The GI effects of iron pills were profound for me. Feosol triggered such awful diarrhea, I felt sick and couldn't leave the house as it was nearly continuous. I actually started to lose weight. After four days, I had to stop, hoping to also stop the diarrhea, but instead, the side effects of the iron somehow interfaced with my gallbladder-less system and produced bile salt diarrhea.
For three weeks straight.
I had to resort to cholestryamine powder to get things under control.
I had't needed it since the surgery to take out my gallbladder, but thanks to iron pills, I had to take it for weeks.
When everything seemed to go back to normal, I tried Slow Fe which was constipating.
Then I ate some popcorn, which didn't go over so well. My system immediately had a negative reaction to it.
Within 40 hours or so, I had a fever along with severe abdominal pain and was in the ER wondering if I had appendicitis (the pain referred to the right).
And that is how I developed diverticulitis.
Without iron pills, knowing how bad my gut felt on them, I firmly believe I would have never gotten sick.