Patients who took the drug for a year had lower levels of LDL cholesterol, which can cause plaques that lead to heart disease, the study found.
Researchers from the US said efforts to slow the onset of diabetes may have the added benefit of reducing the risk of heart disease.
In the study, published in the Journal of Clinical Endocrinology & Metabolism, researchers randomised 1,645 people with impaired glucose tolerance to metformin 850mg or placebo twice daily, or intensive lifestyle changes.
These changes saw patients aim to lose 7% of body weight through diet and exercise.
After a year, patients on metformin and the lifestyle programme had fewer LDL cholesterol particles in their blood and more HDL cholesterol, which reduces heart disease risk.
Researchers said the lifestyle changes and metformin treatment both trigger weight loss, which can reduce LDL cholesterol levels. However, it remains unclear how metformin affects cholesterol levels in other ways, particularly how it influences HDL cholesterol levels.
Dr Ronald Goldberg from the University of Miami, Florida said: 'Preventing or slowing the development of diabetes with these treatments also improves the cholesterol and triglyceride profile of a person's blood. Thanks to the added benefits of existing diabetes interventions, we stand a better chance of lowering the risk of heart disease in this patient population.'