People who took a cheap diabetes drug after testing positive for Covid-19 had a 40 percent lower risk of getting long Covid, a US-based study said Friday.

The finding was hailed as a potential "landmark" in the fight against the still little-understood condition, which the World Health Organization estimates affects one in 10 people who get Covid.

The study said it was the first randomised, placebo-controlled phase 3 trial -- considered the gold standard in research -- to show that taking a drug can prevent long Covid.

It tested a drug called metformin, which was originally developed from the French lilac flower, and has been the most common medication used to treat type 2 diabetes across the world for decades.

This means the drug is known to be safe, as well as being inexpensive and widely available.

The study covered 1,126 overweight or obese people in the United States, half receiving metformin and half a placebo in the days after testing positive for Covid.

After 10 months, 35 of the participants who took metformin were diagnosed with long Covid, compared to 58 for the placebo group, representing a 40 percent reduction in risk.

The trial was conducted between December 2020 and January 2022, meaning it included the Omicron variant, which research has suggested causes long Covid at a lower rate than previous strains.

The team behind the COVID-OUT trial had previously shown that metformin reduced coronavirus patients' risk of emergency department visits, hospitalisations and death by more than 40 percent.

Carolyn Bramante, a researcher at the University of Minnesota and the new study's lead author, told AFP that "our data show that metformin reduces the amount of SARS-CoV-2 virus" in patients.

The research was published in the Lancet Infectious Diseases journal.

- 'Profound' -

Jeremy Faust, a doctor at Harvard Medical School not involved the study, said in a linked comment piece that if confirmed, the findings "are profound and potentially landmark" for long Covid.

Frances Williams, an epidemiology professor at King's College London, pointed out that 564 people had to take the drug "to prevent 23 hypothetical cases".

"This means 24 people would need to take metformin to prevent one case of long Covid," she said, adding that this was a lot of drugs to stop such a poorly understood condition.

The researchers cautioned that they did not test metformin on people who had already been diagnosed with long Covid, so the findings did not mean it could be used to treat the condition.

The study also found that the antiparasitic drug ivermectin, which was the subject of misinformation throughout the pandemic, as well as the antidepressant fluvoxamine did not prevent long Covid.

Tens of millions of people have been estimated to have had long Covid, in which numerous and sometimes debilitating symptoms last or recur three months after infection that can then drag on for years.

The most common symptoms include fatigue, breathlessness and a lack of mental clarity called brain fog.