Hi Charles, simillar to the chap belows situation, I am a bodybuilder and have started taking calcium magnesium and zinc suppliment tablets, along with Brewers yeast B12, multivits which include iron and zinc and glucosamine sulphate, creatine and L-glutamine, plus tri-carbs and protein shakes...my stool has gone black and tar like in the past couple iof 2weeks, when for the last few days I stopped everything it has today cleared up...just want to know whats causing this is it the magnesium?