Metformin in Preventing Androgen Deprivation Therapy Induced Insulin Resistance and Metabolic Syndrome

Inclusion Criteria: * ≥ 18 years of age * ECOG ≤ 1 * Histologically confirmed prostate cancer of early or locally advanced stage, or metastatic prostate cancer with bone involvement only (≤ 5 sites of bone metastases only) * Plan to receive ≥ 6 months continuous androgen deprivation therapy by a GnRH agonist * Patients who are to receive antiandrogens with GnRH agonists are not excluded from the study. But the form, dose and duration of antiandrogen treatment should be recorded. * Adequate renal function (Creatinine ≤ 177mMol/L and GFR \>30 mls/min ) * Adequate hepatic function (Bilirubin must be ≤ 1.5 x upper limit of normal range, ALT and ALP must be ≤ 2.5 x upper limit of normal) Exclusion Criteria: * Visceral involvement * \> 5 sites of bone metastases * History of confirmed diabetes mellitus (patients with impaired fasting glucose or impaired glucose intolerance will not be excluded) 12 * Treatment with medications that may alter glucose or insulin level within 3 months (including insulin, oral hypoglycemic agents, systemic corticosteroid of any dosage, atypical antipsychotic drugs of any dose) * Malignant disease other than prostate cancer at the time of enrolment * Bilateral orchiectomy * Previous androgen deprivation therapy by a GnRH agonist or anti-androgen within last 12 months(patient who had a GnRH agonist more than 12 months ago are allowed if their testosterone levels are in the normal range at the time of recruitment) * Chemotherapy within 6 months * History of lactic acidosis * Cardiac or respiratory insufficiency, severe infection that is likely to increase the risk of lactic acidosis * Medical or psychiatric conditions that compromise the patient's ability to give informed consent