Gemara

R. Chanina permitted to drink cress on Shabbos.

Objection: This is obvious!

Mishnah: On Shabbos one may eat any food or drink any drink for a cure.

Correction: Rather, he permitted to grate and drink cress on Shabbos.

Question: What is the case?

If there is (mortal) danger, obviously it is permitted. If there is no danger, it is forbidden!

Answer: The case is, there is danger. The question was whether or not it really cures. One may be Mechalel Shabbos only for genuine cures.

Shabbos 111a - Mishnah: If one's teeth hurt, he may not Yigma (ingest) vinegar on Shabbos. He may eat vegetables dipped in vinegar; it is no problem if he gets healed.

Contradiction (Beraisa): He may not Yigma (put in his mouth) vinegar and spit it out, but he may ingest and swallow.

Resolution (Abaye): Also the Mishnah forbids (only) to put in his mouth and spit out.

Rishonim

The Rif (Shabbos 40b) and Rosh (Shabbos 14:8,10) bring the Heterim to eat foods and drinks for a cure, the Heter to swallow vinegar, and the Isur to put vinegar in one's mouth and spit it out.

Rambam (Hilchos Shabbos 21:20,21,22): A healthy person may not heal himself on Shabbos lest he come to grind ingredients (to make medicine). One may not eat or drink things that healthy people do not eat or drink, nor things that cause diarrhea. On Shabbos one may eat (or drink) any food or drink that healthy people eat, even if it heals and this is why he eats it. If one drank Chiltis (a plant whose leaves are used as a spice; it is soaked and used for Refu'ah) before Shabbos, he may continue to drink it on Shabbos, even in places where healthy people do not drink it.

Magid Mishnah: He may continue to drink Chiltis because if he will stop he will get sick.

Poskim

Shulchan Aruch (OC 328:37): One may eat (or drink) any food or drink that healthy people eat, even if it harmful for some things (text of Bach, Magen Avraham, Gra) and it is evident that he eats for Refu'ah.

Kaf ha'Chayim (213,214): Many learn from here that one may inhale dust of tobacco smoke. Others forbid this. One may smell oregano to help a runny nose, for we are not concerned for smells.

Kaf ha'Chayim (218): Some say that if a medicine needs to be cooked one may cook it before Shabbos and take it on Shabbos, for one will not forget and come to cook on Shabbos. This is wrong. All Refu'os were forbidden lest one grind spices. The Rambam permits only when one started the medicine before Shabbos and ceasing on Shabbos will make him sick.

Shulchan Aruch (ibid.): Anything that healthy people do not eat is forbidden. This is only if he has a slight pain but he is primarily healthy. If he has no pain it is permitted.

Source (Beis Yosef DH Kol): (Refu'ah was forbidden lest someone grind ingredients to make medicine.) If someone is healthy, there is no concern.

Rebuttal (Magen Avraham 43): The Tur permits to drink for thirst. This implies that one may not drink for a cure even if he is healthy. Even a healthy person may not bathe in bad water (Yerushalmi, and Shulchan Aruch 328:44) or open one's eye in salty water (Shabbos 108b) or stand in the mud near the Deyumeses River (Shabbos 147b) because it is clearly for Refu'ah (to become stronger). The Gemara (Berachos 38a) asks why one may drink Zeisum ha'Mitzri; it does not answer that this is for a healthy person. The Rambam and Tosfos do not allow a healthy person to intend for Refu'ah.

Defense (Mishnah Berurah 119 and Bi'ur Halachah DH Aval): If one has a slight pain but he is primarily healthy, he may not eat these foods even for hunger. We are concerned lest people say that he ate them for Refu'ah. This is like Rashi. Many disagree and permit a Choleh to eat them for hunger. The Rambam (21:20) forbids a healthy person to eat them for Refu'ah, he refers to someone with a slight pain. It is not clear whom the Halachah follows.

Kaf ha'Chayim (219): Shitah Mekubetzes (Berachos 38a DH v'Shavin) permits a mixture of two foods, even though healthy people do not eat this. This is when healthy people eat each food by itself, but they do not eat them mixed.

Rema: Similarly, if he is Nofel l'Mishkav (so sick that he needs to lie down) it is permitted.

Source (Gra DH v'Chen): We are more lenient for a Nofel l'Mishkav than for a healthy person, e.g. he may tell Nochrim to do Melachah for his needs.

Beis Yosef (ibid.): Since his needs may be done only through Nochrim, we are not concerned lest a Yisrael grind ingredients for him.

Rebuttal (of Rema and Beis Yosef - Taz 25): Needs of a Choleh who is not dangerously sick may be done by a Nochri. The Rema himself (328:17) allows the Choleh to help only in an insignificant way (the Nochri could have done everything himself). Swallowing medicine is the primary action, and the Yisrael does it by himself!

Bach (DH Kol): The Beis Yosef (DH u'Mah she'Chosav Rabeinu veha'Ramban, near (9) in Darchei Moshe) does not allow a Choleh to pick up the medicine. The Nochri must do everything, the Yisrael can help only in an insignificant way, like Ravina who opened and closed his eyes to enable the Nochri to paint them. This is unreasonable. Ingesting the medicine is not insignificant; it is the primary action, and it is forbidden mid'Rabanan! The Rema holds like the Beis Yosef, and permits a Nofel l'Mishkav only if the Nochri feeds him the medicine.

Rebuttal (Magen Avraham 44): The Bach learns from Ravina that the Choleh may transgress Shevus to cure himself, but others may not. This is wrong. Ravina was not Nofel l'Mishkav.

Mishnah Berurah (121): The Rema permits a Nofel l'Mishkav to take medicine himself (unlike the Bach explains). The Halachah allows a Yisrael to do Shevus with an action for such a Choleh only through a Shinuy (328:17). This refers only to things that resemble Melachah. Refu'ah does not resemble Melachah. It is permitted to healthy people.