Gemara

29b (Mishnah): One who sells garments with Sha'atnez may sell normally, as long as he does not intend for [protection from] the sun or rain.

110b: To cure jaundice, one drinks two [ingredients mentioned above] in beer. This [also] causes sterility.

Question: This is forbidden!

Answer: A woman may use this cure.

133a (Beraisa - R. Yoshiyah): "Besar" permits circumcision, even if he will cut off Tzara'as on the foreskin.

(Abaye): He holds like R. Yehudah, who forbids Devar she'Eino Miskaven.

After Abaye heard that R. Shimon agrees about a Pesik Reishei, he retracted.

Question: According to Abaye [before he retracted], what does R. Shimon learn from "Besar" written regarding an eight-day old?

Answer (Rav Mesharshiya): This permits a father who intends to cut his son's Tzara'as.

If someone else can circumcise, the father may not, for we can fulfill the Aseh without transgressing the Lav.

Zevachim 91b (Shmuel): A voluntary Korban of wine is sprinkled on the fire (on the Mizbe'ach).

He learns from "v'Yayin Takriv... Isheh."

Question: The wine will extinguish the fire [where it lands]. This is forbidden!

Answer #1: The Torah forbids only totally extinguishing the fire.

Answer #2: Extinguishing is permitted for a Mitzvah (e.g. a Nedavah of wine).

Question #1 (Beraisa #1 - R. Akiva): A voluntary Korban of wine is poured into conduits (that go through the Mizbe'ach).

Question #2 (Beraisa #2): Nesachim of wine are poured into the conduits.

Suggestion: Perhaps they are thrown onto the fire!

Rejection: It says "Lo Sichbeh."

Answer: Shmuel holds like R. Shimon, who permits Davar she'Eino Miskaven. The Beraisos are like R. Yehudah, who forbids this.

Rishonim

Rosh (12:1): The Aruch permits l'Chatchilah a Pesik Reishei d'Lo Nicha Lei, e.g. cutting Tzara'as off the foreskin during Milah without benefit, e.g. it is not his son. There is not even an Isur mid'Rabanan. He brought a proof from 133a. "Besar" permits a father who intends to circumcise and cut his son's Tzara'as. If someone else can circumcise, he is permitted l'Chatchilah, even though it is a Pesik Reishei, since he does not benefit from it. This is not a proof. The question was only according to the Hava Amina that R. Shimon permits a Pesik Reishei. In the conclusion, the father is like anyone else. Also, even according to the Aruch's proof, R. Shimon could agree that it is forbidden mid'Rabanan.

Rosh and Tosfos (103a DH Lo): The Aruch brought another proof from Zevachim. Even though it is a Pesik Reishei to extinguish, it is permitted because he does not benefit. This is wrong. Shmuel permits in a way that is not a Pesik Reishei, e.g. he throws small drops or on wood or on limbs. Perhaps he will not extinguish.

Rosh (14:9): The She'altos (Emor 105) says that only R. Yehudah forbids a cure that also sterilizes. R. Shimon permits, for he intends only for Refu'ah. We rule like R. Shimon regarding Shabbos, but everywhere else the Halachah follows R. Yehudah, who forbids Davar she'Eino Miskaven. This is wrong. A Nazir may shampoo, even though it is a Pesik Reishei that hairs will fall it (Shabbos 81b). R. Yochanan derived from here that the Halachah follows R. Shimon. In Bechoros 34a, regarding the Isur to let blood from a Bechor, Shmuel rules like R. Shimon. We asked about why he needed to rule like R. Shimon, since he already said so. If Shabbos is unlike other Isurim, he needed to rule like him also regarding other Isurim! Rather, there is no distinction.

Rosh (ibid.): The Aruch says that R. Shimon agrees about a Pesik Reishei only if it is Nicha Lei. This refers only to Shabbos, for the Torah requires Meleches Machsheves (intended labor). Regarding other Isurim, e.g. castration, it does not depend on Nicha Lei. In Zevachim, we asked that extinguishing is a Pesik Reishei, even though it is not Nicha Lei.

Ran (41a DH Aval): The Aruch permits a Pesik Reishei d'Lo Nicha Lei. He brought a proof from the Heter for sellers to wear Sha'atnez, as long as they do not intend for protection from the sun or rain. This is not a proof. The Torah forbids only clothing of Sha'atnez, which Stam is for Hana'ah. Without Hana'ah, it is a mere load. This is why we permit [sitting on] felt, which does not warm (Beitzah 15a).

Rosh (ibid.) and Ran (40b DH Ela): Even R. Shimon, who permits Davar she'Eino Miskaven, forbids a man forbidden to drink a potion for jaundice, since it is a Pesik Reishei.

Poskim

Shulchan Aruch (EH 5:12): One may not give to drink to a person a potion to sterilize him. A woman may drink such a potion.

