<<Disclaimer: Verify this information before applying it to your situation.>> Hello All, Large daily doses of iron supplements (1200+ mg daily) can result in loose dark stools and sometimes diarrhea. The more normal daily doses (300 to 900 mg daily) tend to be constipating. Each person's threshold for iron will vary. I find when I'm faithful to my gluten free diet, I can tolerate alot more iron. Cathy Norwalk, CT