Diabetes and tumour diseases are serious health problems in the elderly population. Several studies have suggested that patients with diabetes are at significantly higher risk of cancer. On the other hand, epidemiologic evidence suggests that some drugs used to treat hyperglycemia are associated with reduced risk of cancer. Metformin, the drug applied in type 2 diabetes, has been shown to reduce incidence and mortality due to malignant tumours in various localities. A hypothesis is presented which seeks to explain the anticancer effects of metformin. Empirical data from the scientific literature is used to support the hypothesis that metformin restricts the growth of human cancer stem cells (CSCs). The LKB1/AMPK (liver kinase B1/AMP activated protein kinase) mechanism of the antineoplastic action is pleiotropic, involving the activation of the LKB1/AMP pathway, which inhibits mTOR signalling, inhibits STAT-3 phosphorylation, arrests cell cycle at the G1 phase, inhibits cyclin D1 activity, reduces insulin levels, as well as IGF-1, TNFα, and IL-6 levels which reduce proliferation leading to apoptosis of neoplastic cells. In various tumours, including gynaecological cancers, metformin eliminates cancer stem cells (CSCs). Repositioning metformin as a possible antineoplastic option seems promising. The present authors suggest here that metformin could be a valuable tool for clinicians to help reduce cancer risk in elderly adults with diabetes.