Shabbos, Chapter 14, Mishnah 3. This mishnah deals with the laws of taking medicine on Shabbos.
At the foundation of the matter stands a rabbinic prohibition: the Sages forbade taking medicine under ordinary circumstances, out of concern that it would lead to the grinding of ingredients, that is, crushing the medicines or their components. The symbol of the pharmacist is the mortar and pestle, since that is how medicines were prepared in ancient times. For this reason one does not take medicines on Shabbos, lest a person come to grind various ingredients and transgress the melachah of grinding, one of the thirty nine melachos.
The wording of the mishnah and its explanation:
"Ein ochlin eizovon" - one may not eat a certain type of herb, which people would take in order to rid themselves of a kind of worm found in the digestive system. Even though it is swallowed in the normal manner of eating, the mishnah forbade it with the reason "lefi she'eino ma'achal beri'im" - it is not something eaten by healthy people. From here it may be inferred that if it were a food regularly eaten by healthy people, it would be permitted even if it has a medicinal benefit. For something considered the food of healthy people has the status of an ordinary food in every respect, and even if it has a medicinal benefit, it does not look like taking medicine, and therefore the Sages did not decree against it.
"Aval ochel hu es hayotzer" - one is permitted to eat a certain type of plant called yotzer, which was used to rid oneself of a kind of worm found in the liver, because healthy people also eat it.
"V'shoseh abuv ro'eh" - one is permitted to drink a certain beverage in which various plants were steeped. This drink was used by someone who had drunk uncovered water, out of concern that a snake had injected venom into it, but since healthy people would also drink it, taking it on Shabbos was permitted.
The rule and its exceptions:
"Kol ha'ochalin ochel adam lirefuah v'chol hamashkin shoseh" - any ordinary food may be eaten even for medicinal purposes, and any ordinary beverage may be drunk, because these are ordinary foods and drinks.
And two items were excluded from this rule: "chutz mimei dekalim v'chos ikarin":
Mei dekalim - certain waters that flow from a spring located between two identified palm trees.
Kos ikarin - a beverage in which certain roots were steeped.
Even though healthy people sometimes took them as well, these two were forbidden because they are taken to cure jaundice. Since it is clearly evident that the person is suffering from jaundice and is being cured of it through them, it is obvious to everyone that taking them is for a medicinal benefit.
The mishnah continues: one may not drink manure water that was prepared in order to heal. However, that very same manure water is permitted to drink when a person drinks it merely to quench his thirst, since he is not taking it for a medicinal purpose and it is not evident that it has a medicinal benefit.
"V'sach shemen vered" - a person is permitted to anoint himself with a certain oil in which roots were steeped. "Shelo lirefuah" - when he takes it not for a medicinal purpose, since his intent is not for healing, it is permitted on Shabbos.