In this article we report a large-scale chemical screen in adult Drosophila to find inhibitors of stem-cell–derived tumors. To our surprise, we found that some Food and Drug Administration-approved chemotherapy drugs have the dual property of reducing growth of stem-cell–derived tumors while also stimulating hyperproliferation of their wild-type counterparts. Since hyperproliferation is one of the hallmarks of cancer cells, this side effect could contribute to refueling the growth of the very tumors that these chemotherapeutics are intended to inhibit. We show that this side effect is driven by the evolutionarily conserved Janus kinase-signal transducers and activators of transcription (JAK-STAT) inflammatory pathway, raising the possibility that the JAK-STAT pathway may also be activated in humans who are treated with some chemotherapeutics.