Almost six decades after its discovery, metformin still remains the gold standard drug for the management of type 2 diabetes mellitus, owing to its euglycemic property and cost-effectiveness. It is the most commonly prescribed and utilised oral antidiabetic drug worldwide. However, no two persons with the same phenotypical characteristics, such as age, gender, body weight and others, tend to respond similarly to equivalent doses of metformin. As such, the book first focuses on the various genetic polymorphisms that influence the therapeutic effects of metformin. Physicochemical, pharmacological, and pharmacokinetic properties of metformin will also be reviewed. The side effects of this drug can be dangerous. Metformin-induced lactic acidosis is a lethal disorder. The first significant deviation is gastrointestinal tract involvement. Common infection or the effect of metformin itself causes vomiting and diarrhoea, with subsequent hypovolemia causing pre-renal acute kidney injury, which in turn raises metformin levels up to toxic values. Development of kidney failure is often unexpected and fast. Following this, the authors explore the differences between healthy and cancer cells and how this may influence the metformin effect on these cells. In conclusion, the molecular mechanisms underlying the anticancer effects of metformin in breast and ovarian cancers are summarised, and the potential anticancer applications of metformin-based combinatorial chemotherapy are discussed.