The first time I did the miralax prep I drank it all and paid big time. I was up all night violently ill. Also, I was severly dehydrated afterward. So much so it took several nurses and several painful attempts to find a vein for the IV.
So, most recently, I only drank 5 of the 8 glasses and I was good! No cramping at all and GI didn't have any complaints. However, I am rather thin (5'5" and under 120lbs), so I figured I was safe in not drinking a glass or two. Why should I have to drink the same amount as someone double my weight? I think the answer is I shouldn't, hence why I was so sick when I did drink it all. In my opinion, colonoscopy prep shouldn't be a one size fits all kind of thing.