Magnesium citrate
"Much like others, I write this review from the blissful toilet where I have finally found relief. I'm in my second trimester, and have had constipation for the entire pregnancy. Would go a little bit every three days, but never anything compared to what I used to be able to do. This week hit Peak Constipation. I had a blockage that I couldn't pass for six days, and developed constantly bleeding hemorrhoids.  Gained five pounds, couldn't eat, was nauseous, and had taken Miralax for five days in a row. Caved and took Mag Citrate two hours ago and have finally found relief. Can't recommend this any more highly!  (Also, FYI, my doc recommends it to pregnant constipated women who get this bad)"