Drug repurposing is a strategy that searches for new therapeutic uses for existing medicines. Instead of starting from scratch with a new molecule, researchers investigate drugs that are already approved or have passed substantial safety testing, aiming to shorten development time and reduce cost and risk.

A central motivation is the high failure rate and expense of traditional drug discovery. Repurposed candidates often have known pharmacokinetics, toxicity profiles, and manufacturing processes. This allows faster transition into clinical trials, especially in urgent situations such as emerging infectious diseases or rare conditions with no approved therapy.

Modern repurposing combines clinical insight with computational and high throughput methods. Approaches include systematic screening of compound libraries against new targets, mining electronic health records for unexpected beneficial effects, and integrating genomic, transcriptomic, and proteomic datasets to match disease signatures with drug action profiles. Network biology and artificial intelligence are used to map how drugs perturb cellular pathways and to predict alternative indications.

Successes include identifying existing antivirals, anticancer agents, and central nervous system drugs for new indications. However, many promising candidates fail in later stage trials when efficacy does not translate from models to patients. Intellectual property and commercial incentives are also significant barriers, particularly for off patent molecules, which may limit investment despite scientific promise.

Regulators have created pathways that can streamline development for repurposed drugs, but robust clinical evidence remains essential. Overall, drug repurposing is seen as a powerful complement to conventional discovery, capable of accelerating treatment options by exploiting the untapped potential of known compounds.