Inclusion Criteria:
* Age \> 18 years.
* Patients with primary or recurrent suspectedTa or T1, G1 or G2 (low grade) urothelial carcinoma of the bladder with no suspicion of carcinoma in situ.
* Patients must have at least 1 lesion but no more than 5.
* There must be one index lesion measuring between 0.5 and 1.0 cm in its greatest dimension.
* Bimanual examination immediately following cystoscopyshould be carried out and no mass should be felt.
* Adequate renal function (eGFR \>50 ml/min/1.73m2 according to CKD-EPI). (47)
* Adequate liver function (bilirubin \<1.5 times upper limit of normal, ALAT or ASAT \<2.5 the upper limit of normal).
* Eligible patients must be fully informed of the investigational nature of the study and written signed informed consent must be obtained prior to any study specific investigations.
* Mentally, physically, and geographically able to undergo treatment and follow up.
Exclusion Criteria:
* Patients with positive cytology (suspected for high-grade urothelial carcinoma, TPS 4 or TPS 5) or suspected grade 3 tumours.
* Patients with diabetes mellitus receiving metformin or having received metformin in the past 6 months.
* Patients who have received intravesical treatment (chemotherapy or immunotherapy) within the last 3 months.
* Patients that are currently receiving other anti-cancer therapy.
* Patients with existing urinary tract infection or recurrent severe bacterial cystitis.
* Patients that need to be treated with a transurethral catheter.
* Patients with urogenital tumours with histology other than urothelial carcinoma (i.e. squamous cell or adenocarcinoma) or with urothelial carcinoma involving the upper tract or the prostatic urethra.
* Patients with a history of other primary malignancy (other than squamous or basal cell skin cancers or cone biopsied CIS of the uterine cervix or prostate carcinoma treated curatively with normal PSA values at inclusion) in the last five years.
* Patients with active, uncontrolled impairment of the renal, hepatobiliary, cardiovascular, gastrointestinal, urogenital, neurologic or hematopoietic systems that, in the opinion of the investigator, would predispose to the development of complications from the administration of metformin.
* Patients who are using loop diuretics, cimetidine, ranitidine, cetirizine, trimethoprim, vandetanib, kinidine and/or HIV medication, for which no reasonable alternative is available.
* Women who are pregnant or lactating. Individuals of reproductive potential may not participate unless agreeing to use an effective contraceptive method for themselves and/or their sexual partner.
* Patients with ECOG-WHO performance status of 3 or 4.
* Patients with a known history of alcohol abuse.
* Patients with a known hypersensitivity to metformin.
* Patients who in the investigator's opinion, cannot comply with provisions of the protocol or do not understand the nature of the study.
