Coffee during Shacharis

Shalom!

Thank you for your question.

This question involves two issues: one, drinking during davening, and the second, saying a bracha during davening.

With regard to drinking: ther is a Halchik debate wether eating and drinking during daveining is considered a hefsek, (Ramban Milchamot Pesachim page 103). Nevertheless, we do not drink during davening out of respect for the davening.

With regard to the bracha: reciting a bracha during davening is considered a hefsek, as we find in the Mishneh Berurah, Siman 494 se’if katan 10, relating to the question of whether it is permitted to smell spices while praying. Doing so would require one to interrupt one’s prayers in order to recite the required blessing before smelling the spices. The Mishneh Berurah writes that it is forbidden to do so, because almost any interruption in the course of the prayers is forbidden, including an interruption for another prayer or blessing.

Therefore, in practice, one may not drink during Davening even if he recited the Bracha beforehand.

However if one needs to do so for medical reasons then lechatchilla (ideally) one should recite the bracha on the drink before beginning to pray. This is because, in most cases, the blessing one says over food or drink remains valid as long as one does not leave the room where the blessing was made, if one wasn't able to do so, then if one can he should try to wait untill after "Yishtabach" before "Yotser or" to recite the bracha. 

Wishing you much success.

See:  Shulchan Aruch Harav, OC 494:17; Magen Avraham 494:9; Kaf Hachaim, OC 494:57.

Azamroh Lishmecho Issue # 322-323