The drink tends not to be as bad as bowel prep. It's just some clear contrast, and it tastes like liquorice or orange, depending on the contrast, if my memory is correct.
The injection might be Buscopan, which is a pain-relieving med that also slows down the gut, which will make it easier for them to get the images they need.
The drink can have mild laxative properties, so it might be a good idea to take a change of undies, just in case. However, the Buscopan slows things down internally, so my feeling is that you will probably be ok afterwards
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Good luck with it. I don't mind MRIs: they're much more relaxing than many of the other tests we do. I often doze off during mine
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Ivy.