Taking a common diabetes medication after testing positive for coronavirus may reduce the risk of obese and overweight people developing long Covid by 40%, new research suggests.

The study found that 6.3% of people who took metformin, a drug used to control blood sugar in people with type 2 diabetes, within three days of testing positive for the virus reported a long Covid diagnosis within 10 months.

This was compared with 10.4% of those who were given a placebo instead of the diabetes drug.

According to the researchers, the trial is the first to suggest medication taken during this phase of Covid-19 may be able to reduce the risk of long Covid.

However, the trial did not look at the effect of metformin on those who already had a long Covid, so no conclusions can be drawn about using the drug as a treatment for the condition.

First author Dr Carolyn Bramante, University of Minnesota Medical School in the US, said: “Long Covid is a significant public health emergency that may have lasting physical health, mental health and economic impacts, especially in socioeconomically marginalised groups.

“There is an urgent need to find potential treatments and ways to prevent this disease.

“Our study showed that metformin, a medication that is safe, low cost and widely available, substantially reduces the risk of being diagnosed with long Covid if taken when first infected with the coronavirus.

“This trial does not indicate whether metformin would be effective as a treatment for those who already have long Covid.”

The 1,126 people who took part in the trial were not admitted to hospital, were at a higher risk of severe Covid (overweight or obese), over the age of 30, and had tested positive for the virus within the last three days but had no known previous Covid infection.

The patients were given either metformin or an identical placebo pill, and then followed up for 10 months with data gathered via questionnaire every 30 days.

Co-author David Odde, University of Minnesota biomedical engineer, said: “Previous studies have found that metformin stops the SARS-CoV-2 virus from replicating in the lab, which is consistent with predictions from our mathematical modelling of viral replication, so that might be what is causing the reduction in both severe Covid-19 and long Covid diagnoses seen in this trial.”

The researchers acknowledge some limitations to the study published in The Lancet Infectious Diseases, including that the trial excluded those with a BMI under 25 and those younger than 30.

It is, therefore, unknown if these findings could be generalised to those populations.

However, some experts suggest further research is needed, and there may be other explanations for the findings.

Dr Stephen Lawrence, associate clinical professor, University of Warwick, said: “The researchers have suggested that the observed benefit is attributed to metformin interfering with replication of the Covid virus.

“However, an alternative explanation for the positive outcome following two weeks of treatment with metformin may be that it is addressing a tendency to hyperglycaemia which is considered a risk factor for adverse outcomes following infection with Covid in people with established type two diabetes.

“Poor blood glucose level control is associated with deleterious outcomes following Covid infection.

Ewan Pearson, a professor of diabetic medicine at the University of Dundee, said: “Overall, the study outcomes are convincing, although it should be noted that the number of people developing long Covid was relatively small (n=93) and the study population were largely those with private healthcare and were predominantly white.

“As metformin reduces risk of developing diabetes it is notable that the rate of development of diabetes was not reported; with symptoms of diabetes potentially overlapping with those of long Covid.”

Dr Frances Williams, professor of genetic epidemiology at King’s College London, said: “The authors indicate that treatment with metformin is safe and affordable.

“However, we need to also acknowledge well-established research, citing the importance of monitoring, kidney function and the frequency of gastrointestinal side effects in people taking this medication.

“This study demonstrates useful information supporting the principle of early intervention for people with obesity, who have yet to develop diabetes.”