In says in Shulchan Aruch Yoreh Deah Siman 81 SIf 7 that it's muter to drink chalav isha (the milk of a Woman) as long as it's "pirush" from the body (i.e. it was already extracted and now inside of a vessel). However for a gadol (adult) to nurse from a woman is like nursing from a "sheretz" (a creepy crawler) and is ossur.
Seemingly many poskim understand this as being muter even lichatchila (see Shach Sif-Katan 17 and what the Caf HaChaim brings in Siman 81 Sif-Katan 47). However the Pri Megadim in Sifsei Das Sif-Katan 17 seems to say that it's muter only for a choleh (sick person -- even a choleh shain bo sacana -- one that is not in a dangerous situation in terms of their health). His wording is:
עש״ך ר״ל אפילו חולה שא״ב סכנה ומיהו מההיא דלקמן סי׳ פ״ז ס״ד אין מוכח דשם אסור לבשל בלא אכילה משום מ״ע
We have now many poskim that seem to hold that it's muter lichatchila, and the Pri Megadim that seems to be saying it's muter only for a choleh. Although I can't say with a certainty what the halacha is l'maseh, the majority of the poskim hold it's muter lichatchila. However what is the reasoning behind both sides? Especially that of the Pri Megadim that it's muter only for a choleh.