The nematode Caenorhabditis elegans is a popular model organism that is used to study longevity and aging. One drawback of nematode lifespan assays is the labour intensive separation of offspring from adults during the reproductive period. To circumvent this, the worm community frequently adds 5-fluoro-2'-deoxyuridine (FUdR), a drug that induces parental sterility, to the nematode culture. Here, we report that FUdR causes a significant artefactual increase in the longevity of tub-1 mutants which is not observed in wild-type nematodes. This suggests that the effect of FUdR is not neutral and the mechanism of action may result in misleading data, or indeed the misinterpretation of gerontogenes.