A cheap antidepressant reduced the need for hospitalisation among high-risk adults with Covid-19 in a study hunting for existing drugs that could be repurposed to treat coronavirus.

Researchers tested the pill used for depression and obsessive-compulsive disorder because it was known to reduce inflammation and looked promising in smaller studies.

They've shared the results with the US National Institutes of Health, which publishes treatment guidelines, and they hope for a World Health Organization recommendation.

"If WHO recommends this, you will see it widely taken up," said study co-author Dr Edward Mills of McMaster University in Hamilton, Ontario, adding that many poor nations have the drug readily available. "We hope it will lead to a lot of lives saved."

The pill, called fluvoxamine, would cost $4 for a course of Covid-19 treatment. By comparison, antibody IV treatments cost about $2,000 and Merck's experimental antiviral pill for Covid-19 is about $700 per course. Some experts predict various treatments eventually will be used in combination to fight the coronavirus.

Researchers tested the antidepressant in nearly 1,500 Brazilians recently infected with coronavirus who were at risk of severe illness because of other health problems, such as diabetes. About half took the antidepressant at home for 10 days, the rest got dummy pills. They were tracked for four weeks to see who landed in the hospital or spent extended time in an emergency room when hospitals were full.

In the group that took the drug, 11% needed hospitalisation or an extended ER stay, compared to 16% of those on dummy pills.

The results, published Wednesday in the journal Lancet Global Health, were so strong that independent experts monitoring the study recommended stopping it early because the results were clear.

Questions remain about the best dosing, whether lower risk patients might also benefit and whether the pill should be combined with other treatments.

The larger project looked at eight existing drugs to see if they could work against the pandemic virus. The project is still testing a hepatitis drug, but all the others — including metformin, hydroxychloroquine and ivermectin — haven't panned out.

The cheap generic and Merck's Covid-19 pill work in different ways and "may be complementary," said Dr Paul Sax of Brigham and Women's Hospital and Harvard Medical School, who was not involved in the study. Earlier this month, Merck asked regulators in the US and Europe to authorize its antiviral pill.