I've had one, and it hasn't lasted in my memory as being particularly grim.
What has lasted in my memory though is the -er - after-effects.
Once I'd had the scan (or whatever it was), they showed me to a small toilet and told me to poop it all out before I went home.
Although I consider myself relatively well-balanced, pooping with attendant sound effects in a place where everyone can hear me is high on my list of things I can't cope with. So I only achieved about 50%, before rushing home to my own loo and bliss.
The problem was that whatever the 50% I saved for home was, it sat for years at the bottom of the toilet bowl column, defying all attempts at removal.
So my advice would be: Stick your fingers in your ears, think of England, and when they tell you to poop, Poop!
Good luck with the test and the results