They have a nasty side effect of reducing gastric emptying which makes the "fast for 6hrs prior to an Op" questionable.
As they are dosed 1/week, they hang around a long time - the effect lingers for up to 4 times that. Currently no one knows what the real answer is, so the recommendations are likely to change as these drugs are better understood.
Another recommendation is to fast from solid food for 2 days but can drink water or clear fluids - sort of like the colonoscopy prep period without the bowel prep.
