The main purpose of this study is to assess the efficacy of metformin in abrogating androgen deprivation therapy (ADT) induced insulin resistance as measured by homeostasis model assessment (HOMAIR) in men with non-metastatic prostate cancer.

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