PO (by mouth)
PO means by mouth (orally). It is short for the Latin per os.
How to read it
The American Heritage Dictionary and the Minnesota Board of Pharmacy both give PO as per os, “by mouth”. ISMP’s 2019 guidelines for electronic prescribing list PO among the acceptable route abbreviations, with IV, IM, SUBQ and NAS, written in capitals without spaces or periods, and suggest “oral” for text meant for patients. The opposite, nothing by mouth, is NPO.
Also searched as: per os, p.o., po meaning, po medical abbreviation.
These pages explain what medical abbreviations and symbols stand for. They are not medical advice and give no doses: a pharmacist or prescriber can explain what any label means for you.
Why “per os” is on ISMP’s list
PO itself is not error-prone on ISMP’s list, but the Latin written out is. ISMP lists “per os” because the os is mistaken for OS, the abbreviation for the left eye (oculus sinister); its recommended wording is “PO, by mouth, or orally”.
Other route abbreviations
The same Minnesota list gives p.r. as by rectum and SL as sublingually, under the tongue. ISMP lists SC (subcutaneous) as mistaken for SL.
More prescription abbreviations
Browse symbols drawn with letters and numbers or squares and rectangles.
Sources
- PO (American Heritage Dictionary entry), The American Heritage Dictionary of the English Language, Fifth Edition, HarperCollins Publishers, PO, abbr. 1
- Partial list of prescription abbreviations, Minnesota Board of Pharmacy, Partial list of prescription abbreviations, Minnesota Board of Pharmacy, p.o.; p.r.; SL
- ISMP List of Error-Prone Abbreviations, Symbols, and Dose Designations (2024), 2024 edition, Institute for Safe Medication Practices, Abbreviations for Route of Administration: Per os
- Guidelines for Safe Electronic Communication of Medication Information (2019), 2019 edition, Institute for Safe Medication Practices, Route abbreviations
Last reviewed September 30, 2026 by Rory Hansen. How we research