Medicine on Shabbat
One of several rabbinic decrees that our Sages enacted in order to guard the sanctity of Shabbos concerns the use of medications. In the opinion and experience of the Rabbis, easy access to medicine could lead to the transgression of certain Shabbos Labors. While issuing the decree, however, the Rabbis were lenient in certain cases of those suffering pain or distress.  (See the footnote for some background. The details of what's permissible and what's forbidden is described below:
If one has a minor condition
- If someone has a minor condition which hurts such as a tooth ache, throat ache, head ache, cold, and cough it is forbidden to take any medicine such as pills or drops. However, someone who is in a lot of pain and because of it he is in bed or his body is weakened such as a migraine it permitted to take a medicine. 
If one has a serious sickness
- If one has fever, feels weak all over, or feels bad enough to require bed rest, he can be classified as a “patient not dangerously ill” and he is permitted to take oral medications. 
- Since “requiring bed rest” and “weak all over” are subjective terms, it is up to each individual to determine his personal pain threshold. There is no requirement to be overly stringent when judging the degree of illness. 
Cleaning a wound
- It is permitted to clean and bandage a wound or to pour hydrogen peroxide over it. 
Using a band aid
- It is permitted to put on a band aid on a wound. 
- According to Ashkenazim, one should not remove a band aid on Shabbat if there is hair in the area of the band aid. However, if it is painful one may remove it.  However, Sephardim are more lenient as long as there is a need to remove it, it's permissible. 
- Braces may be worn on Shabbos because there is no medicine for aligning teeth properly. 
- One may remove a splinter even if it may bleed, but one may remove if it will certainly bleed unless it is very painful. 
- One may remove a insect sting and rinse with ice water to prevent irritation but may not soak the soak the area in water. 
- One may take a walk in order to help digestion. 
Foods and drinks
- Even someone with a minor ailment may eat food which healthy people eat even if one’s intention is for improving one’s health. For example, someone who has a head ache may eat honey, a lemon, or suck on candy. Someone who has a hoarse throat may swallow a raw egg. 
- Foods and drinks are permitted even when they are consumed for medicinal purposes. For example, one may have tea for a sore throat on Shabbos. 
- If the purpose of the vitamin is to serve as a food supplement it is permitted to take such a vitamin on Shabbat. 
- If the purpose of the vitamin is to strengthen a weak body or to relieve certain symptoms, in the opinion of many poskim, one may not take those vitamins on Shabbos. 
- One should make sure to schedule a non emergency surgery in the first three days of the week but not on or after Wednesday. However, after the fact and if one did the surgery at such a time and there is a situation of Pikuach Nefesh it is totally permissible to violate Shabbat like any other sick person in danger.  Sephardim could be lenient to schedule such a surgery on Wednesday but not past Thursday. 
- It is permissible for a pregnant woman who is past her term to receive a hormone infusion to induce labor if the doctor feels that there danger to the life of the mother or fetus. 
- If according to the doctor a women has to have a caesarian section and she can choose to schedule the surgery, she should schedule for the first three days of the week and not Wednesday through Friday. 
A doctor on call
- A doctor who is on call and there is a sick patient under his or her care, some say that the doctor should stay in the hospital or nearby so that the doctor will not have to travel on Shabbat, while others are lenient if it will ruin the doctor and his family’s oneg Shabbat. 
- Rabbi Doniel Neustadt on Torah.org
- Rashi (Shabbat 53b D"H Gezerah) explains that there is a rabbinic decree not to do an medical practice on Shabbat because one might come to violate the prohibition of Tochen (grinding) the ingredients for the medicine. Maggid Mishna (Shabbat 2:10) draws a distinction between one is sick but isn't in danger of his life and a person who is pain. Tur and Shulchan Aruch 338:1 rule that a healthy person who is in pain may not do any activity of healing because of the rabbinic decree.
- Shemirat Shabbat KeHilchata 34:1,3. Rabbi Doniel Neustadt on Torah.org writes that although contemporary poskim debate whether nowadays we can be more lenient with taking medication on Shabbos because of the change in technique, the general consensus is to reject this argument. See Minchas Shabbos 91:9; Ketzos ha-Shulchan 134:7; Chelkas Yaakov 4:41; and Tzitz Eliezer 8:15-15. Listen to Rabbi Aryeh Lebowitz on YUTorah.org.
