Could a decades-old antidepressant be a secret weapon against COVID-19? A few scientists think so, after two small studies showed that fluvoxamine, typically prescribed for obsessive-compulsive disorder, prevented serious illness in all participants who took the pills soon after developing symptoms.

It's an exciting notion: A $10, two-week course of this drug could reduce death and hospitalizations. The drug could be used to fight ongoing outbreaks and would be a particular godsend for lower-income countries that may have to wait years for vaccines. But fluvoxamine, as well as other old drugs showing potential against COVID-19, face hurdles to full evaluations.

Drug companies have no incentive to spend millions to test new uses for cheap, off-patent drugs. And early enthusiasm for COVID-19 treatments that later flopped has "made people gun-shy," said Dr. Jeffrey Klausner, a professor of preventive medicine at the University of Southern California.

In particular, former President Donald Trump's premature promotion of hydroxychloroquine likely stymied efforts to find other generic cures. The Food and Drug Administration granted emergency use of the malaria drug in March, then revoked the authorization less than three months later after evidence showed it was more likely to harm than help patients.

"We doctors who want to use evidence-based medicine feel somewhat burned by the hydroxychloroquine experience and really want to see good studies before we actually jump on the bandwagon," said Dr. Paul Sax, clinical director of the division of infectious diseases at Brigham and Women's Hospital in Boston.

And that poses a Catch-22. Until recently, the National Institutes of Health, the largest public funder of biomedical research, has shown little interest in studies of repurposed drugs. Without big money, it's hard to do the research needed to show whether existing treatments could work against COVID-19.

As a result, efforts to repurpose drugs have fallen to philanthropists. "We're missing out on public health benefits of the drugs we already have because we're relying almost entirely on capitalism and private industry to make advances," said Elaine Lissner, founder of the San Francisco-based Parsemus Foundation.

Repurposing is a long shot, yet compared to creating drugs and vaccines, the approach has clear advantages during a fast-moving pandemic. "If it works and it's on the shelf, you don't have any development time," said Dr. Lisa Danzig, a specialist in infectious diseases. Danzig was "very excited" last April by news that a team of researchers had identified 69 possible drugs that, when used early on, could counteract infections with SARS-CoV-2. "I'm thinking, if we can rapidly test some of these in clinical trials, we can have answers by October." Yet these studies struggled to get off the ground.

Kaiser Health News is a national newsroom that produces in-depth journalism about health issues. Together with Policy Analysis and Polling, KHN is one of the three major operating programs at the Kaiser Family Foundation.