Now Foods Magnesium Ascorbate Powder, 8-Ounce
Magnesium Ascorbate is a buffered (non-acidic) form of Vitamin C that will not contribute to gastric irritation in acid-sensitive persons.  Magnesium Ascorbate is synthesized from a combination of Ascorbic Acid and Magnesium to form Magnesium Ascorbate.  From the FDA's website: "Some scientific evidence suggests that consumption of antioxidant vitamins may reduce the risk of certain forms of cancer. However, FDA has determined that this evidence is limited and not conclusive."