Gemara

(R. Yochanan): If a sick Yisrael might die, he may not be cured by a Nochri. If he surely will die (if not treated), he may be cured by a Nochri.

Question: Ben Dama, the nephew of R. Yishmael was bitten by a snake. R. Yishmael did not allow Yakov Minah (an avid idolater) to cure him.

Answer: We are more stringent about Minim, lest one be drawn to heresy.

Ben Dama thought that "va'Chai Bahem" permits this (one should not die in order to fulfill Mitzvos). R. Yishmael holds that this does not apply in public.

Yakov Minah applied a potion to the thigh of R. Avahu.

Pesachim 25a (R. Yakov citing R. Yochanan): One may use anything to heal, except for wood of an Asheirah.

Question: If there is [mortal] danger, even wood of an Asheirah should be permitted. If there is no danger, all Isurim should be forbidden!

Answer: Even though there is danger; Asheirah wood is forbidden:

(Beraisa - R. Eliezer) "Uv'Chol Nafshecha" - you must love Hash-m even if it requires forfeiting your life.

Ravin citing R. Yochanan: One may heal through any means, except through idolatry, Arayos or murder.

Sanhedrin 74a (R. Yochanan): Chachamim concluded that if a person is told to transgress or else be killed, he transgresses. There are three exceptions (for which he must forfeit his life), idolatry, Arayos, and murder.

Question (Beraisa - R. Yishmael): If one is told to serve idolatry or be killed, he transgresses. "Va'Chai Bahem" - do not die for Mitzvos.

Suggestion: Perhaps this applies even in public!

Rejection: "V'Lo Sechalelu Es Shem Kodshi v'Nikdashti".

Answer: Chachamim concluded like R. Eliezer.

(Beraisa - R. Eliezer): The Torah needed to say (regarding "v'Ohavta Es Hash-m Elokecha") both "Uv'Chol Nafshecha" and "Uv'Chol Me'odecha" to teach that (even) if one's life is dearer to him than his money, he must love Hash-m with all his life (use of idolatry contradicts love of Hash-m). (Even) if his money is dearer to him than his life, he must love Hash-m with all his money (use all of it to avoid transgressing, if necessary).

Rishonim

Rif (Avodah Zarah 9a): R. Yishmael holds that "Va'Chai Bahem" applies even if one is forced to do idolatry. The Halachah follows R. Eliezer who requires Mesiras Nefesh for idolatry.

Question: R. Yishmael did not allow Yakov Minah to cure Ben Dama from mortal danger, for people are drawn after heresy. When R. Avahu was sick, he used a cure from Yakov Minah. Why was this allowed?

Answer (Ran Pesachim 5b DH b'Chol): Yakov Minah wanted to do for Ben Dama a cure dependent on Idolatry. He used a standard cure for R. Avahu. This supports Tosfos, who says that to the Isur to be healed through an Asheirah is only if the cure requires an Asheirah. However, the Yerushalmi forbids wood of an Asheirah in any case. Even though using such wood is only a Lav ("Lo Yidbak b'Yadcha Me'umah Min ha'Cherem"), one may not transgress any Lav of idolatry, even to save a life. Chachamim hold that the same applies to Arayos and murder. Chachamim did not allow a man mortally sick due to desire for a certain girl even to talk with her in back of a fence (Sanhedrin 75a), due to the Lav "Lo Sikrevu Legalos Ervah."

Ran (DH Im): R. Yishmael explains "b'Chol Levavcha uv'Chol Nafshecha" to mean wholeheartedly.

Question: Love of Hash-m is a basis of all Mitzvos. We should expound uv'Chol Nafshecha to require Mesiras Nefesh for all Mitzvos!

Answer (Ran ibid.): Since it also says "Va'Chai Bahem", it is we apply va'Chai Bahem to all other Mitzvos, and uv'Chol Nafshecha to require Mesiras Nefesh for Idolatry. This is because denial of idolatry is the basis of Ahavas Hash-m and is like acceptance of the entire Torah.

