Prophylaxis with fluoroquinolones in high-risk neutropenic patients is currently under scrutiny due to their toxicity and the potential of selecting multirresistant bacteria. In this setting, the search for an alternative prophylactic drug is a priority. The FOVOCIP study aimed to evaluate the efficacy and safety of fosfomycin compared to ciprofloxacin in this population. This was a multicentre, randomised, phase-3, non-inferiority, open-label trial performed in 11 centres in Spain. Adults diagnosed with acute leukaemia or recipients of a Haematopoietic Stem Cell Transplant were randomised to receive oral fosfomycin or oral ciprofloxacin as prophylaxis. The primary endpoint was rate of febrile neutropenia. Secondary endpoints included safety, including microbiological safety and gut microbiota changes .