Studies on fruit flies show how biological sex influences the effect of rapamycin, the currently most promising anti-ageing drug. Researchers from the Max Planck Institute for Biology of Ageing in Cologne and University College London report in Nature Aging that the drug only extends the lifespan of female fruit flies, but not that of males. In addition, rapamycin slowed down the development of age-related pathological changes in the gut only in female flies. The researchers conclude that biological sex may be a decisive factor in the effectiveness of anti-ageing drugs.