The medication, which is thought to cost around 3p per pill, has been shown to reduce inflammation in the body, a key cause of death among those who become infected with the virus.
Chinese researchers say death rates among those with COVID-19 who had been taking metformin were significantly lower.
They study involved looking at the health records of 104 people who were admitted to hospital between January and March with coronavirus and had been taking metformin prior to becoming ill.
They compared the group to a further 179 people who had not been taking the diabetes drug.
The outcome showed only three people from the metformin group died and 22 in the non-metformin drug group passed away.
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Similar results were replicated in another study carried out in America, which involved more than 6,000 obese, older people with diabetes and who became infected with COVID-19.
Among those people, only 2,000 had already been taking metformin for their diabetes. However, their findings only showed positive benefits among women, lowering the mortality rates in women from 21 to 24 per cent.
The University of Minnesota researchers said although diabetes and obesity increased the risk of death, metformin helped to protect against it.
Dr Christopher Tignanelli, a lead author of the study and a critical care physician, said: “If we can understand the difference between men and women that really drove this that gives us a key treatment avenue that we can really go after.”