1) Tefillin may not be worn if one does not have full control over his bodily functions. Therefore, a person experiencing a stomach virus, or who is taking a laxative or diarrhetic medication, is exempt from the obligation of tefillin, and is in fact forbidden from wearing tefillin. This applies even if he feels well and is fully functional. It is forbidden for a person in such a condition to wear tefillin, except for the recitation of Shema and the Amidah if he determines that he can maintain bodily cleanliness throughout this recitation.[199]

2) A person who suffers from an illness other than a stomach virus, and experiences discomfort to the point where he cannot settle his mind, is exempt from the obligation of tefillin, as he will likely be unable to keep his attention directed toward his tefillin. If he wishes, however, he may wear tefillin despite his condition. The prohibition of heseh ha’da’at, diverting one’s attention away from the tefillin while wearing them, applies only to distractions involving frivolity and the like. Therefore, in a case where one’s mind is distracted due to physical discomfort, he may wear tefillin if he so wishes. If a patient experiences discomfort but feels capable of maintaining focus on his tefillin, he is obligated to wear tefillin[.200]

3) One who feels unable to restrain himself from passing air should not wear tefillin. If, however, he feels capable of restraining himself during the recitation of Shema, then he should don tefillin after concluding the berachah of ahavat olam (that immediately precedes Shema), with a berachah. At that point he should recite Shema and then remove his tefillin.[201]

4) If one feels capable of restraining himself until he can remove his tefillin shel rosh, but is unsure whether he can restrain himself until he removes both tefillin, then he may nevertheless wear both. If it happens that he passes air after he removes his shel rosh but before he removes his shel yad, no violation is committed since the requirement of maintaining bodily cleanliness applies mainly to the tefillin shel rosh.[202]

5) One who does not have control over his bladder, or who has an external catheter to collect his urine, may nevertheless wear tefillin, provided that his outer garments are clean and do not emit any foul odor.[203]

6) The obligation of guf naki — bodily cleanliness while wearing tefillin — also requires that one refrain from thinking thoughts relating to sexual desire while wearing tefillin.[204] A person who feels unable to refrain from such thoughts is nevertheless required to wear tefillin, and he should make every effort to subdue his evil inclination and avoid thoughts of immorality. Still, he should wear his tefillin only during prayer, and not at other times.[205]

7) One who has no garment with which to cover his chest should not wear tefillin, as it is disrespectful to the tefillin to wear them with one’s chest exposed.[206]

8) Whenever a person is exempt from the tefillin obligation due to some temporary condition, such as illness or the absence of a shirt, if the condition passes that day he must ensure to wear tefillin. Thus, if a person did not wear tefillin in the morning due to a stomach flu and recovers from his condition before sundown, he must wear tefillin.

9) Women are exempt from the obligation of tefillin, just as they are exempt from all מצוות עשה שהזמן גרמא, meaning, mitzvot that apply only in certain times (and tefillin is not worn on Shabbat or festivals). A woman who wishes to nevertheless don tefillin should be discouraged from doing so.207 (As with regard to all mitzvot, a tumtum or adroginus — somebody whose genitals are concealed, or who features both male and female organs — must wear tefillin.208)