Australian scientists have helped unravel a decades-old mystery about how the most common diabetes drug works.
The drug is metformin, the active ingredient in more than a dozen medicines taken by hundreds of thousands of Australians with type 2 diabetes.
Until now, no one has been able to explain how it lowers blood sugar, say researchers at St Vincent's Institute of Medical Research in Melbourne, who worked in collaboration with colleagues at McMaster University in Canada.
The answer is that it reduces fat in the liver, according to their research published in the journal Nature Medicine.
This work has the potential to help develop more effective treatments for type 2 diabetes, which affects four per cent of Australians, says St Vincent's researcher Professor Bruce Kemp.
Many people taking metformin have a fatty liver, which is frequently caused by obesity, he says.
"Our study indicates that metformin doesn't directly reduce sugar metabolism, as previously suspected, but instead reduces fat in the liver, which in turn allows insulin to work effectively."
The research is important, although it will not have immediate benefits for patients, says Associate Professor Jonathan Shaw of the Baker IDI Heart and Diabetes Institute.
"This is an interesting paper which will help in our understanding of metformin."
Prof Shaw says it not common for scientists to not fully understand drugs before they are prescribed, but it isn't unique to metformin.
"Drugs that have their clinical role well established can certainly be prescribed, have their effects and side-effects known, but still not have their mechanism properly understood."