I unfortunately have had lots of experience with colonoscopies and their
preps.

It is my understanding that both the Phospho-Soda and the Colyte stay in the
GI tract and are not well absorbed.  I was diagnosed with an inflammatory
bowel disease about the same time my four-year-old was weaning and had many
discussions about the drugs being prescribed.  The only warnings I recall
were to do with dehydration because the preps pull water into the intestines.
 The Colyte goes straight through, literally. The prep is exhausting:  clear
liquids and diarrhea for 24 hours, at least.
The IV meds used here during the colonoscopy are usually Fentanyl and Versed.
 Fentanyl is used for preemie babies. I can't imagine that either of these
drugs would be reason to temporarily wean.
I know we have a gastroenterologist on lactnet, I hope she responds to your
question.
Since colonoscopy is not a routine procedure, I suppose these women are
having some significant symptoms.  If either one is diagnosed with an
inflammatory bowel disease--Crohn's or ulcerative colitis--I have references
concerning pregnancy, lactation and drug treatments.  Let me know if I can
help.

Elizabeth Novelo Puzar, IBCLC