Acute pancytopenia due to megaloblastic arrest in association with co-trimoxazole

Three patients who were taking co-trimoxazole developed acute pancytopenia due to megaloblastic arrest, and two of the three died while pancytopenic. One patient had a pre-existing megaloblastic anaemia. In the other two patients, there was no macrocytosis or neutrophil hypersegmentation despite the severe megaloblastosis. Because specific treatment is needed urgently, it is important to distinguish megaloblastic arrest from drug-induced hypoplasia.