AC and HS (before meals and at bedtime)
AC means before meals, from the Latin ante cibum. HS means at bedtime, from hora somni. Written together, AC and HS (or ACHS) mean before meals and at bedtime.
How to read it
The American Heritage Dictionary and the Minnesota Board of Pharmacy give a.c. as ante cibum, “before meals”, and the Minnesota list gives h.s. as hora somni, “at bedtime”. Its opposite number for meals is p.c., post cibum, “after meals”. Joel Weaver, editor of Anesthesia Progress, listed a.c., p.c. and h.s. in 2006 among the Latin abbreviations modern prescribing discourages, in favour of “before eating a meal”, “after eating” and “at bedtime”. ISMP’s 2019 guidelines for electronic prescribing accept HS written in capitals as a frequency abbreviation. Every night at bedtime is QHS.
Also searched as: achs meaning, achs medical abbreviation, ac medical abbreviation, ante cibum, hs 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.
HS and half-strength
ISMP’s 2015 and 2021 lists of error-prone abbreviations had HS for half-strength and hs for bedtime, each mistaken for the other. The 2021 list recommended “half-strength” for the first and HS in capitals for bedtime. The Minnesota Board of Pharmacy’s list notes the same double meaning of h.s.
AC and AD
One letter away, a.d. is auris dextra, the right ear, on the Minnesota list. ISMP lists AD, AS and AU (right, left and each ear) as error-prone because they are mistaken for OD, OS and OU, the eyes.
More prescription abbreviations
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Sources
- a.c. (American Heritage Dictionary entry), The American Heritage Dictionary of the English Language, Fifth Edition, HarperCollins Publishers, a.c., abbr.
- Partial list of prescription abbreviations, Minnesota Board of Pharmacy, Partial list of prescription abbreviations, Minnesota Board of Pharmacy, a.c.; a.d.; h.s.; p.c.; not recommended
- It’s time to throw out old-fashioned Latin abbreviations, Joel M. Weaver, Anesthesia Progress 2006;53(1):1–2, American Dental Society of Anesthesiology, Paragraph 4
- ISMP List of Error-Prone Abbreviations, Symbols, and Dose Designations (2021), 2021 edition, Institute for Safe Medication Practices, Abbreviations for Frequency/Instructions for Use: HS, hs; Route: AD, AS, AU
- Guidelines for Safe Electronic Communication of Medication Information (2019), 2019 edition, Institute for Safe Medication Practices, Frequency abbreviations
Last reviewed September 30, 2026 by Rory Hansen. How we research