Oral administration (per os) is a route of administration where a substance is taken through the mouth.
Per os (pron.: /ˌpɜrˈoʊs/; P.O.) is an adverbial phrase meaning literally from Latin "by mouth" or "by way of the mouth." The expression is used in medicine to describe a treatment that is taken orally. The abbreviated P.O. is often used on medical prescriptions. P.O. is also occasionally (and grammatically incorrectly; os is a neuter noun of the 3rd declension and thus the accusative is the same as the nominative) rendered per orem, which is sometimes corrupted to per oram.
Oral administration generally includes buccal, sublabial and sublingual administration, as well as enteral administration and that through the respiratory tract, unless made through e.g. tubing so the medication is not in direct contact with any of the oral mucosa.
Facilitating methods 
Concomitant ingestion of water facilitates in swallowing tablets and capsules. If the substance has disagreeable taste, addition of a flavor may facilitate ingestion. Substances that are harmful to the teeth are preferably given through a straw.
Dating and time-stamping of pharmaceutical ingestion can be automatically documented by concomitantly administrating an ingestible sensor, whose consumption can be detected by an externally worn adhesive monitor.
See also