Gemara

(Shmuel): All Peturim in the Mishnayos of Shabbos are forbidden, with three exceptions, which are permitted - 'trapping' a deer (that is already trapped), trapping a snake, and Mafis Morsa (piercing an abscess).

107a (Mishnah): If one pierces an abscess to make an opening, he is liable. If it is to let out the pus, he is exempt.

Question: What is the source that it is permitted?

Answer (Mishnah): A hand needle may be moved, for it is useful to remove a thorn.

107b (Mishnah): If one punctures an abscess on Shabbos:

If it was to make an opening, he is liable. If it was to let out pus, he is exempt.

(Rav Yehudah): This is like R. Shimon, who exempts Melachah she'Einah Tzerichah l'Gufah.

(Mishnah): If one traps a snake on Shabbos:

If this was so it will not bite him, he is exempt;

If this was to use it for a cure, he is liable.

(Rav Yehudah): This is like R. Shimon, who exempts Melachah she'Einah Tzerichah l'Gufah.

Rishonim

Rambam (Hilchos Shabbos 10:17): If one pierces a boil to widen the opening of the wound like doctors do, with intent for Refu'ah, he is liable for Makeh b'Patish. This is the doctor's Melachah. If he pierced it in order to let out the pus inside, it is permitted.

Ra'avad: Other texts say that (if he intended for Refu'ah) he is liable for Boneh (building).

Tosfos (3a DH ha'Tzad): Bahag asked that here Shmuel exempts Melachah she'Einah Tzerichah l'Gufah like R. Shimon, and permits due to pain. On 42a, we say that Shmuel holds like R. Shimon regarding Davar she'Eino Miskaven, but like R. Yehudah regarding Melachah she'Einah Tzerichah l'Gufah! R. Tam answered that Shmuel taught that according to R. Shimon, who exempts Melachah she'Einah Tzerichah l'Gufah, in these three cases one is exempt and it is permitted, like he proved on 107a. Shmuel himself disagrees. However, we hold like R. Shimon, for Rava rules like him, and he is Basra.

Tosfos (107a DH umi'Mai): In Sanhedrin (85a), the Gemara concluded that the Mishnah that permits using a needle to remove a thorn is like R. Shimon. Even if it will Vadai wound, it is permitted for it is Melachah she'Einah Tzerichah l'Gufah. However, the Gemara asked 'perhaps it will make a wound!' I.e. it assumed that the Mishnah is like R. Yehudah, and permits only when perhaps no wound will result. If so, how can we learn from here to Mafis Morsa, which Vadai wounds? The Ri says that even according to the Hava Amina, presumably the Mishnah is like everyone. R. Yehudah normally forbids Davar she'Eino Miskaven even if it is not a Pesik Reishei (an inevitable consequence). Here it is permitted, due to pain. Likewise, Mafis Morsa, which is Melachah she'Einah Tzerichah l'Gufah, is permitted according to R. Shimon, due to pain.

Poskim

Shulchan Aruch (OC 328:28): If one pierces a boil to widen the opening of the wound like doctors do, with intent for Refu'ah, he is liable for Makeh b'Patish. This is the doctor's Melachah.

Beis Yosef (DH Im): The Tosefta (Eduyos 1:7) says that one who widens the opening of the wound for the sake of Refu'ah like doctors do, is liable. He is liable for Boneh, or for fixing a Kli. Fixing a wound is just like fixing a Kli.

Bi'ur Halachah (DH Kedei): Seemingly, one who intended to widen is liable only if he used a Kli. (This is not Gozez (shearing) due to removing skin, for it is like a fingernail that mostly detached. It does not nurture from the body. The Pri Megadim says so. Tosefes Shabbos disagrees. The Pri Megadim's proof from Rashi is not solid. Perhaps he merely peeled the skin up, but none is was detached). It is not normal to do so by hand. Even so, if he intends to remove pus, it is permitted (with a Kli). However, the Magen Avraham (307:7) and the Taz here (Sa'if Katan 23) connote that we do not permit Shevus (an Isur mid'Rabanan) in the normal way due to pain. If so, the Heter in order to remove pus is only by hand, but not with a Kli (Pri Megadim).

Shulchan Aruch (ibid.): If he pierced it in order to let out the pus inside, it is permitted.

Beis Yosef (DH v'Im): We permit when he wants to remove pus now, and does not care whether or not it closes immediately. Since there is no Tikun, and Chachamim did not decree, due to pain.

Magen Avraham (32): He is not concerned if it closes immediately. Therefore, it is good to do it through a Nochri (Agudah 121).

Machatzis ha'Shekel: This is because sometimes perhaps he wants it to remain open.

