Eating Before Prayers
It is forbidden to eat or drink anything except for water in the morning before praying. This rule applies not only on a weekday, but also on Shabbat and Yom Tov. However, foods and drinks that are consumed for medical purposes are permitted. (Shulchan Aruch Orach Chaim 89:3)
The Talmud explains (Berachot 10b) based on the verse, “You may not eat upon the blood,” that it is forbidden to eat before one prays. Doing so is considered an act of arrogance. The Zohar goes so far as to compare one who eats before praying to one who indulges in sorcery. (Zohar Vayakhel 215b, brought in Kaf HaChaim)
It is forbidden even to just taste food and drink before prayers (Rambam Laws of Prayer 6:4, brought in Mishneh Berurah). It is permitted to drink water since there is no sign of arrogance in drinking it. The same applies to tea and coffee, which are usually drunk in order to help one focus in the morning. The early halachic authorities forbade having milk and sugar in tea and coffee. However, nowadays, since most people cannot drink these drinks without milk and sugar, it is permitted (Halichot Shlomo 2:2; also see Kaf HaChaim in the name of Ikrei HaDat for a discussion about adding sugar — perhaps today the same would apply for milk as well). One should nevertheless be careful to not congregate together with other people when drinking before prayers, since it can lead to delaying one’s prayers, and may involve needless conversation when people should be praying.
Though one is not allowed to eat before saying kiddush on Shabbat and Yom Tov morning, this prohibition does not apply until after one prays the Morning Prayers. As such, one is also allowed to drink tea and coffee before prayers on these days as well. (Mishneh Berurah)
One who is sick and must therefore eat before prayers is allowed to eat even foods that are tasty, even though normally these foods are associated with a degree of arrogance. This rule applies also to someone that is not too sick (Mishneh Berurah). It would seem that one who needs to take medicine upon rising and cannot wait until after prayers, and also cannot take them on an empty stomach, would also be allowed to eat before praying.