The widely available diabetes drug metformin may reduce the risk of emergency room visits, hospitalisation, and death from COVID-19 by more than half if taken within four days of the start of the symptoms, according to a study.

The researchers from the University of Minnesota in the US noted that metformin, also known as Glucophage, has been used in the 1950s as an anti-viral called "Fluamine."

In more recent research, the drug has been found to affect inflammation pathways, they said.

Since Covid -19 involves viral inflammation, the research team thought the combination of anti-inflammatory plus antiviral action was intriguing enough to test the drug.

"This was really intriguing to us early on when we learned that people who take metformin were catching COVID-19 less and being hospitalised less," said Elaine Lissner, founder of California-based nonprofit Parsemus Foundation.

The study, published on Thursday in the New England Journal of Medicine, compared three medications that were considered promising -- fluvoxamine, an antidepressant that had shown strong results in previous studies, ivermectin, the object of much interest, and metformin.

The study included 1,323 participants most at risk of serious outcomes -- adults over 30 and with body mass index (BMI) over 25 kilogrammes per square metre (kg/m2) -- to get lifesaving results more quickly.

The researchers noted that metformin, an exceedingly inexpensive drug, if given to high-risk people with COVID right away, can reduce serious outcomes by more than half.

"Metformin is an incredibly common drug, taken by millions of people all over the world. There is no stigma like with an antidepressant, and it is a lot easier to take than the other two," Lissner said.

According to the researchers, in 2019, metformin was the fourth-most commonly prescribed medication in the US, with over 85 million prescriptions written for over 17 million patients.

The US drug regulator FDA guidelines indicate that metformin should not be given to people with advanced kidney disease, and should be taken with food, or in extended-release formulation, to avoid stomach upset, they said.

Side effects are minor and include loose stools. Metformin is also considered safe for use in pregnant women, the researchers added.