Magen Avraham (33): This is a Melachah she'Einah Tzerichah l'Gufah. Chachamim did not decree in a case of pain. According to the opinion that obligates for Melachah she'Einah Tzerichah l'Gufah, when he does so to remove the pus, this is not the final Melachah. He can never come to a Chiyuv for Makeh b'Patish. Alternatively, this is Davar she'Eino Miskaven. Some people have a hole in the arm. If it is sealed somewhat, it is not clear whether one may put a legume inside so it will open. Here he wants it to remain open. Or, perhaps it is permitted, since it was already open, like in Siman 314:3. (Where there already was a hole, e.g. a spigot, one may re-open it even with a drill.) The same applies to a wound that already opened. This requires investigation. The Shulchan Aruch connotes that one may not widen the hole. Sefer ha'Terumah permits to remove pus from a scald called Kvitra, even though there is blood inside, for it is deposited, like in an abscess. (The blood does not come out due to a wound.)

Mishnah Berurah (89): One must be careful not to press the wound and extract new blood.

Magen Avraham (33): Keneses ha'Gedolah brings that Sefer ha'Zichronos forbids changing the garment or paper over the wound, for this draws out the pus, and also perhaps blood will exude and color the garment. See Sa'if 48 [which forbids putting a garment on a wound emitting blood] and what I wrote (Sa'if Katan 53, that if it is tight and draws out more blood, he is liable). I say that here is different, since there is a hole inside. (Machatzis ha'Shekel - the Magen Avraham agrees with Sefer ha'Zichronos' law, but says that Sa'if 48 is not a proof, for here is more lenient.) All the more so one may not put an ointment on a wound without a hole. It seems that one may not scratch a boil that emits blood. This is unlike pus, which is deposited.

Kaf ha'Chayim (171): If a hole in the arm partially closed, one may not put in legumes to widen it. If it did not close, one may not put in legumes to keep it open.

Mishnah Berurah (88): One may remove a splinter with a needle, as long as he is careful not to draw blood, for this is a wound (Sanhedrin 84b). Even though this is Mekalkel (destructive), all agree that it is forbidden. Even though they permitted an Isur mid'Rabanan regarding boils due to pain, here, since one can remove the thorn without drawing blood, one should not transgress without need.

Sha'ar ha'Tziyon (63): Chemed Moshe challenged the Magen Avraham from Tosfos, who connotes that in the conclusion, even if it Vadai wounds, it is permitted. I answer that the Magen Avraham permits only if it is impossible to remove the thorn without wounding, e.g. it is very deep inside, like the case of Mafis Morsa, but when it is possible (without a wound), it is forbidden to wound.

Bi'ur Halachah (316:3 DH Nechashim): The Gemara brought that the Petur for Mafis Morsa is according to R. Shimon, who exempts Melachah she'Einah Tzerichah l'Gufah. The Rambam obligates for Melachah she'Einah Tzerichah l'Gufah, yet he brought the Mishnah verbatim! The Magid Mishneh answered that Shmuel argues with Rav. I.e. the Rambam obligates for piercing a boil when he intends to widen the opening, like doctors do. This is Makeh b'Patish, for it finishes the Melachah. If he pierced it just to let out the pus, he does not widen it. This is not final Melachah at all. Even R. Yehudah agrees to this. The Gemara proved that it is like R. Shimon from the Reisha, which obligates only if he intended to make an opening, i.e. to widen it like doctors do. If he widens it without this intent, rather, just to remove pus, he is exempt for it is Melachah she'Einah Tzerichah l'Gufah, like R. Shimon. If not, the Mishnah should have said "if he makes an opening" he is liable. This would connote even if he does not need it. All agree that an opening to remove pus is exempt and permitted. It is not Makeh b'Patish, since he does not widen the opening. Bahag's question forced the Rambam to explain this way. The Rambam was not satisfied with Tosfos' answer.

Kaf ha'Chayim (167): One should puncture it only with a needlepoint, but not with his fingernail, lest he tear off skin, which is Gozez. However, Tosfos (brought in the Magen Avraham - PF) at the beginning of Siman 340 holds that Gozez is a Melachah she'Einah Tzerichah l'Gufah. If so, if it is impossible in any other way, one may tear off a little skin, to avoid pain. However, the Rivash forbids according to everyone. He holds (303) that it is an Isur Torah. Tosefes Shabbos says so. The Mishbetzos Zahav (21) says that one is not liable for tearing skin off a wound, just like a nail that mostly detached, for it does not nurture any more.

Kaf ha'Chayim (169): Even if he is healthy, one may open it to remove the pus, due to pain.