The Rambam says [Shabbos 2/1]:
The [laws of] the Shabbos are suspended1 in the face of a danger to life,2 as are [the obligations of] the other mitzvos. Therefore, we may perform - according to the directives of a professional physician of that locale3 - everything that is necessary for the benefit of a sick person whose life is in danger.
When there is a doubt whether or not the Shabbos laws must be violated on a person's behalf, one should violate the Shabbos laws on his behalf, for the Shabbos laws are suspended even when there is merely a question of danger to a person's life. [The same principles apply] when one physician says the Shabbos laws should be violated on a person's behalf and another physician states that this is not necessary.
[Note: Spell check doesn't like the way I spelled Shabbos. But the alternatives they offered me were pretty shabby - "shabby" "shabbiest" "shabbier" "shabbily". So I went with "Shabbos"]
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1] The Rambam uses the expression דחויה, which, as evident from Pesachim 77a and Hilchos Bi'as HaMikdash 4:15, implies that although a prohibition is not enforced, it has not been completely lifted. In contrast, the term הותרה implies that no trace of the prohibition remains.
The Kessef Mishneh and other sources question the implications and the appropriateness of the Rambam's choice of words. [In a teshuva (Vol. 1, 689), the Rashba illustrates the difference between these terms. A person was in need of meat on Shabbos and there was non-kosher meat available. If the Shabbos laws are דחויה, it would be correct to eat the non-kosher meat. If the Shabbos laws are הותרה, it would be preferable to slaughter a kosher animal on the sick person's behalf. The Shulchan Aruch (Orach Chayim 328:14) states that, in such a situation, one should slaughter a kosher animal, implying that the laws are הותרה. (Note, however, the Shulchan Aruch HaRav 328:16 and the Mishnah Brurah 328:39.)]
See the Sochchover in his Avnei Nezer, Orach Chayim 455, who explains the concept of הותרה as meaning that, with regard to this person, it is as if the laws of the Shabbos were never commanded. (See also Chiddushim UVeurim BaShas, Vol. 3, which explains that although the Rambam stated that a threat to Jewish life overrides the observance of all the mitzvos of the Torah in Hilchot Yesodei Torah, Chapter 5, he emphasizes this concept with regard to the Shabbos laws for the following reason. The other prohibitions are דחויה in the face of a threat to life, while the Sabbath laws are הותרה.
2] Yoma 85b uses the expression, "Violate one Shabbos on his behalf, so that he will be able to observe many Shabbosos [in the future]." This expression, however, is not halachically exact. Even when one knows that the person will not live to observe many other Sabbaths, as long as he is alive we are obligated to be מחלל שבת on his behalf.
3] Rav Kapach explains that the Rambam implies that we may rely on a local physician and need not seek the advice of a greater expert who lives further away.
[Based on Rav Eliyahu Touger chabad.org]
What would be the halacha if one doctor said that he should definitely NOT eat? The other doctor said that he MIGHT have to eat. The doctor who says that he need not eat is ודאי while the doctor who said that he might have to eat is ספק. To whom do we listen?
Apparently, he should EAT!! ספק נפשות להקל!!! Similarily said the Maggid Mishna [שביתת עשור ב ח] in the name of the Yerushalmi that if the ill person said "I can fast!!" and the doctor said "I don't know if you can fast" we tell him him eat. So the ספק of the doctor trumps his ודאי [so paskened the Shulchan Aruch 518/5].
But the Aruch Hashulchan tries to prove that the Rambam doesn't pasken like the Yerushalmi. For he rules [שם] that if the ill person said that he need not eat while an EXPERT doctor said that he needs to eat then we allow him to eat. Implying that if the doctor who says that he needs to eat is a non-expert doctor then we DON'T allow him to eat. Why not?? What a non-expert doctor says should create a ספק and we should allow him to eat. So it seems that if it is only a ספק that he needs to eat, we don't permit him to eat. That would seem to contradict the Yerushalmi which says that if there is a ספק then he eats.
But see what the אורה ושמחה says about this:
רופא אמר לחלל עליו וכו'. לכאורה מוכח דאפילו זה הרופא האומר לחלל עליו את השבת אינו אומר אלא מכח הספק מפני שאינו יודע ודאי שאינו צריך והשני אומר ודאי אינו צריך ג"כ צריך לחלל עליו את השבת, וכה"ג איתא במגיד משנה בפ"ב מהל' שביתת עשור (ה"ח) בשם הירושלמי "חולה אומר יכולני לצום והרופא אומר איני יודע נעשה ספק נפשות וספק נפשות להקל", הובא גם להלכה בשו"ע (או"ח סי' תרי"ח ס"ה יעו"ש). אבל ראיתי בערוך השולחן (סי' תרי"ח סעי' ז' ח') שהכריח שהר"מ לא פוסק כהירושלמי מדכתב בהל' שביתת עשור (שם) שאם החולה אומר איני צריך והרופא הבקי אומר צריך מאכילין אותו, ומינה משמע שאם הרופא אינו בקי שומעין לחולה, ואם ס"ל כהירושלמי לא יהיה גרע הודאי של האינו בקי כספקו של הבקי עיי"ש, ולפי"ז צריך לומר דדוקא כשהרופאים חולקים זה בזה אזלינן לקולא אבל כשאחד אומר ברי שאי"צ לחלל והשני אומר שמא אין מחללין עליו את השבת, ומ"מ יש לחלק דשאני התם שלב יודע מרת נפשו והודאי של החולה קובע יותר מן הספק של הרופא, אבל בב' רופאים אין הודאי של הרופא קובע נגד הספק של הרופא השני, וצ"ע. אמנם גוף דברי הערוה"ש יש להם פירכא ואין שום הכרח שהר"מ לא פוסק כהירושלמי, דשפיר יש לומר דגם הר"מ ס"ל להלכה כהירושלמי ששומעין לרופא אפילו כשהרופא מסתפק והחולה אומר ברי כיון שהרופא הוא רופא בקי, ואעפי"כ ס"ל שרופא שאינו בקי גרע טפי ואין חוות דעתו כלום כי דבריו של רופא שאינו בקי לאו דברים הוא כלל וודאי שלו אינו עושה אפילו ספק נגד דברי החולה עצמו, ודו"ק.
