Prepositional Phrases are an important component of the English language.
They consist of a preposition (Prep) (Click on the Red book for common prepositions ) and its object (Obj. of the Prep.), normally anoun or pronoun:

In	the	morning
Prep		Obj. of the Prep. (noun)

Prepositional phrases are used to give important information, such as time (at 3 o’clock), place (in the doctor’s office), duration (for a week), etc.
In the particular situation we’re dealing with in this unit: taking about symptoms and taking prescriptions, prepositional phrases come in very handy to indicate amount and frequency. Look at the following examples:

In the morning/afternoon/evening With food/water At night Before/after meals/going to bed Between meals