Beis Shmuel (13) and Levush (12): He may not drink for Refu'ah without intent for sterility, since it is a Pesik Reishei. We permit a Pesik Reishei d'Lo Nicha Lei on Shabbos, but not for other Isurim, like the Rosh and Ran say.

Chasam Sofer: According to those who permit a Pesik Reishei for an Isur mid'Rabanan (see Magen Avraham 314:5), why can't a man drink a potion to cure jaundice? Rashi forbids Gerama mid'Oraisa. He could forbid causing castration to a female (e.g. through a potion) mid'Rabanan. Most Poskim permit it.

Shulchan Aruch (OC 320:18): Some permit [to remove and re-insert] flax plugging the hole in the side of a barrel. It is a Pesik Reishei d'Lo Nicha Lei that he will squeeze.

Question (Birkas ha'Zevach 91b DH Lo Kashya, citing the Maharal): What was the proof from the question about extinguishing? We conclude that Shmuel holds like R. Shimon, who permits! Also, the Rosh (12:1) said that the Aruch brought a proof from Zevachim!

Answer #1 (Birkas ha'Zevach, brought in Machatzis ha'Shekel 20): The Gemara answered that Shmuel holds like R. Shimon, but it relies also on the initial answers (we permit partial extinguishing, or for a Mitzvah). Because the Isur became weaker, we permit due to Pesik Reishei d'Lo Nicha Lei, even though this alone does not permit other Isurim [other than Shabbos].

Answer #2 (Magen Avraham 20): this is wrong. The Rosh brought a proof from Zevachim. This shows that the Aruch said so regarding all Isurim! Rather, we rule unlike Shmuel. What is the reason? It is due to the question of extinguishing, even though it is a Pesik Reishei d'Lo Nicha Lei, which the Aruch permits. Rather, the Aruch permits only regarding Shabbos. The Poskim who rule unlike the Aruch must say like the Kesef Mishneh [that we rule unlike Shmuel because the Stam Mishnah is like R. Akiva. He had a tradition that wine is poured into conduits. It is not due to extinguishing.]

R. Akiva Eiger (Yoma 34b:7): What was the proof from the verse that wine goes in conduits? The Torah permits Davar she'Eino Miskaven also in other Isurim! Even if Shmuel holds like Abaye, who says that R. Yehudah forbids Davar she'Eino Miskaven in other Isurim mid'Oraisa, this is difficult for Rav, who holds that all permit. Like whom is the Beraisa?!

Note: Rav could say that the Beraisa is an Asmachta, or that Rav is a Tana and argues with the Beraisa. Perhaps those who argue with R. Akiva's conclusion must say so.

R. Akiva Eiger: This is a proof for the Aruch. Tosfos holds that a Pesik Reishei is considered intent. When it is Nicha Lei it is Tzerichah l'Gufah, and when it is Lo Nicha Lei it is Einah Tzerichah l'Gufah. Therefore, in other Isurim the Torah forbids Pesik Reishei d'Lo Nicha Lei, and in Shabbos it is forbidden mid'Rabanan, like any Melachah she'Einah Tzerichah l'Gufah. The Aruch permits also regarding other Isurim. He must say that Melachah she'Einah Tzerichah l'Gufah is when one intends to do the Melachah, but he does not need it itself, e.g. extinguishing due to bandits, and not to make coal, or digging a pit without intent to put anything inside. If he does not intent to do the matter, but it is a Pesik Reishei d'Nicha Lei, it is as if he intended for it. If it is not Nicha Lei, it is called Eino Miskaven. Therefore, also R. Yehudah forbids Pesik Reishei d'Lo Nicha Lei only mid'Rabanan. We do not find that he argues with R. Shimon about whether or not he is called Miskaven. However, Kerisus 20b says that R. Yehudah obligates for a Pesik Reishei d'Lo Nicha Lei!

R. Akiva Eiger: Rather, the Aruch holds that one could say like Tosfos, that it is called intent. Or, one could say that it is Eino Miskaven. Rav holds that R. Yehudah forbids Davar she'Eino Miskaven in other Isurim only mid'Rabanan. Sprinkling wine on the Mizbe'ach is a Pesik Reishei d'Lo Nicha Lei. Rav considers this intent, so even R. Shimon holds that wine goes to the conduits. Shmuel says that the Beraisa is only like R. Yehudah, for it is called Eino Miskaven, and R. Yehudah forbids this mid'Oraisa in other Isurim, and R. Shimon permits.

Note: R. Akiva Eiger continues at length, and answers Tosfos' questions against the Aruch based on this idea, that there is no proof whether or not Pesik Reishei d'Lo Nicha Lei is considered intent, and Amora'im argue about this.

Mishnah Berurah (53): [Some permit a Pesik Reishei d'Lo Nicha Lei regarding Shabbos, for the Torah forbids only Meleches Machsheves. The Rosh holds that for other Isurim, all forbid Pesik Reishei d'Lo Nicha Lei mid'Oraisa.