Scientists found that metformin, used to treat type 2 diabetes, reduced the proliferation of tumour cells in the laboratory.
It also prevented or delayed resistance to the chemotherapy drug doxorubicin.
Other laboratory experiments and tests on mice showed that metformin reversed protein markers associated with multiple drug resistance (MDR).
This meant the drug might help resistant breast cancers to start responding to treatment again, said the researchers.
Previous work has shown that metformin has an anti-proliferative effect on many kinds of cancer cell, they added.
Multiple-study analyses of diabetic cancer patients treated with metformin have hinted at the drug boosting survival and preventing new tumours.
The team led by Dr Terra Arnason, from the University of Saskatchewan, Canada, wrote in the journal Public Library of Science One: “Our demonstration that metformin can prevent MDR development and re-sensitise MDR cells to chemotherapy in vitro (in the laboratory) provides important medical relevance towards metformin’s potential clinical use against MDR cancers.
“This may be the first indication of a clinical role for metformin in the long-term management of cancer, where individuals may be maintained on oral metformin to extend remission times, or prevent drug resistance from developing.”
The drug is thought to act through a number of pathways involving histones, proteins that package DNA and also play a role in gene regulation.