Some maintain that one may take modern, mass-produced medications on Shabbat for any type of ache or pain, as there is no real concern that anyone will grind anything in order to produce the medicine. However, most poskim maintain that even nowadays one who is only mildly ill or bothered by an ailment may not take medicine on Shabbat. There are two reasons for this. First, according to many, no rabbinic enactment may be nullified except by a larger and more prominent beit din than the one that passed the enactment. Second, some people still prepare household remedies, so there are cases where the reason for the enactment still applies.
In practice, as long as the ailment is merely irritating but not painful, it is proper to be stringent and avoid taking mass-produced medications. However, if the ailment causes pain, one may take medication, because some maintain that the Sages never prohibited taking medication when pain is involved. Even though many maintain that the Sages’ enactment applies even when pain is involved, in the case of mass-produced medication, where there is no concern that a private individual would try to prepare it himself, it is proper to be lenient. It is worth noting that when the technical law allows leniency, it is proper to act accordingly so as to fulfill the mitzva of oneg Shabbat.
Therefore, if one is bothered by an ailment of the eyes or ears, he should not use drops. However, if the ailment causes him pain, he should use the drops. The same applies to a runny nose: if it is merely irritating, one should not use nose drops, but if it causes pain, he should. Similarly, one may take sleeping pills to relieve insomnia, since without them he will suffer pain. Perhaps we can suggest that if one is pained to the point that he would be willing to walk a kilometer in order to get medicine, it indicates that he is truly suffering and may thus take mass-produced medications. However, if he thinks it is unnecessary to go to that much trouble, it indicates that he has a mere ailment, and he should thus refrain from taking medicine.
Some are lenient concerning all medicine, even for regular pain. This is the opinion of R. Mordechai Benet, who argues that sometimes the Sages were permissive when one is in pain, even in a case resembling a melakha she-eina tzerikha le-gufah (SA 328:28; above, 14:2); certainly, then, they would permit taking medicine in a case of pain. Minĥat Shabbat 91:1 and She’arim Metzuyanim Ba-halakha 91:3 take this position as well. Radbaz 3:540, appearing earlier than any of them, states that the prohibition on taking medicine is even less severe than a shvut di-shvut. Therefore, when any pain is involved, one may be lenient.
Thus there are two disagreements here: 1) Does the enactment apply to mass-produced medicine? 2) Does it apply to people who are in pain? It is true that in each of these disagreements, most poskim feel that one should be stringent. Nevertheless, since these disagreements concern a rabbinic prohibition, one who is lenient has an opinion to rely upon. Furthermore, in a situation where there are two reasons to be lenient, such as if the medicine is mass-produced and the person is in pain, then it is a twofold doubt that affects oneg Shabbat. Accordingly, one may be lenient even le-khatĥila. (We should add that when the medication simply relieves pain but does not cure the illness, some poskim maintain that it is not considered medication for purposes of the rabbinic enactment. See Tzitz Eliezer 8:15:15:21 as well as 14:50, and Yalkut Yosef 328:52.)