Rambam (Hilchos Yesodei ha'Torah 5:6): The law of Refu'ah is like that of being coerced. If doctors said that there is no permitted cure for a mortally sick person, he transgresses any Mitzvah except for Avodah Zarah, Giluy Arayos and murder. If he transgressed, Beis Din gives him the appropriate punishment.

Rosh (Sanhedrin 7:3): Yoma 82a says that only idolatry, Arayos and murder override Piku'ach Nefesh. It does not cite R. Yishmael and Rava, who exclude idolatry; this shows that the Halachah does not follow them.

Rosh (Avodah Zarah 2:9): One may not be cured by a heretic, even if he is an expert doctor, for he whispers the name of idolatry and one can be drawn after it and come to believe that it is real. The same applies to any Nochri who whispers the name of idolatry, even if he is not a heretic. If one knows that the Nochri whispers, but does not know whether he whispers the name of idolatry, he may be cured. The Isur to be healed through an Asheirah is only if the cure requires an Asheirah. R. Yishmael did not allow Yakov Minah to cure Ben Dama from a snake bite, but R. Avahu was cured from him, i.e. through a potion that did not depend on idolatry. In Avodah Zarah (54a) Rava holds like R. Yishmael, who says that idolatry is not more stringent than other Aveiros. In Kesuvos (19a) Rava holds like Chachamim, who require Mesiras Nefesh for idolatry in every case. It is not clear which of these we should rely on; the Rif and She'altos are stringent, like Chachamim.

Tosfos (27b DH She'ani): We forbid when the cure depends on particular water, e.g. through a Shed. Sometimes Shedim have powers, to deceive (unworthy) people. E.g. there was a house of idolatry that seemed to bring rain (55a).

Poskim

Shulchan Aruch (YD 155:1): If an idolater wants to cure a Yisrael through a Lachash (incantation), it is permitted, unless he knows that he mentions the name of idolatry. Then, it is forbidden even if he will die. A Stam Lachash of an Apikoros is forbidden, for surely he mentions the name of idolatry.

Shach (8): We discuss a Nochri Apikoros. It is forbidden even if he is an established doctor.

Gra (10): Shabbos 75a forbids learning from a Gedufi (Min) even in order to understand and give rulings.

Shulchan Aruch (2): If an idolater came to cure a Yisrael, and he said 'take this water or from tree Ploni, which are of idolatry', it is forbidden. If he said ''take this water or from tree Ploni', and he did not mention idolatry, even if the water or tree are found only of idolatry, it is permitted, since he did not attribute the cure to idolatry. Some forbid this, even if he said Stam 'bring leaves', and they are of idolatry.

Bach (Kuntres Acharon): The Tur forbids when the Nochri says 'you will be cured only through this water.' I think that this is even if he does not mention idolatry, just we infer this in one of two ways. 1) It is a common water or tree, yet the Nochri says to take specific water or from a particular tree, which is of idolatry. 2) This is the only such water or tree around, and he said 'take from this, of idolatry.' It is permitted only if there is no other such water or tree around, and he said only 'take this.' Then, nothing shows that the cure depends on idolatry. The Tur learned this from the Rosh.

Shach (9): The Bach said that he explains unlike the Shulchan Aruch and Drishah (3). I say that they agree with him.

Taz (3): The Tur teaches that if he merely mentions the idolatry to say where the water is found, it is permitted, unless he says that only this water will cure. If he specifies water that is found only of idolatry, it is as if he specified that only idolatry will cure, so it is forbidden.

Shulchan Aruch (157:1): Regarding idolatry, Arayos and murder, even in private when there is no decree, even if the Nochri intends for his own benefit, one must be Moser Nefesh.

Rema: If one was obligated to be Moser Nefesh, but transgressed, he was Mechalel Hash-m. Nevertheless, he is exempt (from punishment).

Gra (18): This is like the first answer in Tosfos (Sanhedrin 61b). The second answer says that (in a usual case) one who transgressed is liable.