Polyethylene glycol 3350 with electrolytes
"I've been taking this stuff for two hours and NOTHING.  I can't drink this much liquid in a 2 to 4 hour period.  It takes me hours to drink a 16 oz bottle of water.  At least right now, it isn't making me sick.  That remains to be seen.  And I've only taken half of what I'm supposed to.  The flavor is disgusting.  If you must use this, I suggest keeping a bottle of water near.  As soon as you finish a glass, rinse your mouth out immediately, so the taste doesn't linger.  If this doesn't work, then I don't know what the gastroenterologist will do tomorrow.  Probably send me home, telling me I didn't follow their instructions."