Gemara

Question: May one castrate a dog?

Answer (Ben Zoma): "Uv'Artzechem Lo Sa'asu" - do not do this to anything in your land.

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

Question: This is forbidden!

(Beraisa - R. Chanina): One may not castrate a person. We read "Uv'Artzechem Lo Sa'asu" like 'Bachem Lo Se'asu' (do not become sterile).

Answer #1: One may not intentionally castrate. Here he intends to be cured, and the castration results by itself;

(R. Yochanan): To castrate a rooster one shears its mane, and it is castrated by itself:

Rejection: Rav Ashi taught that it is not really castrated. It merely mourns the loss of its glory (but one may not even cause castration)!

Answer #2: A Seris (one who was already castrated) may use this cure.

Objection: But R. Chiya bar Aba taught that all agree that one is liable for Mechametz after Mechametz (of Menachos), for it says "Lo Se'aseh Chometz" and "Lo Se'afeh Chometz" (to Mechayev for every step of baking, even if the dough already became Chometz);

All agree that one is liable for castration after castration - "u'Ma'uch v'Chasus v'Nasuk v'Charus (the verse discusses blemishes of castration; it ends ... uv'Artzechem Lo Sa'asu)";

Question: If one is liable for Kores (disconnecting the testicles), all the more so for Nosek (Tosfos - totally cutting them off, but they are still in the sac; Rashi - removing them from the sac). Why did the Torah need to say v'Nasuk?)

Answer: This teaches that one is liable for Nosek after Kores.

Answer #3: Someone too old to have children may use this cure.

Objection: But R. Yochanan said 'These (cures discusses in Gitin) returned me to my youth (fertility)!'

Answer #4: A woman may use this cure (she is not commanded to have children).

Question: According to R. Yochanan ben Berokah, who says that "Va'Yomer Lahem Peru u'Rvu" was also said to women (they are also commanded to have children), how can we answer?

Answer #5: A woman who is sterile or too old to have children may use this cure.

Yevamos 65b (Mishnah): A man is commanded about Peru u'Rvu, but not a woman;

R. Yochanan ben Brokah says, "Peru u'Rvu" was said to both of them.

Yehudis, R. Chiya's wife, had an excruciating pregnancy. Afterwards, she disguised herself and asked R. Chiya whether or not a woman is commanded about Peru u'Rvu. R. Chiya said that she is not. She drank a sterility potion.

Toras Kohanim (on 22:25): (The Isur of) castration applies to females;

R. Yehudah says, it does not apply to females.

Rishonim

Rif and Rosh (Shabbos 40b and 14:9): It is forbidden to castrate a person - "Uv'Artzechem Lo Sa'asu".

Question: Why is a man forbidden to drink a potion for jaundice? Even if it may cause sterility, the Halachah follows R. Shimon, who permits Davar she'Eino Miskaven!

Answer #1 (She'altos, brought in Rosh, ibid): We read "Lo Sa'asu" like 'Lo Se'asu'. This is like R. Yehudah, who forbids Davar she'Eino Miskaven. The Halachah follows R. Shimon only regarding Shabbos, but not regarding other Mitzvos.

Rebuttal and Answer #2 (Rosh and Ran DH Ela): The Halachah follows R. Shimon regarding all Mitzvos. R. Shimon agrees here, for it is a Pesik Reishei. The Aruch says that this is only if the person is happy with the result. This is only regarding Shabbos, for only Melachah Machsheves is forbidden. In other Isurim a Pesik Reishei is forbidden in any case.

Rif and Rosh (ibid.): One is liable even for castration after castration. "U'Ma'uch v'Chasus v'Nasuk v'Charus" teaches that one is liable for Nosek after Kores.

Rambam (Hilchos Isurei Bi'ah 16:10): One may not ruin the reproductive organs in a person, animal or bird, whether Tamei or Tahor, in Eretz Yisrael or Chutz la'Aretz. "Uv'Artzechem" forbids Yisraelim to castrate themselves or others. One is lashed for this, even for castration after castration.

Hagahos Maimoniyos (2): One may eat a castrated animal. The Torah forbids it for a Korban. This implies that it is permitted to people (like it says regarding crossbred animals and Mechusar Zeman - Chulin 115a).

Rambam (11): One who castrates a female person, animal or bird is exempt.

Magid Mishnah: In Toras Kohanim, R. Yehudah says that there is no castration of females. The Rambam explains that there is no Chiyuv for castration of females, since it did not say it is permitted.

Rebuttal (Gra EH 5:25): The Rambam rules like the first Tana there, who includes castration of females from "Ki Mashchasam". There are no lashes because it is not included in "Lo Sa'asu".

Rambam (12): It is forbidden to give to drink to a person, animal or bird a potion that sterilizes. One is not lashed for this. Also, if he tied down a man and incited an animal to cut his organs, or stuck the man in water or snow until the organs were ruined, he is not lashed, unless he overtly castrates. It is proper to lash mid'Rabanan for causing castration.

Source (Magid Mishnah): The Gemara in Shabbos teaches that one may not cause castration. One who does not touch the organs is not lashed.

Source (Gra 28,29): Lashes are only for the Lav "u'Ma'uch v'Chasus...", i.e. cutting the organs. "Mashchasam" forbids a potion.

Migdal Oz: One is not lashed because it is a Lav she'Ein Bo Ma'aseh.

Rambam (ibid.): A woman may drink such a potion.

Source (Beis Yosef EH 5 DH v'Asur): The Tosefta (Yevamos 8:2) permits for a woman and forbids for a man. Also the episode with R. Chiya's wife suggests that a woman is permitted.

Rosh (ibid.): Even castration of Tamei animals is forbidden, like it says in Chagigah.

Poskim

Shulchan Aruch (EH 5:11) One may not ruin the reproductive organs in a person, animal or bird, whether Tamei or Tahor, in Eretz Yisrael or Chutz la'Aretz. One is lashed for this mid'Oraisa, even for castration after castration. One who castrates a female person or animal or bird is exempt.

Shulchan Aruch (12): It is forbidden to give to drink to a person or animal or bird a potion to sterilize. One is not lashed for this. A woman may drink such a potion.

Beis Shmu'el (13): A man may not drink such a potion even for a cure, even if he already fulfilled Peru u'Rvu and even if he is a Seris (sterile).

Bach (DH v'Ishah): Tosfos (Shabbos 111a DH bi'Zkenah) says that it is impossible to castrate females (directly. The only way is through a potion.) The Rambam forbids castrating females, but he permits a female to drink a potion! We must say that it is permitted only if she has more labor pain than other women, or for the sake of a cure.

Rebuttal (Taz 7): Tosfos means that there is no Isur to castrate a female. It is possible (directly). The Poskim permit through a potion even if she did not have pain.

Shulchan Aruch (13): If one tied down a man and incited an animal to cut his organs, or stuck the man in water or snow until the organs were ruined, he is not lashed, unless he overtly castrates. It is proper to lash mid'Rabanan for causing castration.

Rema: One may shear a rooster's mane, even though this castrates it, since he does not touch the organs of semen.

Beis Shmu'el (15) and Gra (31): This is because it is not really castrated, it merely refrains from mating. One may tie down the tails and organs of females and males to prevent mating (Shabbos 53b-54a).