High-dose intravenous vitamin C has previously been shown to selectively kill cancer cells by increasing hydrogen peroxide and now it has been found to potently kill cancer stem cells. The new study screened a variety of compounds in order to find those that were most potent at killing cancer stem cells, which are the most resistant to any type of cancer treatment. Interestingly, vitamin C was found to be 10 times more potent at killing cancer stem cells (cultured in a dish) than 2-deoxyglucose, a drug that prevents cancer cells from using glucose via glycolysis. While this study was done in a culture dish, other studies have been done in animals and humans. It is important to realize that intravenous vitamin C increases blood levels that are 100-500 times higher than levels that can be achieved with oral ingestion. It is this VERY-high concentration of vitamin C in the blood that results in the generation of hydrogen peroxide that selectively kills cancer cells because normal cells effectively remove it but cancer cells cannot. Also, early phase 1 clinical trials showed that the combination of high-dose, intravenous vitamin C with standard chemotherapy or radiation was well tolerated and improved patient outcome. Larger clinical trials are now underway.