Q. I was under the impression that one can take the pre-fasting pills. This (quotation bellow from dailyhalacha@aol.com) clearly says not
“One should not say that one is eating to have strength for the fast, and one may not take a pill that makes it easier to fast, because one would be preparing on Shabbos for Sunday which is not permitted.”
Please clarify??
A. Quoting question 203:Q. May one take pills on Shabbos Chazon (either The Kollel’s Tylenol/Advil or Kali Tzom), in order that it should be easier to fast on Tisha B’Av?
A. Some Poiskim maintain that when Tisha B’av coincides with Shabbat, one should avoid consuming on Shabbos medicine to make the fasting easier on Sunday, mainly out of concern for the prohibition of “hachono” or preparations done in Shabbos for the sake of weekdays. They would recommend for someone in real need of medication, to take those pills on Tisha B’av without any liquids. (Ohr Lenesivosi shiur 37:3, Halacha Uminhag p.88).
Horav Shlomo Miller’s Shlit”a opinion is that you may take that medication on Shabbos. His reasoning is that it does not constitute a hachono, since you do it bederech achilo, or as part of the eating process which is permitted in Shabbos. After all we do eat more on this particular Shabbos at the third Seudah, and we may probably eat meat, something we may normally not do, all done in preparation for tomorrows fasting. However, since it is done in the usual way of eating and we don’t declare that it is for the purpose of fasting easier tomorrow, it is permitted. The medicine, as it is not needed right away, would be regarded only as additional food and not as taking medication on Shabbos.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a