I used this as directed by my oncologist. The first bottle did nothing. Halfway through the second bottle it kicked in.

Prior to the development of Go Lytly for colonoscopies, patients were directed to use citrate of magnesia with two Dulcolax. (Do not try that without a doctor's approval). I had my first upper and lower GIs in the 70s and was working in the pharmacy of major chain drug store at the time where I set that same prep up for patients often.