Definition of a.c.

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a.c.: Abbreviation on a prescription meaning before meals; from the Latin "ante cibum", before meals. This is one of a number of hallowed abbreviations of Latin terms that have traditionally been used in writing prescriptions.

Some others:

b.i.d. = twice a day (from "bis in die", twice a day)
gtt. = drops (from "guttae", drops)
p.c. = after meals (from "post cibum", after meals)
p.o. = by mouth, orally (from "per os", by mouth)
p.r.n. = when necessary (from "pro re nata", for an occasion that has arisen, as circumstances require, as needed)
q.d. = once a day (from "quaque die", once a day)
q.i.d. = four times a day (from "quater in die", 4 times a day)
q._h.: If a medicine is to be taken every so-many hours (from "quaque", every and the "h" indicating the number of hours)
q.h. = every hour
q.2h. = every 2 hours
q.3h. = every 3 hours
q.4h. = every 4 hours
t.i.d. = three times a day (from "ter in die", 3 times a day)
ut dict. = as directed (from "ut dictum", as directed)