Some scientists and medical practitioners have suggested that vitamin C may have a role in the treatment of advanced cancer. Scientists know that high concentrations of vitamin C can kill cancer cells, at least in laboratory test tubes. However, studies of its effectiveness in people have been inconclusive. To help interpret future studies, researchers must determine whether the way vitamin C is administered affects the results. Previous studies have used vitamin C given by mouth (oral administration) or by injection into a vein (intravenous administration). The potential effect of vitamin C on cancer is probably greater if more of the vitamin reaches the bloodstream and produces a higher concentration, or “blood level,” of the drug.