Nanomaterials are central to nanotechnology and have many useful properties. Cerium(IV) oxide nanoparticles (nCeO) have catalytic and antioxidant activity, and hold promise in industrial and medical products. However, the potential biological hazards of nCeO have not been addressed. We therefore investigated the in vivo effects and drug interactions of nCeO. Tail vein administration of nCeO did not induce liver and kidney injury in mice but induced liver or kidney damage when co-administered with paraquat, cisplatin or acetaminophen. Therefore, our findings indicate that nCeO are potentially hepatotoxic and nephrotoxic due to drug interactions.