May one who has a localized pain, such as a headache, foot pain, back pain, and the like, take a pain killer on Shabbos?
*Important note: This Halacha is dealing with the very specific case of taking pain killers for a non-lethal localized pain. It does not refer to illnesses, such as fever, or other medical conditions, or people who do not have any pain, illness or medical condition, which contains its own article.
The Sages prohibited taking medicine on Shabbos due to a decree that this can lead one to grind medicinal herbs which is Biblically forbidden due to the grinding prohibition. This prohibition applies even today. This prohibition applies even if one is slightly ill, and even if one is in great pain. However this restriction only applies if one has a mere ache and is able to strengthen himself and walk like a healthy person, even if it hurts him tremendously. However, if his entire body feels weak or sick as a result [of the pain] then even though he strengthens himself and walks, and it goes without saying if he has become bedridden, then some Poskim rule that he may take medicine, and so is the final ruling.
Very painful to point of weakness: If one is experiencing pain to the point that it drains one’s energy and makes one’s entire body feel weak, then any pain reliever may be taken on Shabbos to relieve the pain.
Painful, but able to function-Pain relievers that are not regularly taken unless sick [i.e. prescription drugs, Tylenol, ibuprofen]: All pain relievers which are not commonly taken by people unless they feel sick, or are in pain, may not be taken on Shabbos, even if one is in pain, so long as he can still walk and function like a normal person. [Thus, all prescription drugs, and all OTC [i.e. Over-The-Counter] pain relievers which healthy people do not take daily as preventive medicine, may not be taken. An example of such pain relievers are all forms of opioids, Tylenol, ibuprofen, and most over the counter pain killers.]
Painful, but able to function-Pain relievers that are taken regularly even by healthy people [i.e. Aspirin]: Pain relievers which are regularly taken even by healthy people as preventive medicine, such as aspirin to lower the risk of heart disease, are debated in whether they are defined as medicine, and as to whether it may be taken by one who is in pain. Some Poskim rule that Aspirin may be taken in any circumstance being that it has become a common drug used by even healthy people in order to prevent heart disease. However, the majority of Poskim rule that even such medicines are forbidden to be taken for medicinal purposes, just like all other medicine, and so is the implied ruling of Admur in his Shulchan Aruch. This is especially in light of the fact that it has been discovered that aspirin has negative side effects and thus many people avoid taking it unless they need to, thus eroding the entire basis of the lenient approach. Practically, it is forbidden to be taken unless one is in great pain, which has weakened his body, as explained above.
It is forbidden for one who has localized pain to take pain killers on Shabbos unless he is in pain to the point that it depletes his energy and makes him feel weak in his entire body, in which case any pain killer may be taken regularly.
If one is unsure as to whether he is weak in his entire body due to the pain, may he take the medicine?
Some Poskim rule it is permitted to do so.
Word of Advice-Dissolving the medicine from before Shabbos:
One who feels that he may need to take pain killers on Shabbos, but will not reach the state of being weak in his entire body due to the pain, may dissolve the pain killer in water before Shabbos and then drink the solution on Shabbos upon need. [One is to dissolve the pill in juice or colored soda water, in a way that the solution will not be recognizable that it contains medicine.]
