Time Based Prescription Abbreviations

Abbreviation	Meaning / Intended Meaning	Notes About Confusion
a	before
A.M.	morning
ac	before meals
achs	before meals and at bedtime
ad lib	freely; as much as desired
alt.	alternate
alt. h.	every other hour
am, A.M.	in the morning; before noon
ante	before
ap	before dinner
ATC	around the clock
bid, BID	twice a day
BT	bedtime	In U.S., 'hs' or 'HS' is more commonly used for bedtime
h, or hr.	hour
hs or HS	at bedtime, hours of sleep	Do not misinterpret as 'half-strength'
mane	in the morning
n or noct.	in the night
noct. maneq.	night and morning
o.d.	once per day	Preferred in the UK; Can also mean "overdose" or "right eye"; better to spell out
p	after
p.r.n., prn	as needed
pc	after meals
PM	evening
q	every
q12h	every 12 hours
qd, q1d	daily
q2h	every 2 hours
q3h	every 3 hours
q4h	every 4 hours
q6h	every 6 hours
q6PM, etc	every evening at 6 PM
q8h	every 8 hours
qam	every morning
qd, QD	every day	Mistaken as q.i.d; Instead write "daily" or "every day" per Joint Commission's "Do Not Use" List of Abbreviations
qh	every hour
qhs	each night at bedtime	Can be confused with "qh" meaning every hour; better to spell out
qid	four times a day
qn	Nightly or at bedtime	Not commonly used in U.S.; 'hs' more common for bedtime
qod, QOD, q.o.d	every other day	Can be mistaken as qd (daily) or qid (four times daily); Instead spell out "every other day" per Joint Commission's "Do Not Use" List of Abbreviations
rep	repeats
stat	immediately
TID, t.i.d.	three times a day
tid ac	three times a day before meals
TIN, t.i.n.	three times a night
TIW, tiw	3 times a week	Spell out; can be confused with TID (three times a day)
yr	year