Zejula (niraparib) is a PARP inhibitor used as maintenance treatment to prevent the recurrence of certain cancers, including ovarian, fallopian tube, or peritoneal cancer. It is given after chemotherapy if the cancer has responded. Zejula is used in patients with specific genetic markers, like an abnormal "BRCA" gene. Niraparib works by blocking the PARP protein, preventing DNA repair in cancer cells, leading to their death and slowing tumor growth.