Microbes in the gut can dramatically boost the effectiveness of a common anticancer drug, at least in mice. That’s the finding of Laurence Zitvogel of Gustave Roussy Cancer Campus in Vellejuif, France, and colleagues, who looked at the anticancer antibiotic cyclophosphamide in treated mice. In those mice lacking a protein that restricts the growth of two spices of gut bacteria (Enterococcus hirae and Barnesiella intestinihominis), the drug was almost twice as effective at reducing tumour size. The work suggests that gut bacteria could be used to optimise cancer therapies.