One way they have of cleaning you out is with Phospho-Soda, as I had done several years ago. However, the Wikipedia article (cited below) seems to suggest that they have discontinued this. Anyway, in case they do that, here is my experience:
I had a sigmoidoscopy several years ago. They gave me Phospho-Soda to clean me out, exactly as described in this Wikipedia article. My experience was that there was nothing particularly unpleasant about it, other than being starvingly faint with hunger by the time of the procedure. (As we started the procedure, the technician kindly gave me a little single-serving bottle of apple juice.)
It goes something like this: You drink the glop they give you, twice, several hours apart, the day before. There is nothing particularly vile about it. Then you stay within arm’s distance from your bathroom the rest of the day and night!. You are also supposed to drink some very large amount of water with it, or at various time intervals.
This produces a case of industrial-strength diarrhea, very quickly. But it’s not painful, like you would expect. No cramps or other discomfort, at least not as I experienced it. You just get the runs.
And I do mean you get the runs! And you DO need to stay right near your bathroom.
At first, it just comes out mud, like you would expect diarrhea to be. But after a while, as you get cleaned out inside, it gets clearer and clearer. After several hours of this, all you have have is a torrent of substantially clear colorless fluid cascading out your rear, like a knocked-over fire hydrant.
This stuff works by causing a major imbalance in your body fluid electrolytes, with the result that water is drawn out of your bloodstream, through the intestinal lining, into the intestines, whereupon it flows down and out the back end. There is a real and serious possibility of getting seriously dehydrated. That’s why you MUST drink plenty of water. It really does clean you out like drano.
My understanding (at the time) was that this could possibly cause some kidney damage, which was occasionally even known to produce total and irreversible kidney failure. The product literature acknowledged this, but claimed that it was VASTLY RARE, out of all the millions and millions of times the product was used this way. If you have any already-existing renal problems, this isn’t the way to go.
I got the impression (just an impression, it didn’t actually say this) that maybe the rare cases of serious kidney damage perhaps happen because the patient didn’t drink enough water.
Anyway, the above-cited Wikipedia article states that Phospho-Soda was recalled a few years ago because of this, with the implication that it isn’t used any more to clean you out. If so, that may be too bad. It was thorough and painless, as opposed to some of the other stuff they use, that others have described up-thread. And once you get all cleaned out, there is no further problem with it – at least in my case, it took several hours (this was the evening before), and then I was able to go to bed comfortably without discomfort, and no “accidents” or having to get up even ten minutes all night.
So, I’ll leave you with that to think about. I guess you won’t know what variety of drano they will give you until you get it.