Drug repurposing is the process of finding new medical uses for drugs that were already developed or approved for a different condition. Instead of building a medication from scratch, researchers take something with a known safety profile and test whether it works against a disease it was never designed to treat. This approach can cut development timelines roughly in half and costs a fraction of what it takes to create an entirely new drug.
Why It Matters: Cost and Speed
Developing a brand-new drug from the ground up takes an average of 10 to 17 years and can cost as much as $2.8 billion. Repurposed drugs, by contrast, can reach approval in 3 to 12 years at nearly half the cost. The reason is straightforward: much of the expensive early work is already done. Safety testing in humans, manufacturing processes, and dosing information already exist. That means researchers can often skip directly to testing whether the drug works for the new condition.
A 2024 analysis of FDA approvals between 1985 and 2024 found that the average time between a drug’s first approval and its approval for a repurposed use was about 7.2 years, though individual cases ranged from the same year to 39 years later. That’s considerably faster than the 12 to 15 years it typically takes to bring a completely novel drug to market for its first indication.
How Researchers Find New Uses for Old Drugs
Some of the most famous repurposing discoveries happened by accident, when patients or doctors noticed unexpected side effects. But today, much of the work is systematic and driven by technology.
One major approach uses network analysis, where researchers map the relationships between proteins, genes, diseases, and drugs in massive interconnected databases. By analyzing these networks with graph algorithms, they can identify places where an existing drug might interact with the biological machinery of a completely different disease. For example, a network-based approach identified the anti-malaria drug hydroxychloroquine as a candidate for reducing cardiovascular disease risk. Another tool called DeepDTnet, which integrates 15 different types of biological networks, predicted that a cancer drug could potentially treat multiple sclerosis.
Similarity-based methods take a different angle. They compare drugs based on their chemical structure, their side effect profiles, or how they change gene activity in cells. If two conditions trigger similar patterns at the molecular level, a drug that treats one might work for the other. A network analysis using this kind of intermediate biological data identified sildenafil (originally a heart drug, later Viagra) as a potential candidate for Alzheimer’s disease research.
Famous Examples
Sildenafil: From Chest Pain to Erectile Dysfunction
Sildenafil is perhaps the most well-known repurposing story. In 1986, Pfizer researchers identified the compound as a potent inhibitor of a specific enzyme, developing it as a treatment for angina (chest pain caused by reduced blood flow to the heart). By the early 1990s, it was looking less promising for angina. But during clinical trials, some volunteers reported penile erections as a side effect. That unexpected observation led Pfizer to pivot, and the drug became Viagra. It was later repurposed again for pulmonary hypertension, a serious condition involving high blood pressure in the lungs.
Thalidomide: From Scandal to Cancer Treatment
Thalidomide was originally marketed in the late 1950s as a sedative. It was pulled from the market after it was found to cause severe birth defects. Then in 1964, a doctor prescribed it as a sedative to a patient with a painful skin condition related to leprosy and noticed a dramatic resolution of the skin eruptions within 48 hours. The FDA approved thalidomide for that leprosy-related condition in 1998, with strict safety controls.
The story didn’t end there. Researchers recognized that thalidomide could block the growth of new blood vessels, a process that certain cancers depend on. It also modulates the immune system in ways that attack myeloma cells. In 2006, thalidomide became the first new agent in over a decade approved for treating a type of blood cancer called multiple myeloma, completing what researchers have called a “remarkable renaissance.”
Baricitinib: From Arthritis to COVID-19
Baricitinib was approved in 2018 for adults with moderate to severe rheumatoid arthritis. When COVID-19 emerged, researchers recognized that the drug’s ability to dampen overactive immune responses could help critically ill patients whose bodies were mounting a destructive inflammatory attack on their own lungs. Clinical trials confirmed the benefit, and it became one of the repurposed treatments used during the pandemic. It has also been studied for conditions ranging from lupus to alopecia.
The Regulatory Shortcut
In the United States, repurposed drugs often use a specific FDA pathway called a 505(b)(2) application. This allows a company to submit a new drug application that relies partly on data it didn’t generate itself, including the FDA’s own previous findings that the drug is safe and effective for its original use. The company doesn’t need to repeat every safety study from scratch. Instead, it can submit “bridging studies” that connect what’s already known to the new proposed use.
This pathway was designed to encourage innovation without forcing companies to duplicate research that already exists. An applicant identifies the previously approved drug, points to the existing safety and effectiveness data, and then provides whatever new evidence is needed to support the change, whether that’s a new indication, a new dosage form, or a new route of administration.
Why More Drugs Aren’t Repurposed
Despite the advantages, drug repurposing faces serious commercial obstacles that have nothing to do with science. The core problem is intellectual property. Most repurposing candidates are older drugs whose original patents have expired, meaning generic versions are already available. A company that invests millions in clinical trials to prove a new use may find that doctors simply prescribe the cheap generic version “off-label” for the new condition, leaving the company with no way to recoup its investment.
This creates what researchers call a “free rider” dilemma: no company wants to spend the money proving a new use if every competitor can benefit from the results without sharing the cost. In many health systems, doctors prescribe by active ingredient rather than brand name, and pharmacists dispense the cheapest available version. If the pharmacist doesn’t know or doesn’t need to know the intended use, the company that funded the research can’t charge a higher price for the repurposed version.
Companies can try to patent a new formulation, dosage form, or derivative of the drug. But national policies that promote generic prescribing and reference pricing systems often prevent meaningful price differentiation between uses. In expert surveys, 75% of the most involved researchers identified developing private-sector incentives as the main challenge, and 60% of experts overall pointed to insufficient commercial incentives as the primary financial barrier to repurposing research. About 56% also cited insufficient public funding.
Repurposing for Rare Diseases
Drug repurposing holds particular promise for the roughly 6,000 to 8,000 recognized rare diseases, only about 5% of which have an approved treatment. Traditional drug development for rare conditions is often not commercially viable because the patient population is so small. Repurposing existing drugs lowers the financial bar enough that some of these diseases become feasible targets.
The Orphan Drug Act provides additional incentives, including market exclusivity, for drugs that treat conditions affecting small populations. One example is propranolol, a common blood pressure medication that received orphan drug designation in 2008 and was approved in 2014 for treating infantile hemangioma, a type of blood vessel growth in infants. The drug had been in use for decades for heart conditions, so its safety profile was well understood, which simplified the path to approval for this new pediatric use.