Only a few decades ago, all entries in medical records were in handwriting. Necessary haste made abbreviations very desirable. Rx is just one of many. "s" with a line over it meant "sine = (Latin)without." "c" with a line over it meant
(Latin) cum = with. "q" meant each or every. I wonder if there is anywhere a collection of them. I hope wofahulicodoc,Alex Wiliams and doc_comfort will make additions to this.

http://mlanet.org/resources/medspeak/medshort.html

>http://mlanet.org/resources/medspeak/medshort.html

ha! Anu's liver has been chopped, qv.

That sneaky underline escaped my notice.
I know some not included there. E.G.: qs ad = quod sufficit = enough to (yield desired volume). And note there is not a single one with bar on top. Some of them I never happened to see. It used to be a pain when new people imported ones
nobody else had ever seen before, or worse still invented some.
Sorry to have minced the maestro's hepar.

chopped

actually, cut and pasted...

only a few decades, ago all entries in medical records were in handwrithing
as opposed to nowadays when only most of them are. Some of the longer ones are dictated but we still use lots of x's.
Dx =diagnosis
Hx =history
Abx =antibiotics.
We even use them in speech. The other day I heard a nurse say "We D/c'd the qHS IV and started oral Abx q4h."

?

All day long I've been trying without success to remember
something about the "tail" of the R being looped back over itself. There was a story to it, but I have no idea how to find it now. Just that it originally was not an "X". But I am so much older than my colleagues (if I may so call them)
that a lot of the old mystique was long gone when they took pharmacology.I'll bet none of them ever made oleum percomorphum. Not even sure how to spell that now. Or pills of aloes. There ain't no such thing as a pill any more. Pills were made by triturating ingredients in a mortar with a pestle, and taking a small amount of the goo and rolling it into a small ball or ovoid between both palms. Remember Oliver Wendell Holmes,Autocrat of the Breakfast Table, and a notted phsysician, saying that if all the medications were thrown into the sea it would be all the better for mankind, and all the worse for the fishes.

I have a vague idea that the Rx was somehow meant to represent the eye of Horace, or something like that.

I think most of us now just accept that it's short for Recipe, which I'm pretty sure it's not.

Can I be any more uncertain?

AHD4 says it is indeed an abbrev. of the Latin recipe, though I still don't get where the x comes from. And, the R is indeed capitalized, I learned:

http://www.bartleby.com/61/6/R0360600.html

where the x comes from

Way I got it in my JDM® it's not so much an x as just a cross bar on the tail of the R. Probably to indicate that there's more to it than just the R.

http://www.health.ufl.edu/shcc/rx.gif

Here's what Straight Dope has to say:
http://www.straightdope.com/mailbag/mrx.html

Incidentally, I'm reminded of a Puerto Rico guy in my class getting bullshit when lecturer said in South America, the sign for pharmacy was a big "606". The symbol for salvarsan, the first cure for syphilis. (Because it was the six hundred and sixth compound in a series of formulas
Paul Ehrlich had tested for treatment of infedctions. A Jap student of his named Kitasato, discovered it would cure rabbit syphilis. But he never got any credit, so far as I know. Everybody thinks Paul Ehrlich was the discoverer. I read a book by Paul Ehrlich's ex-secretary, who said Paul Ehrlich didn't even want the Jap in his lab, and had him retest all previous formulas that he had found to have no value,just to get the Jap out of his way. But the Jap was the first person able to infect babbits with syphilis. But he didn't get the Nobel Prize.