Forgot, you probably know this, but the important part about the Epsom Salts is they dilate the bile duct. The timing of the diarrhea shouldn't affect how well that happens I don't think. It would still dilate just fine. Because, working on logic here, whether or not a person can consciously hold in diarrhea would be a separate mechanical/muscle thing, unrelated to the body's dilation reaction to the Epsom Salts , which would be an uncontrollable chemistry thing.