 Admur 328:1, 19-20, 43; Michaber 328:1; Mishneh Shabbos 109b, 111a and Gemara 53b
 See Ketzos Hashulchan 134 footnote 7; Piskeiy Teshuvos 328:2
 Admur 328:20 in parentheses; 328:43; Michaber 328:37; Mishneh Shabbos 109b
 Admur 328:43; Michaber ibid
 1st and Stam opinion in Admur 328:19 “A sick person that is bedridden due to his illness and is not in danger or [a person] that has an ache that pains him to the point that his entire body is weakened due to it, in which case even though he is able to walk he is considered like one who is bedridden, then…..if one needs to eat foods which are recognizable that they are administered for healing, then although doing so is a Rabbinical prohibition due to [a decree] that one may come to grind up herbs as will be explained [in Halacha 43], nevertheless since (this is something which is impossible to do through a gentile and he is) sick throughout his entire body, [therefore] [the Sages] permitted for him to do so.”; Rama 328:37; Beis Yosef 328; M”A 328:44; Implication of Maggid Mishneh 21:20 and Ran 14; See Biur Halacha 328:37 “Vechein”
Other opinions: Some Poskim rule it is forbidden to take medicine on Shabbos even if one is bedridden due to the pain or illness. [2nd opinion in Admur ibid; Taz 328:25; Bach 328; Olas Shabbos 328:42]
 Admur 328:19 “Regarding the final ruling, in a [dispute over a] Rabbinical prohibition one may follow the lenient opinion.”; 328:43 “It has already explained above [Halacha 19] that there are opinions that permit [him to take medicine].” See Biur Halacha 328:37 “Vechein” who leans to be stringent; See See Ketzos Hashulchan 134 footnote 7; Piskeiy Teshuvos 328:2
 See Ketzos Hashulchan 138:31 “It is difficult to allow medicine to be taken even by a strong head ache unless one feels week in his entire body”; SSH”K 34:3 and 6; Piskeiy Teshuvos 328:2 [old]
 Opinion of Rav Shteif, recorded in Beir Moshe 2:32
 SSH”K 34:3 and 6 forbids all medicines in all cases of pain, unless one is bed ridden or his entire body is weak, as is the simple ruling of SHU”A; Piskeiy Teshuvos [new] 328 footnote 499 that majority of Poskim argue on the above leniency; Minchas Yitzchak 3:35 rules that although aspirin is taken by healthy people, nevertheless, since its main intent is for medication, therefore it may not be taken for a mere ache. However, if one is in great pain, he leans to allow it.; Ketzos Hashulchan 138:31 writes regarding headaches rules that “it is difficult to allow medicine to be taken even by a strong head ache unless one feels week in his entire body”. Thus, implying that he too does not hold of the aspirin allowance [which had already been around for about 30 years before him writing this ruling]. This is furthermore understood from his ruling in Ketzos Hashulchan 134 footnote 16, brought in next footnote. The Beir Moshe in 1:33 forbids this completely, although in Beir Moshe 2:32 he writes that after hearing testimony that some Gedolei Haposkim have allowed it, nullifies his opinion to the allowance, although questions if it is still relevant today being that people do not use it when healthy due to negative side effects; See Beir Sarim 3:43; Minchas Asher 2:38
 Opinion of Admur: Seemingly according to Admur taking pain killers to relieve pain is forbidden even if the fact were that healthy people always take it. This is learned from Admur in 328:34 in which he writes: “However when he does eat raw egg or resin [foods that are taken even by the healthy] intending for healing then it is forbidden even if he is completely healthy and does not have any ache [and is merely taking the medication in order to gain more strength]”, and as infers the Ketzos Hashulchan 134 footnote 16 from here that those foods eaten for a type of healing which is not defined as healing [such as egg to sweeten ones voice and likewise pills to prevent illness], then when done for actual healing it is nevertheless forbidden even though it is no recognizable for others. Thus, according to Admur since aspirin is taken by healthy people for a [non-Halachic] health purpose, it may not be taken for a Halachic health purpose such as to relieve pain even though it is not recognizable that one is taking it for this purpose.
 Shut Rav Akiva Shlezinger 1:122; Piskeiy Teshuvos 328:2
 Beir Moshe 1:33; See Ketzos Hashulchan 138: footnote 31; Minchas Yitzchak 6:28; Igros Moshe 2:86 allows only in cases that this is the normal way of taking the medicine, that it is normally taken in a drink; Sheivet Halevy 3:36; Shearim Hametzuyanim Behalacha 91:2
 The reason: As any action which is being done for healing but is not evident to others that it is being done for healing may be done on Shabbos. [See Admur 328:43; Ketzos Hashulchan 134 footnote 16]