Chovel Beatzmo-The prohibition against damaging or injuring one’s body:^[1]

It is [Biblically^[2]] forbidden to hit one’s body^[3], or to embarrass it^[4], or to pain it in any form of pain as a person does not have possession over his body at all to be allowed to do so.^[5] Thus, it is forbidden for one to hit one’s friend even if the friend gives him permission to hit him.^[6] Furthermore, one is obligated to guard his body to not cause it loss, destruction or injury.^[7] [Some Poskim^[8] learn that this is learned from the verse [Bereishis 9:5] of Veach Es Dimchem Lenafshoseichem Edrosh. However, some Poskim^[9] learn also this prohibition against self-affliction from the verse of “Hishamer.”]

Fasting: Due to the above prohibition, it is forbidden for one to pain his body even through abstaining from eating or drinking something [that he needs].^[10] This is with exception to one who does so for the sake of repentance, in which case this pain is in truth good for him as it saves him from purgatory. Therefore it is permitted to fast for reasons of penitence, even if one’s body is not healthy enough to handle it.^[11] However, one who is healthy and able to handle the fasts, may fast even not out of repentance, but simply to refine his soul for G-d, as there is no better good than this.^[12] Certainly, one may abstain from a certain food for the sake of serving G-d, such as if one is addicted to eating and drinking or other lusts, then he may force himself to abstain completely from these lusts until he returns to a balanced eating habit.^[13]

For the sake of money:^[14] It is permitted for one to cause pain to his body for the sake of Parnasa.

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^[1] Admur Hilchos Nizkeiy Guf Vinefesh Halacha 4 “It is forbidden for one to hit one’s friend even if the friend gives him permission to hit him.  The reason for this is because a person does not have possession over his body at all to be allowed to hit it , or to embarrass it , or to pain it in any form of pain.  ”; Hilchos Shemiras Guf Vinefesh Halacha 14; See Encyclopedia Talmudit Erech “Chovel” pp. 681-682; See Likkutei Sichos Shoftim vol 34 p. 106

[2] Rama Y.D. 236:2; Shach 236:3; 215:11; Beis Yosef Y.D. 236; Rashba 616; Ran Shavuos 27a; 25; Sefer Chassidim 676; See Yabia Omer 1 Y.D. 38; Sefer Marcheshes 20; Shulchan Chaiy Pesicha; See Shemiras Haguf Vihanefesh [Lerner] Mavo Chapter 1:4; See Encyclopedia Talmudit ibid

Other opinions: Some Poskim rule that this prohibition is only Rabbinical. [Meiri Bava Kama 91a] See Encyclopedia Talmudit ibid.

[3] Admur ibid and Hilchos Shemiras Guf Vinefesh Halacha 14; Rama Y.D. 236:2; Shach 236:3; Rambam Chovel 5:1 and Rotzeiach 1:4; Mishneh Bava Kama 90b; Radbaz Sanhedrin 18

[4] Admur ibid; Rashal in Yam Shel Shlomo Bava Kama 8:59

^[5] Vetzaruch Iyun why this reason was not mentioned at all in the next chapter regarding why one must beware from danger.

[6] Admur ibid; Shut Harivash 484; See Tosafos Bava Kama 91b

[7] Admur Shemiras Guf Vinefesh Halacha 14

[8] See Admur Hilchos Nizkeiy Guf Vinefesh Halacha 4; Torah Temima on verse ibid; Sefer Chassidim 675; Levush Y.D. 116:1; Shoel Umeishiv Tinyana 3:122; Aruch Hashulchan Y.D. 116:1; See Encyclopedia Talmudit Erech “Miabeid Atzmo Ladaas” Vol. 38

[9] Binyan Yehoshua on Avos Rebbe Nasan 3:4; Urim vetumim 27:1; Aruch Hashulchan C.M. 420:43’ Kuntrus Likkutei Imrim 13 of Chofetz Chaim

[10] Admur ibid and 155:1; Rambam Deios 3:1; Rivash 186; Rebbe Elazar Hakapar Bava Kama 91b; See Michaber 571:1 and M”A 571:1

[11] Admur ibid; M”A 571:1; Taz 571:1; Reishis Chochmah Shaar Ahava 11:77

[12] Admur ibid; Michaber 571:1; Taanis 11b; M”A 571:1; Tanya Igeres Hakodesh 3; See Likkutei Sichos 21:130 and 132; Vol. 2 p. 531

[13] Admur 155:1; Rambam Deios 2:2; Nedarim 13:23; Shemoneh Perakim 4

[14] Admur Hilchos Nizkeiy Guf Vinefesh Kuntrus Achron 2 who questions the ruling of the Rashal who prohibits it, based on Pesachim 113a and Yaakov Avinu of whom the Torah states that he made his body suffer in cold for the sake of making a living.

Other opinions: Some Poskim rule that it is forbidden for one to shame himself, or damage his body, even for the sake of money. [Rashal in Yam Shel Shlomo Bava Kama 8:59]