A drug used to treat recurrent ovarian epithelial, fallopian tube, or primary peritoneal cancers that have gotten better after treatment with anticancer drugs that included platinum. Niraparib tosylate monohydrate is used to help keep the cancer from coming back again. It is also being studied in the treatment of other types of cancer. Niraparib tosylate monohydrate blocks an enzyme involved in many cell functions, including the repair of DNA damage. Blocking this enzyme may help keep cancer cells from repairing their damaged DNA, causing them to die. Niraparib tosylate monohydrate is a type of poly (ADP-ribose) polymerase inhibitor. Also called Zejula.