1) MEDICATION THAT CAUSES STERILITY
QUESTION: The Gemara asks that drinking "Kos Ikrin," even for medicinal purposes, should be forbidden because it causes sterility, and causing sterility ("Sirus") is forbidden by the Torah.
Why should it be forbidden when a person drinks it for healing? Since he intends to heal himself and not to make himself sterile, it is a Davar she'Ein Miskaven, and it should be permitted.
(a) TOSFOS (DH Talmud Lomar) cites the SHE'ILTOS who writes that the Gemara asks that it should be forbidden only according to Rebbi Yehudah, who maintains that a Davar she'Ein Miskaven is forbidden. However, if this is the Gemara's intention, then l'Halachah one should be permitted to drink Kos Ikrin, because the Halachah follows the view of Rebbi Shimon, who maintains that a Davar she'Ein Miskaven is permitted, and yet none of the Poskim permit one to drink Kos Ikrin. The Sh'eiltos answers that we rule like Rebbi Shimon only with regard to Davar she'Ein Miskaven on Shabbos. In all other areas of Halachah, though, we rule like Rebbi Yehudah with regard to Davar she'Ein Miskaven.
(b) Tosfos cites the RI who argues with the She'iltos and says that even according to Rebbi Shimon, one may not drink Kos Ikrin, because it is a Pesik Reshei.
(c) The ROSH (14:9) adds that this type of Pesik Reshei is forbidden even according to the opinion that maintains that a Pesik Reshei that one does not want ("d'Lo Nicha Lei") is permitted on Shabbos (ARUCH, cited by Tosfos to 103a, DH Lo Tzerichah). Such a Pesik Reshei is permitted only with regard to the laws of Shabbos, when the additional exemption of Meleches Machsheves applies. In all other areas of Halachah, Meleches Machsheves does not apply and therefore a Pesik Reshei that one does not want remains forbidden.