Drug repurposing is the development of existing drugs for new uses: given that 9 in 10 drugs that enter drug development are never marketed and therefore represent wasted effort, it is an attractive as well as inherently more efficient process. Three repurposed drugs can be brought to market for the same cost as one new chemical entity; and they can also be identified more quickly, an important benefit for patients whose diseases are progressing faster than therapeutic innovation. But repurposing also requires a fresh look at configuring pharmaceutical R&D, considering clinical, regulatory and patent issues much earlier than would otherwise be the case; a holistic gedanken experiment almost needs to be undertaken at the very start of any repurposing development. In addition to new ways of thinking, the discovery of repurposing opportunities can take advantage of artificial intelligence techniques to match the perfect new use for an existing drug. And while repurposing of medicines has been in the mind of every doctor since Hypocrates, modern clinical practice will simply have to adapt to new repurposing techniques in an age where the number of known diseases is increasing much faster than the healthcare dollars available.