide adenine dinucleotide phosphate (NADPH). Erythrocytes are particularly susceptible to oxidative stress, because unlike most cells, they lack other NADPH-producing enzymes. Rapid destruction of large numbers of erythrocytes in individuals with G6PD deficiency can occur after therapy with certain drugs, including the 8-aminoquinolines used in treating malaria. These episodes can range from mild to life-threatening depending on the dose of the drug, the variant of G6PD deficiency, age, and pre-existing or coexisting morbidities. In malaria treatment, people with G6PD deficiency may experience serious haemolytic episodes if treated with 8-aminoquinolines, such as primaquine; thus, testing for G6PD deficiency before drug administration is essential for patient safety.