My daughter has done the Dulcolax/Miralax prep with all 3 of her colonoscopies.......we mix the Miralax with 64 oz. of gatorade (use a flavor you don't like! Or at least one that you can live without..........whatever flavor we've used, by the end of the prep she never wants to drink that flavor gatorade again!), the first time we didn't know any better and she drank it all, the last two times I had her stop drinking after about
2/3 of the way through, when only clear liquid was coming out of her. The notations on her colonoscopies have always read "bowel prep: excellent."
Her GI had explained to me that the reason why they like to use this prep is because the miralax is purely topical, and does not get into your system the way other bowel preps might.
Best wishes and good luck to you!