Not sure I can tolerate prep.

My colonoscopy is on the 7th of Nov, and I honestly don't think I can tolerate the Suprep prescribed by my Dr. I'm thinking of drinking a couple of those bottles of the magnesium citrate they sell over the counter instead..I know that it also doesn't taste great, but if memory serves me, it was more tolerable then the other large volume preps. I'm going to pick up the Suprep but in case I just cannot keep it down, which is very probable, I want to have a back up. Has anyone done this?