- Mishna Brurah 328:121 permits taking an oral medication without any shinui (alteration from the normal procedure). Yalkut Yosef (Shabbat, vol 4, pg 129) and 39 Melachos (Rabbi Ribiat, vol 2, pg 492) agree with the Mishna Brurah. Halachos of Refuah on Shabbos (Rabbi Bodner, pg 55) and Rabbi Doniel Neustadt on Torah.org write that most poskim agree with the Mishna Brurah. See, however, Sh"t Igrot Moshe 3:53, Tzitz Eliezer 8:15:15, and Minchat Yitzchak 1:108, 6:28.
- Rabbi Doniel Neustadt on Torah.org. See also Sh"t Tzitz Eliezer 14:50-7 and 17:13.
- Rabbi Doniel Neustadt on Torah.org based on S"A 328:23
- Shemirat Shabbat KeHilchata 34:3
- Shemirat Shabbat KeHilchata 35:30 (in the new edition) writes that a band aid shouldn’t be removed on Shabbat in an area where there’s hair because removing the band aid will certainly pull out hairs. However, the Shemirat Shabbat KeHilchata writes that it’s permissible to use a Benzine to remove the band aid so it won’t rip out any hairs that’s assuming the cream was set aside before Shabbat and isn’t Muktzeh. Nonetheless in the footnote he quotes Rav Shlomo Zalman saying that if it’s painful it may be removed because it’s a pesik reisha delo nicha leih (פסיק רישא דלא ניחא ליה), keleacher yad (כלאחר יד), and mekalkel (מקלקל).
- The Yalkut Yosef (Shabbat 4 pg 179, kitzur S”A 328:101, 340:6) writes that if there’s a need, it’s permissible to remove a band aid from an area of hair on Shabbat because it’s a pesik reisha delo nicha leih (פסיק רישא דלא ניחא ליה) for a Derabbanan. In the footnote he writes that even though the Or Letzion (vol 2, pg 259) is strict, his father (Rav Ovadyah, in Haskama to Lev Avraham), Rav Yitzchak Elchanan (Bear Yitzchak Siman 15), and Rav Shlomo Zalman (from Shemirat Shabbat KeHilchata) are lenient.
- Rabbi Doniel Neustadt on Torah.org writes that it is permissible to wear braces on Shabbat because the goal of the treatment or procedure can only be achieved without the use of medicine.
- 39 Melachos (Rabbi Ribiat, vol 2, pg 478)
- 39 Melachos (Rabbi Ribiat, vol 2, pg 478)
- 39 Melachos (Rabbi Ribiat, vol 2, pg 477)
- Shemirat Shabbat KeHilchata 34:4
- Rabbi Doniel Neustadt on Torah.org, 39 Melachos (Rabbi Ribiat, vol 2, pg 477)
- Rabbi Doniel Neustadt on Torah.org
- Rabbi Doniel Neustadt on Torah.org writes to see Igros Moshe, O.C. 3:54, Minchas Shelomo 2:37 and Shemiras Shabbos K’hilchasah 34, note 86, quoting Rav S.Z. Auerbach. See, however, Tzitz Eliezer 14:50, who takes a more lenient approach concerning vitamins on Shabbos.
- Shemirat Shabbat KeHilchata 32:33 based on the idea of Baal HaMoer quoted in S”A 248:4. Shemirat Shabbat KeHilchata (chap 32 note 99) quotes Rav Shlomo Zalman who says that if an expert doctor who is needed for this surgery is only available on Wednesday through Friday it is permissible to schedule the surgery then. For background of this topic see here.
- Yalkut Yosef (Shabbat, vol 1, pg 60)
- Shemirat Shabbat KeHilchata (chap 32, note 98 (note 100, in new one)).For background of this topic see here.
- Shemirat Shabbat KeHilchata 36:4 based on the principle of the Baal HaMoer. For background of this topic see here.
- Sh”t Igrot Moshe 1:131 and Shemirat Shabbat KeHilchata 30:26
- Shemirat Shabbat KeHilchata (chap 40 note 71) in name of Rav Shlomo Zalman Auerbach explaining that since there is a mitzvah of oneg Shabbat one is not obligated to leave one’s home on Friday in order to avoid violation of Shabbat. For background of this topic see here.