English term (edited): q.d. quaque die "once a day" vs. o.d. omni die "daily dose"

in practical terms, how it's used: OFTEN quasi-synonyms - the medicine is to be taken every day, in one daily dose.

BUT there IS a difference in meaning

q.d. = quaque die "once a day" / "once every day" that daily dose has to be taken as once / in one dosing.

o.d. = omni die "every day" as in "that much for the whole day", usually but not necessarily - in one dosing.

that could be meaningful difference if the medication is administered continuously (like "i.v." intravenal route) - as administering the daily dose of medicine through a slow continuous drip is hardly taking the daily dose "once a day" - like when taking pills "once a day".

FROM
British National Formulary (BNF)
Key information on the selection, prescribing, dispensing and administration of medicines:

Latin abbreviations
Directions should be in English without abbreviation. However, Latin abbreviations have been used when prescribing.

The following is a list of appropriate abbreviations. It should be noted that the English version is not always an exact translation.

a. c. = ante cibum (before food)
b. d. = bis die (twice daily)
o. d. = omni die (every day)
o. m. = omni mane (every morning)
o. n. = omni nocte (every night)
p. c. = post cibum (after food)
p. r. n. = pro re nata (when required)
q. d. s. = quater die sumendum (to be taken four times daily)
q. q. h. = quarta quaque hora (every four hours)
stat = immediately
t. d. s. = ter die sumendum (to be taken three times daily)
t.i.d. = ter in die (three times daily)

https://bnf.nice.org.uk/about/abbreviations-and-symbols/

Medical Definition of Quaque die (on prescription)
Medical Editor: Charles Patrick Davis, MD, PhD
Reviewed on 3/29/2021

Quaque die (on prescription): Seen on a prescription, qd (or, written with periods, q.d.) means one a day (from the Latin quaque die). The abbreviation is sometimes written without a period in capital letters as "QD". However it is written, it is one of a number of hallowed abbreviations of Latin terms that have been traditionally used in prescriptions to specify the frequency with which medicines should be taken.

Other examples include:

-- b.i.d. (bid or BID) is twice a day; b.i.d.. stands for "bis in die" (which means, in Latin, twice a day).
-- t.i.d. (or tid or TID) is three times a day ; t.i.d. stands for "ter in die" (in Latin, 3 times a day).
-- q.i.d. (or qid or QID) is four times a day; q.i.d. stands for "quater in die" (in Latin, 4 times a day).
-- q_h: If a medicine is to be taken every so-many hours, it is written "q_h"; the "q" standing for "quaque" and the "h" indicating the number of hours. So, for example, "2 caps q4h" means "Take 2 capsules every 4 hours."

https://www.medicinenet.com/quaque_die_on_prescription/defin...


ALSO

q.d. every day (from Latin quaque die)
vs
o.d. every day, usually regarded as once daily (from Latin omni die). Generally written in lowercase.

https://en.wikipedia.org/wiki/List_of_medical_abbreviations:...
https://en.wikipedia.org/wiki/List_of_medical_abbreviations:...