I used to have a problem with Mg, but not Vit C. Eventually looked at it from the other direction -- not that taking Mg caused diarrhea, but what else might cause diarrhea if Mg were not artificially low and causing a balancing constipation. Came up with candida and/or lack of beneficial flora. I did not know then about the candida/Hg connection. I found I could take one Trenev Trio probiotic per day, and the diarrhea stopped. However, it's made with sunflower oil, and soon my sinuses were completely blocked, so I switched to Culturelle. Two a day also kept the diarrhea in check, and I could take the Mg (and Vit C) I wanted. A solution while I work on the bigger problem, candida & Hg.