Inclusion Criteria:  To be eligible to participate in this trial, patients must satisfy all of the following inclusion criteria:  Willing to participate and provide informed consent Aged 18-70 years Diagnosis of ADPKD based on radiological +/- genetic criteria as per Kidney Health Australia - Caring for Australians and New Zealanders with Kidney Impairment (KHA-CARI) Guidelines eGFR equal to or greater than 45 mL/min/1.73m2 and <90 mL/min/1.73m2  And have either:  One or more risk factors of progression from the following:  Bilateral kidney length equal to or greater than16.5 cm Total Kidney Volume (TKV) equal to or greater than 750 mL or height-adjusted TKV (htTKV) equal to or greater than 600 mL/m2 Mayo class IC/D/E or Pro-PKD score equal to or greater than 6 OR  Evidence of Active progression  Decline in eGFR equal to or greater than 5 mL/min/1.73m2 in one year Decline in eGFR equal to or greater than 2.5 mL/min/1.73m2 per year over five years or more Increase in htTKV/TKV of equal to or greater than 5% per year on at least 2 measurements in the past year, excluding any initial eGFR effect over the initial 3 months of tolvaptan commencement (if applicable) Note: Tolvaptan therapy must have been in place for at least 6 months with stable dose for at least 3 months.  Exclusion Criteria:  Diabetes mellitus (as per American Diabetes Association definition), or other systemic conditions that may cause CKD independent of PKD (excluding hypertension) Uncontrolled hypertension (Systolic BP >160 mmHg and/or diastolic BP >100 mmHg after a period of rest) Clinically significant heart failure, including but not limited to New York Heart Association Class (NYHA) III or IV  Non-polycystic liver disease, including but not limited to:  Liver enzymes (ALT, AST or Total Bilirubin) >2 times the upper limit of normal, except when a diagnosis of Gilbert Syndrome exists and/or, Child-Pugh classification score equal to or greater than 5 Any contraindication to metformin including abnormal liver function tests or untreated Vitamin B12 deficiency Currently taking metformin Pregnancy or breastfeeding, or planning to get pregnant in the next three years. Comorbidities with potential to contaminate trial outcomes, specifically active cancer, history of other solid organ transplantations, chronic obstructive pulmonary disease (COPD), inflammatory bowel disease, and the presence of stoma. History of dialysis.