C. elegans lifespan extension by osmotic stress requires FUdR, base excision repair, FOXO, and sirtuins

•FUdR extends lifespan of C. elegans adapted to hypertonic stress conditions.•FUdR increases resistance to acute hypertonic stress, thermal stress, and anoxia.•Effects of FUdR are primarily somatic in this context.•FUdR acts via thymidylate synthase inhibition and impacts FOXO, sirtuins, and DNA repair pathways.•FUdR use should be avoided in C. elegans stress or aging studies, or when different genotypes are compared.