Background: Continuous exposure to millimolar (mM) Vitamin C (AA) in vitro kills cancer cells. For superficial bladder carcinoma (SBC), standard chemotherapy is instillation of Bacillus Calmette-Guerin. The recurrence rate with this therapy is 91%. But high dosage vitamins including AA reduced the recurrence to 41%. Aim: To determine the oral dosage of AA that causes the highest concentration of AA [AA] in the bladder. Method: We conducted a clinical trial of 14 people who took various dosages of AA, and analyzed the [AA] in their urine. Results: AA above 2 g twice a day was not absorbed. But that intake produced a bladder [AA] above 1 mM in all participants. Conclusion: Taking 2 g of AA BID will increase [AA] in the bladder to a level likely to kill cancer cells that cause SBC. Taking that dosage 2 consecutive days a week is likely to reduce the recurrence rate of SBC substantially.