Latin expressions. What are they and why are they still used?
Apart from the standard prescribing abbreviations, i.e and e.g. it is good practice not to use these in the medical record in these days of patient access and ever fewer understanding Latin, even such common ones as de novo.
Perhaps the most obvious Latin abbreviation is the capital R with a stroke across the downstroke of the R thus . This is the abbreviation for “R.I.” = Recipe Jove = Take thou in the name of Jove. This prayer is said to have been the most powerful ingredient in many mediaeval medications. It was removed from the standard NHS prescription only in the 1990s. It is still to be seen in pharmacies, often as part of signage or advertising.
Greek was the language of international culture even after the Greek Empire fell to the Romans. It was the common language in the first and second centuries. It was the language in which the Greek philosophers and playwrights communicated. The New Testament was written in the common or “koine” Greek of the marketplace rather than the high “classical” of philosophers. With the spread of the Roman Catholic Church Latin became common throughout Europe.
For many centuries Latin was the language of science in which educated men (and some educated women) expressed their thoughts to others. 
We use many Latin and Greek words and phrases still. e.g. referendum, extortioner, bicycle, leucocyte . Many medical words in European languages are from the same root which can be of use to a British GP trying to understand a letter from a hospital doctor in Ibiza or Athens. Sometimes the numbers in laboratory reports can help to clear doubt.
The inside back cover of the British National Formulary gives a number of the commonest used for prescribing .
Why did they persist for so long?
The phrase "Sig tabs ii tid pc"
can be written in English as an instruction to the pharmacist.
Label this "Take two tablets three times daily after meals.”
If you have a large number of prescriptions to write which form would you choose.?
It is only with the widespread use of computers that this can be reduced to 2 or 3 key clicks – and be more legible.
a.c. – ante cibum - before food
a.m. – ante meridiem - before mid day i.e. in the morning
ab initio - from the beginning
actum est - it is finished
ad extremum - to extremes
ad hoc - with respect to this matter
ad infinitum - to infinity
ad nauseam - till disgust arises
alter ego - one's second half
a posteriori - from effect to cause, empirical reasoning
a priori - from cause to effect, from experience
b.d. – bis die - twice daily
bona fide - in good faith
caveat emptor - the buyer must face the risk
compos mentis - of sound mind
cui bono ? - to whose benefit?
de novo - anew
durante vita - while life lasts
e.g. - exempli gratia - for the sake of example
ex offico - by virtue of his office
i.e. - id est - that is
in memoriam - to the memory of
in pertetuum - for ever
in silico – in silicon (Latin expression, modelled after in vitro and in vivo used in Bioinformatics to indicate the experiment was done using the computer alone.
ISQ – in statu quo - In status quo - in the same state, unchanged
inter alia - amongst other things
in vacuo - in empty space
in vitro – in glass i.e. in a test-tube, rather than in the living organism
in vivo - 'within the human body', as opposed to, say, in vitro
ipso facto - by that very fact
mane - morning
magnum opus - masterpiece
mea culpa - my fault
modo praescripto - in the manner directed
modus operandi - the manner of working
mutatis mutandis - after making the needful changes
N.B. – Nota Bene - note well – this is important
non sequitur - it does not follow
nocte – at night
o.d. - omni die - daily
o.m. – omni mane - every morning
o.n. – omni nocte - every night
obiit - he died
odium medicum - professional hatred of rivals (could not resist despite rare use in medicine !)
p.c. – post cibum -after food
p.m. – post meridiem - after the middle of the day ie after noon
PR (sometimes written pr) – per rectum – implied is (“examination”). (May be used as noun or verb.)
p.r.n – pro re nata - as required
pace - by leave of
q.d.s. - quater die summendus - four times daily
q.i.d - quarter in die - four times daily
q.q.h. - quarta quaque horas - every four hours, i.e 6 times daily.
quae nicent docent - we learn by painful experience
quid pro quo - something in return
stat – statim -immediately
sine die - no day fixed
sine qua non - indispensible
sub judice - under consideration
t.d.s – ter die summendus - take three times daily
t.i.d. – ter in die - three times daily
ultra vires - beyound one's powers
Roman counting ( I,II,III,IV,V etc) is seldom now seen outwith civic memorial tablets or Copyright notices at the end of films, books & TV programs: the use of lower case Latin numerals is seen in the preface to the BNF. It may be also seen in nursing notes e.g. “Bowel moved ii/o”. The advantage of this was that if the patient’s bowel moved again one simply added another i as a “hash mark” rather than score out a 2 and replace it with 3. This is also now frowned upon but older doctors and nurses may use it still. Since they are senior they tend to get away with it.