Niraparib hydrochloride is primarily used in the treatment of certain types of cancer, particularly ovarian, fallopian tube, and primary peritoneal cancers. It is specifically prescribed for patients who have recurrent epithelial ovarian, fallopian tube, or primary peritoneal cancer and who have responded to platinum-based chemotherapy. The drug works by inhibiting PARP enzymes, which play a crucial role in DNA repair. By blocking these enzymes, it prevents cancer cells from repairing their DNA, leading to cell death and slowing the progression of the disease. It is often used as a maintenance therapy to help delay the recurrence of cancer after initial treatment. Additionally, it has shown promise in treating other cancers with specific genetic mutations, such as BRCA mutations, where PARP inhibition can be particularly effective.