I might think that one who took an oath to harm himself etc.
The Gemara (Shavous 27a) teaches that one who takes an oath to harm oneself is obligated to observe that oath since it falls into the category of elective activities. Tosfos explains that our Gemara follows the opinion that maintains that one is permitted to wound himself. The debate of whether a person may wound himself is recorded in the Gemara Bava Kamma (91b). Rambam rules in accordance with the position of R’ Elazar HaKapar who disagrees with that Gemara and maintains that one is not permitted to inflict harm upon himself. Furthermore, even one who, out of piety, refrains from meat and wine is behaving improperly and is called a sinner. Rashba explains that since the Gemara Taanis (11a) relates that Shmuel, an Amora, followed the opinion that inflicting harm upon one’s self is prohibited it must be that this opinion represents the halachically-accepted opinion.
Tur cites the opinion of Ramah who holds that a person is permitted to wound himself. Shitah Mekubetzes explains that the source for this ruling is the practice of R’ Chisda recorded in the Gemara Bava Kamma who would lift his garment so that it should not rip even though it meant that thorns would scratch his skin. Yam Shel Shlomo, however, adds that even according to Ramah one is only permitted to wound himself if there is a need, similar to R’ Chisda’s practice, but when there is no necessity for a person to wound himself it is prohibited.
Shulchan Aruch also rules that it is prohibited for one to harm oneself. Shulchan Aruch Harav sets the guidelines of the prohibition as follows. “One does not have authority over his own body, not to strike oneself, not to embarrass oneself, nor to cause oneself pain. Even pain that is the result of a person denying himself food and drink is prohibited. The only exception to this rule is a person who, as an act of penitence, refrains from eating or drinking something since in such a circumstance it is considered a positive act to save oneself from punishment.”
Meiri
The prohibition to harm oneself is from the Torah from the verse (Bereshis 9,5) “But I will demand (an account) for your blood (if you take) your own life (by suicide)” This establishes that you cannot take your life. However, to harm yourself is Divrei Sofrim and not from the Torah.
Rashba
Prohibition is from the Torah, but does not expound the source.
Chidushei HaRan
Prohibition is from the Torah. A nazir at the conclusion of his nezirus brings a sin offering. According to Rabbi Elazar HaKafar the person sinned by paining himself by prohibiting items.
HaTumim
Devarim 4,9: "רק השמר לך ושמר נפשך מאד" The word השמר indicates a negative mitzvah of not harming oneself.
Avi Ezri
Devarim 25,3: "ארבעים יכנו לא יוסף" Do not add on to harm this person or even yourself.
Aruch LaNer
Devarim 20,19: "לא תשחית" If one does not destroy property (even while at war) one for sure does not harm oneself.
Lechem HaMishneh
Prohibition to harm oneself is a decree from Chaza”l.
(Lubavitcher Rebbe): A person is not the owner of his body (one cannot harm oneself by admission); similarly, money appears to be disconnected from HaShem, but the law allows one to display ownership (to use for good or for bad).
