Few students go into medical research dreaming of one day giving a mouse a tumour or heart disease to a rabbit. But animal experimentation of this kind has become fundamental to modern medicine. For regulatory bodies around the world, treatments that might save human lives must be proven safe and effective in animals with similar conditions before they can be put on the market.

So how is this ethical bargain holding up? Not well enough, suggests a paper in PLOS Biology by Benjamin Ineichen of the University of Zurich and his colleagues. Out of 367 biomedical therapies tested on animals over thousands of studies, a surprisingly low proportion—only 5%—eventually obtained the approval of America’s regulatory agency for drugs, the Food and Drug Administration (FDA), to be used in humans.