When suffering from diarrhea, is there an exemption from prayer and Tefillin?
One who feels a need to relieve his bowels should remove his Tefilin, relieve his bowels and only then daven. In the case he did not do so: If he can hold himself for 72 minutes – the Tfilah is valid and counts, [and in the case one will miss the Time of Tefilah, this is the way to go] If he can't even hold for 72 minutes without relieving himself – the Tefilah does not even count and this person is exempted from Tefilah and Tefilin. [Shulchan Aruch OC 92; 1. See also Mishnah Brura and Biur Halacha ibid]