In the seven years after the trial ended, both the drug and placebo groups - now also eating more carefully and exercising - saw the rate of diabetes fall.
But the most significant drop was among those who had started out on a diet and exercise regime - their risk was over a third lower than the placebo group.
In an accompanying editorial, Dr Anoop Misra, a specialist in diabetes in New Delhi, described the prevention of the disease as "a long and winding road".
'No short cut'
Dr Misra said: "There seems to be no short cut, and a persistent and prolonged intensive lifestyle intervention seems to be the most effective way to travel on it."
But he warned it could not be the only measure: "We need more effective drugs for those who cannot follow intensive lifestyle therapy because of infirmity."
Type 2 diabetes usually appears in people over the age of 40, however increasing numbers of children are being diagnosed with the condition, some as young as seven.
Although obesity is a risk factor, not all people with type 2 diabetes are overweight.
Dr Iain Frame, head of research at Diabetes UK, said: "It is fascinating to read about the 10-year follow up studies and of the importance of lifestyle interventions, with or without metformin, in the prevention of Type 2 diabetes even after 10 years.
"There is clearly no easy route to take to prevent Type 2 diabetes but indications are that with further research into the long-term benefits of good dietary advice, physical activity and, where necessary drug therapies, we may be a step closer into helping people at high risk of developing Type 2 diabetes modify their lifestyle choices that are sustainable in the longer term."