If you drink phospho soda thing make sure you drink loads of liquid along, don't skimp on the liquid at all or it can damage your kidneys. It also tastes nasty. I made the mistake of taking phospho soda along with apple juice and it ruined Apple juice for me for like 2-3 years. Yucky stuff. Though I think nowadays you can choose pills that give you the shits instead of phospho soda if your doctor gives you that option, take it
Other than that they'll knock you out with anesthesia and you won't feel anything. I had an out of body experience and was pissed off they woke me up other than that my ass was just fine, unlike endoscopy where my throat was sore for days