Metformin is a commonly-prescribed drug, used as first-line medical management of type 2 diabetes mellitus (T2DM), but also off-license in non-diabetics for Polycystic Ovary Syndrome (PCOS). Over 120 million people worldwide are prescribed metformin. Despite this, its mechanism of action is not fully understood. Both tolerance of and response to metformin varies greatly from patient to patient, and highlights the need for further research into the pharmacokinetics and dynamics of the drug.