p.o.: Abbreviation meaning by mouth, orally (from the Latin 'per os', by mouth). One of a number of hallowed abbreviations of Latin terms that have traditionally been used in prescriptions. Some others: a.c. = before meals (from 'ante cibum', before meals) b.i.d. = twice a day (from 'bis in die', twice a day) gtt. = drops (from 'guttae', drops) p.c... Found on http://www.medterms.com/script/main/art.asp?articlekey=8310