Magnesium hydroxide
"Took at 9 am hoping to move bowels the same day, after days of horrible constipation and bloating.  MoM claims it works within 6 hours, but nothing happened.  I took another dose at bedtime, still without any movement.  The following morning I have awful diarrhea.  It's so bad it gave me an anal fissure and I've been taking Imodium and putting on lidocaine to ease the pain.  Ridiculous to go from one extreme to the other. Yes I should not have taken a second dose, but thought it would be fine since it did not work at all after 14 hours, and supposedly you can take this daily.  I thought this would just quicken transit time, but no, it stayed it my gut, mushed everything to liquid, and gave me the worst diarrhea ever. Never taking again!"