Magnesium sulfate / potassium sulfate / sodium sulfate
"The Suprep has been easier all around  than the large volume of liquid in my one and only other colonoscopy prep.  I drank the whole 16 ounces in one steady gulping, and chased with a jolly rancher.  I had to wait 90 minutes for any effect, but then it came.  It was pretty thorough, but the main part was over in 30 minutes, followed by a few trips to the BR, but with ample warning.  My second round at 1 AM went down fine, I also had to wait 60 minutes, but this was about the same, 30 minutes, and I could tell it had done it's job.  I stayed up later and watched a tv show to catch the few remaining BR visits.  All in all, it isn't fun, but so much better than the last time.  I would ask for this next time."