The Tzitz Eliezer in 8:15:15:4 suggests a novel approach to the prohibition of Refuah BaShabbos, which is prohibited so that one does not come to crush herbs on Shabbos. The teshuva is too long to translate, so here is my summary of the main points found there.
The Rambam claims that the reason it is permitted to perform certain types of Refuos is because, “They are not using herbs, and he has pain from the ailments.” The obvious question is that the Gemara prohibits standing in cold water if it is obvious that he is intending to cure an ailment, although that is not a cure which involves herbs.
One answer might be that these ailments don’t fit the Rambam’s second criterion, that he has pain. However, this is not likely, because the guy is in pain…. Additionally, the Bach says that it is obvious that the decree of Chazal was not only on cures using herbs, but on all cures.
A more convincing answer is suggested by Rabbi Dovid HaLevi in his monumental work, Turai Zahav. He suggests that the Rambam requires the illness to be treated commonly with herbs. The Gemaros which prohibit curing illnesses even with cures which don’t involve herbs do so because the illnesses were commonly treated with herbs. The Mishnah Berurah in 328:130 also adopts this approach, although he doesn’t mention that it comes from the Taz.
This approach has radical implications for the laws of Refuah nowadays. Today, virtually no one crushes medicinal herbs at home (maybe for recreational use). Therefore, there may be no prohibition of Refuah nowadays at all, because all illnesses fall into the category of not being normally cured by herbs.
However, what’s left is the Rambam’s second criterion, that it’s only permitted if the has pain from the illness. The question is why it would be prohibited if the decree against refuah isn’t applicable. Rabbi Joseph Cairo (Also brought by Rabbi Avrohom Gombiner in his beastly commentary Magen Avrohom) is his awesome Bais Yosef suggests that it’s prohibited because it’s uvdin dechol, weekday activity. Why this is uvdin dechol is a debate between authorities. The Bais Yosef himself says that the issue is that it’s a bother on Shabbos, so presumably when it’s not a bother, (like taking pills), it would be permissible even if he has no pain. The Rabbi Mordechai Yaffe, in his competition to the Bais Yosef, the Levush, says that the refuah itself is uvdin dechol, because it is a weekday activity. If so, refuah nowadays would be prohibited unless the patient is in pain.
However, the entire heter is questionable, because although individuals don’t crush medicinal herbs, pharmaceutical companies do. The Tzitz Eliezer himself holds that because the reason of the gezairah has to do with the individual, it makes sense that Chazal’s gezairah would be based on the practice of individuals. This logic though isn’t certain, because it may be that chazal never foresaw the coming of industrial manufacturing of herbs, so they simply made their gezairah dependant on whether the illness was treated with crushed herbs, irrespective of who crushed them. The Tzitz Eliezer is somewhat vague when he addresses this issue. He just says “It is logical that it would go after the reason….”