Gemara

(Beraisa #1) Suggestion: If one swore to harm himself and did not fulfill it, he should be exempt!

Rejection: "Lehara Oh Leheitiv" - just like Leheitiv is Reshus (optional), also Lehara. This includes one who swore to harm himself.

Bava Kama 91a (Mishnah - R. Akiva): One may not injure himself.

Contradiction (Beraisa #2 - R. Akiva): One may injure himself.

Answer #1 (Rava): One may not bodily injure himself, but he may embarrass himself.

Question: Beraisa #1 permits harming oneself!

Answer #1 (Shmuel): The Beraisa discusses an oath to fast.

Objection (Beraisa #1): What is the case of (swearing to) harm others? 'I will hit Ploni, I will bruise his brain'.

Answer #2 (to both questions): Tana'im argue about this. (Each Tana says that R. Akiva agrees with him.) R. Elazar Hakapar forbids damaging oneself

(Beraisa - R. Elazar Hakapar): "Me'Asher Chota Al ha'Nefesh" - he sinned against his own soul, by denying himself wine. If one is called a sinner for denying himself wine, all the more so one who (fasts and) denies himself all food is called a sinner!

Ta'anis 11a (Shmuel): One who fasts is called a sinner.

He holds like R. Elazar Hakapar.

(R. Elazar): A Nazir pained himself only from one thing, and he is called Kadosh - "Kadosh Yihyeh Gadel Pera Se'ar Rosho". All the more so, one who fasts and pains himself from everything is called Kadosh!

(Reish Lakish): He is called Kadosh - "Gomel Nafsho Ish Chased v'Ocher She'ero Achzari".

Rishonim

Rif and Rosh (3:15): If one swore to harm himself and did not fulfill it, he is liable. Just like Leheitiv is Reshus, also Lehara. This includes one who swore to harm himself.

Rambam (Hilchos De'os 3:1): A Nazir who denies himself wine needs atonement, and all the more so, one who denies himself everything!

Rambam (Hilchos Shevuos 5:17): If one swore to harm himself, e.g. to wound himself, even though he may not, the Shevu'ah takes effect and if he did not harm himself he is liable for Shevu'as Bituy.

Radvaz (Hilchos Shevuos 5:7): The Rambam says that a Shevu'ah not to eat any amount of Neveilah takes effect, because the oath of Sinai does not forbid this. Indeed, the Torah forbids this, but it is not explicit in the Torah, therefore, a Shevu'ah takes effect on it. A proof of this is that Shevuos take effect on Isurim mid'Rabanan, even though Lo Sasur obligates observing them, because the Isurim are not explicit in the Torah.

Pri Chodosh (in Likutim in Frankel Rambam, DH Od (2)): The Rambam rules that a Shavu'ah takes effect on Chetzi Shi'ur, even though we forbid it mid'Oraisa, for it is not explicit in the Torah. However, the Rambam calls a oath to fast on Shabbos or Yom Tov 'Shav', even though the Isur is not explicit in the Torah! It seems that he does not really mean that it is Shav.

Rambam (Hilchos Chovel u'Mazik 5:1): One may not wound himself or another. Even without wounding, a Lav forbids hitting a Kosher Yisrael in the way of fighting.

Question (Lechem Mishneh Hilchos De'os 3:1): Reish Lakish said 'he is called Kadosh.' Rashi explains that he discusses one who fasts. The Rambam and Tosfos explain that he discusses one who does not fast. The Rambam rules like Shmuel and Reish Lakish. Even though in Nedarim (10a), Abaye says that R. Elazar Hakapar is part of a Shitah (Tana'im who hold similarly), and the Halachah does not follow a Shitah, Halichos Olam says that when there is a proof from the Gemara, this overrides the general rule. Tosfos asked a contradiction in Shmuel, and also in Sugyos. Two Sugyos say that R. Elazar Hakapar holds that even a Tahor Nazir is called a sinner, but in Nazir (3a) we say that he said so only about a Nazir Tamei! If so, what forced the Gemara there to say that R. Elazar Hakapar disagrees with the Beraisa? It is difficult to say that this was the Hava Amina, but in the conclusion he need not disagree.

Answer (Lechem Mishneh): The Rambam explains that mid'Oraisa the verse applies only to a Tamei Nazir. Even though the Gemara says that the verse applies even to a Tahor Nazir and it was written regarding a Tamei Nazir since he sins twice, this is an Asmachta. Really, it applies to a Tahor Nazir only mid'Rabanan. Similarly, the Isurim to fast or harm oneself, which are learned from a Nazir Tahor, are mid'Rabanan. Even though an oath takes effect on an Isur mid'Rabanan, the Beraisa connotes that the Tana permits harming oneself even mid'Rabanan. Shmuel established the Beraisa to discuss fasting. This is not like R. Elazar Hakapar, rather, like R. Elazar who argues with him. However, all agree to the law of the Beraisa, that the oath takes effect.

