A three-pence pill already used for patients with type 2 diabetes could be a cheap and effective new treatment for achy knees.

Metformin, which has been around since the 1950s, is taken by millions of people to control blood sugar levels in type 2 diabetes.

Now a trial involving 120 patients is under way to see if it can also treat osteoarthritis of the knee, after studies on mice found it soothed pain and slowed down destruction of the joint.

With osteoarthritis, the drug is thought to work by stimulating the release of an enzyme that protects cartilage against wear and tear, easing inflammation and pain.

Around nine million people in the UK have osteoarthritis, where the protective cartilage within a joint breaks down - leaving bone rubbing on bone, causing pain and problems moving the joint.

It often develops from wear and tear, although other risk factors include being overweight, a family history and sports injuries.

Anti-inflammatory painkillers help but can damage the stomach if used for long periods, while steroid injections to dampen inflammation risk a cortisone flare, where the steroid (cortisone) crystallises inside the joint and triggers more inflammation.

Around 100,000 people a year in the UK have a knee replacement as a result of the condition - major surgery which can leave a foot-long scar, and it can take up to a year to fully recover.

Scientists at Sadat City University in Egypt set up the new clinical trial after earlier studies indicated that the cheap diabetes medication might help with osteoarthritis by inhibiting inflammation, optimising the way the body clears out damaged cells to make way for new, healthier ones and stimulating the release of an enzyme that protects against cartilage wear and tear.

One 2019 study, published in the journal Arthritis Research & Therapy, found that obese patients with osteoarthritis were 70 per cent less likely to need knee replacement surgery if they regularly took metformin than obese volunteers not on the drug.

Another 2020 study in the journal BMJ Open found that mice with early-stage osteoarthritis given metformin in drinking water every day for 12 weeks suffered significantly less cartilage breakdown than those given water alone.

The researchers, from Tianjin Medical University General Hospital in China, concluded that metformin works by stimulating production in the bloodstream of AMP-activated protein kinase, an enzyme that helps to strengthen cartilage and stop it from disintegrating.

In the human trial, patients will be given either 1,000mg daily of metformin plus a 200mg dose of celecoxib, a powerful anti-inflammatory painkiller, or 1,000mg of metformin with a dummy tablet that looks like celecoxib but has no active ingredients.

Patients will be monitored for three months to see if taking the diabetes pill in addition to their normal osteoarthritis medication improves pain and mobility. Results are expected this year.

Philip Conaghan, a professor of musculoskeletal medicine at Leeds University, said: "It looks like there are some potential benefits from metformin in osteoarthritis but the evidence from existing animal and human studies is mixed."

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