הלכה על ברכות 20:30

It is forbidden to busy one's self with his own needs or to get on the road before praying the prayer of 18 [i.e. the Amidah] (and there are those who are lenient [to allow this] after they have said some of the blessings before saying Baruch She-amar, but is good to be stringent in this - Terumat HaDeshen 18) nor may one eat or drink, but one may drink water before prayer, both on a weekday and on Shabbat or Yom Tov, and likewise, foods or drinks for medicinal purpose is allowed.