Avi Ezri (De'os 3:1): Why did the Lechem Mishneh need to say that the Shevu'ah takes effect because the Isur is not explicit in the Torah? The source to forbid harming oneself is from Nezirus, which takes effect! Why did the Rambam include the Isur to harm oneself with the Isur to hit another? An explicit Lav "Lo Yosif" forbids hitting others, and the Isur to harm oneself is only due to the Isur to pain oneself, like fasting! Rather, if it were permitted to pain oneself, there would be no Isur to harm oneself. Since one may not pain himself, one may not harm oneself. I.e. since one does not own himself for this, , Lo Yosif applies. The simple meaning of the verse refers to hitting others, therefore, one is not lashed for hitting himself, but the Isur applies. The Ran explains that a oath takes effect on this because the simple meaning of Lo Yosif refers to hitting others.

Ran (Chidushim '23b DH d'Mukim, and on the Rif 11a DH Gemara): A Shevu'ah is not Chal on an Isur Torah even if there are no lashes or Korban for the Isur. In Yoma, we say that it is Chal on Chetzi Shi'ur according to Reish Lakish, who forbids Chetzi Shi'ur mid'Rabanan, since the Torah permits it. R. Yochanan could not say so. Alternatively, R. Yochanan could agree. The Tana holds that a Shevu'ah, which is severe, is Chal on a light Isur (Chetzi Shi'ur) that has no lashes or a Korban. A better answer is that since the Lav is not explicit in the Torah, a Shevu'ah takes effect on it. A proof is from an oath to harm oneself (27a). It takes effect, in a case similar to harming others, i.e. bruising the brain, even though one may not hit himself, for this is not explicit. It is learned from a Drashah. Surely one may not transgress due to his oath. We transgress even Torah law passively due to an Isur mid'Rabanan, and all the more so (we passively transgress an oath) due to an Isur Torah. However, he brings a Korban for transgressing his oath. The same applies to an oath to eat Chetzi Shi'urh of Isur. Perhaps my proof is not solid. Perhaps the Shevu'ah is Chal according to the opinion that permits harming oneself, but the Halachah follows the one who forbids, and one is exempt for Shevu'as Bituy to do so. However, the Rif and Rambam rule that one may not harm himself, but one is liable for transgressing a Shevu'ah to do so, for we do not find a Tana or Amora who exempts.

Poskim

Rema (YD 236:2): If one swore to hit himself, even though he may not do so, the oath takes effect.

Gra (3): We hold like the Rambam says, that one may not harm himself, like the Stam Mishnah (Bava Kama 90b). Even though Ta'anis 11a holds like R. Eliezer, fasting is different. The Ran holds that Tana'im argue about whether or not one may harm himself, but all agree that a Shevu'ah to do so takes effect.

Bach (DH v'Yesh Ledakdek): Why did the Tur specify that a Shevu'ah to harm Ploni is Shav? The Chidush is that if Ploni will permit this, there is no Isur to do so (Bava Kama 93a, and then it is not Shav).

Note: I did not see a proof that then there is no Isur. Surely, if one may not harm himself, he cannot authorize others to harm himself!

Shulchan Aruch (239:6): A Shevu'ah takes effect on something expounded by Chachamim that is not explicit in the Torah, i.e. do or not do a Mitzvah. An oath to transgress a Lav, even mid'Rabanan, does not take effect.

Gra (17): The Rif and Rambam rule that the Shevu'ah takes effect, even though they rule that one may not harm himself.

Shach (20 and Gra 18): The Shulchan Aruch (238:4) rules like the Rambam, that a Shavu'ah takes effect on Chetzi Shi'ur, even though it is an Isur Torah, for it is not explicit in the Torah! Rather, the Shevu'ah takes effect, but one must ask a Chacham to permit it. One may not fulfill it!

Shulchan Aruch (CM 97:15): Even if a borrower stipulated and wrote in the document that the lender may seize him (if he does not pay), it does not help. The lender may not confine him or force him to work for him.

Rema: Some say that even if he cannot pay, if he stipulated to be confined until he pays, he must fulfill his oath.

Gra (48): R. Elazar Hakapar says that even though Nezirus harms a person, he is obligated to fulfill his vow. Likewise, one who swore to harm himself must fulfill it.

Kaf ha'Chayim (OC 571:12): If a worker vowed to fast, it takes effect, because he can quit his job in the middle.