1. Stomach issues/Illness/Pain/Disturbance of mind:[1]

Diarrhea:[2] One who has a stomach illness [or took a laxative, which leads to constant diarrhea[3]] is exempt from wearing Tefillin [until his stomach returns to proper function that day, or the next day].[4] This applies even if he is not in any pain and walks normally in the streets and marketplaces.[5] He is not to be stringent upon himself to wear Tefillin despite his illness. Nevertheless, he may be stringent upon himself to wear them during Kerias Shema and Tefilah if he feels that he is able to guard himself in cleanliness [and refrain from passing gas during those times].[6] He is to wear them with a blessing between the blessing of Ahava and Shema.[7]

Flatulence:[8] One who feels gassy in the morning and is releasing flatulence and is sure [or estimates[9]] that he is unable to restrain himself from releasing gas until after Shemoneh Esrei, then he is not to wear Tefillin [until his stomach returns to proper function that day, or the next day].[10] If, however, he is confident that he can restrain himself for the recital of Shema, then he is to wear Tefillin with a blessing between the blessing of Ahava and Shema [and recite the Shema] and then immediately remove the Tefillin.

Illness/sick/pain:[11] One who is sick with other illnesses [other than stomach illness] then if he feels pain to the extent that he is unable to concentrate on the Tefillin due to the pain, he is exempt from wearing Tefillin.[12] The same applies to one who is not sick but is in pain [to the above extent] that he is unable to concentrate on the Tefillin.[13] If, however, he does not feel pain to this extent then he is obligated to wear Tefillin despite his illness.

Pain/Cold weather:[14] If one is in pain, even if he is simply suffering due to the cold, if he cannot gather his mind to concentrate on the Tefillin, he is exempt from Tefillin.[15]

One who is stressed and anxious:[16] If one’s mind is not calm and settled, if he cannot gather his mind to concentrate on the Tefillin, he is exempt from Tefillin.[17]

Obligation to calm the mind:[18] In all the above cases of [illness and] pain, if one is able to clam and relax his mind to the point that he can concentrate on Tefillin, then he is obligated to calm his mind and wear Tefillin.[19]

[1] Ketzos Hashulchan 8:21

[2] Admur 38:1; 80:3; Michaber 37:1; Chulin 110a

[3] M”B 38:1 in name of Chayeh Adam

[4] The reason: As Tefillin require a clean body. [Admur 80:1; Shabbos 49a]

[5] Admur ibid; Rama ibid; M”A 37:1

The reason: As he is unable to guard himself properly while wearing the Tefillin, to guard himself in cleanliness [to refrain from passing gas].

[6] Admur ibid; M”A ibid

[7] See Admur 80:3 regarding flatulence

[8] Admur 80:3; Michaber 38:2; Rosh 4:8

[9] See Piskeiy Teshuvos 38:1

[10] The reason: As Tefillin require a clean body. [Admur 80:1; Shabbos 49a] However, some Poskim rule that the prohibition against flatulence in Tefillin is only when one is sleeping, or intentionally, and not when one is gassy. [Toras Chaim Sofer 38:1 in name of Mor Uketzia; Kaf Hachaim 38:5]

[11] Admur 38:2; Rama 38:1; Michaber 38:9; Rambam Tefillin 4:13

[12] The reason: As it is forbidden to remove one’s mind from the Tefillin. [Admur ibid; M”B 38:5]

[13] Admur 38:8; Piskeiy Teshuvos 38:2 based on Mikdash Me’at 39:10

[14] Admur 38:8; M”A 38:11

[15] The reason: As it is forbidden to remove one’s mind from the Tefillin. [Admur ibid; M”B 38:5]

[16] Admur 38:8

[17] The reason: As it is forbidden to remove one’s mind from the Tefillin. [Admur ibid; M”B 38:5]

[18] Admur 38:8; M”A 38:11; Bach

[19] The reason: This is learned from the law regarding a mourner, in which case a mourner is obligated to wear Tefillin from the 2^nd day and onwards despite being in pain and not having calmness of mind. This is due to that he is required to calm his mind and ignore the pain [for the time in which he wears the Tefillin]. [Admur ibid; M”A ibid]

[20] P”M 38 A”A 1; M”B 38:5; Piskeiy Teshuvos 38:2 footnote 18

The reason: As there is no prohibition against removing one’s mind from the Tefillin if he remains with fear of Heaven and is not involved in lightheadedness, and it is only a Mitzvah Min Hamuvchar to do so. [ibid; See Admur 28:1] Vetzrauch Iyun, as Admur explicitly writes that the reason for the exemption is because it is forbidden to remove one’s mind from the Tefillin, which implies that even this type of removal of one’s mind is forbidden. Vetzaruch Iyun how to coalesce this with the ruling of the P”M ibid.

[21] Aruch Hashulchan 38:14; Piskeiy Teshuvos 38:2

[22] P”M ibid; See Admur ibid “He should not be stringent upon himself”

[23] Piskeiy Teshuvos 38 footnote 1

[24] Mikdash Me’at 38:1; M”B 80:4 in name of P”M

[25] M”B ibid

The reason: As if he can only wear them for a mere moment, then seemingly this person is exempt from Tefillin. [Shaar Hatziyon 80:9]

[26] M”B ibid

[27] Minchas Yitzchak 6:13 based on Ashel Avraham 38 and 28; Piskeiy Teshuvos 38:1

Wearing with Chatzitza: See Piskeiy Teshuvos ibid who writes to wear it with a Chatzitza, Vetzaruch Iyun as according to the Rashba and other Rishonim [brought in Chapter 2 Halacha 13A], one is Yotzei even when he wears the Tefillin with a Chatzitza, and hence it would be considered that one is flatulating while wearing Tefillin. Hence, it would seem that the only available option is to wear them out of place, and so indeed writes the Minchas Yitzchak ibid, and makes no mention of Chatzitza at all in his Heter.

[28] Shevet Hakehasi 4:24

The reason: If the patient cannot control his bowels and hence contains diapers and the like, then he may not have Tefillin placed on him while he is dirty. Furthermore, a person in such a state is similar to one who is sleeping, in which case it is forbidden to wear Tefillin.

[29] Piskeiy Teshuvos 38:4

[30] Piskeiy Teshuvos 26:2

[31] M”A 27:11; P”M 27 A”A 11; M”B 27:29; See Admur 38:2; Rama 38:1