Inclusion Criteria:

(Eligible subjects must meet one of the inclusion criteria 1-3 and all of items 4-6)

1. Men with metastatic prostate cancer that require castration therapy with either using
an LHRH analogue or surgical castration are eligible. Complete androgen blockade
using anti-androgen therapy prior to castration or up to 4 weeks following castration
therapy is permitted to prevent disease flare. Thereafter anti-androgen therapy may
continue or be discontinued based on treating physicians preference.

OR

2. Any men with prostate cancer who are candidates for castration therapy despite no
evidence of definite metastatic disease including patient with biochemical failure or
'rising PSA' are also permitted to enter study provided castration therapy is planned
for a minimum of a year. Patients with biochemical failure prior to enrolment should
have also have already received appropriate salvage therapy. Men with prostate cancer
who have already started castration therapy are also permitted to enter study
provided castration therapy was initiated within one month of study entry.

OR

3. Men with prostate cancer previously treated with castration therapy for management of
localized prostate cancer in the adjuvant setting or in combination with radiation
therapy are permitted to enter study provided they currently have known metastatic
disease and have non-castrate testosterone levels (Testosterone > 50 ng1dL).

4. An ECOG performance status of 0-2.

5. Patients will need to have documentation of metastatic disease in bone and/or soft
tissue, and a baseline PSA of 35 nglml. Patients with biochemical failures, with
rising PSA (baseline PSA does not need to be 2 5 nglml to be eligible), without
metastatic disease are also eligible if castration therapy is indicated for a minimum
of a year.

6. Patients must have provided informed consent, be willing to have blood specimens
taken, and exhibit no severe other medical or psychiatric problems.

Exclusion Criteria:

1. Patients with severe medical or psychiatric diseases are INELIGIBLE. (Patients with
stable chronic diseases such as high cholesterol or hypertension ARE eligible.)
Examples of problems that would make patients INELIGIBLE include severe heart
failure, or hypoxia due to severe lung disease.

2. Patients with clinical or biochemical evidence of renal failure or liver failure are
INELIGIBLE. Creatinine and bilirubin needs to be less than or equal to 1.3~up per
limit of normal (ULN), and ASTIALT less than or equal to 2.5 x ULN unless liver
metastasis is present then up to 5 X ULN permitted).

3. Patients already receiving metformin or anti-diabetic medications are INELIGIBLE.

4. If any patient develops symptomatic diabetes requiring drug therapy, he must receive
such a therapy, which may include metformin. This must be documented, and the patient
will not continue on the study.

5. Patients with important infections requiring antibiotics are INELIGIBLE, but patients
who acquire minor infections while on the study may remain on the study.

6. Alcohol abuse problems make patients INELIGIBLE. Patients need to be consuming less
than or equal to 14 units of alcohol weekly.

7. Patients with history or evidence of lactic acidosis or metabolic acidosis will be
excluded.