The simple, quick answer--but not the only (or most romantic) one--is that Rx is an abbreviation for the Latin word "recipere" or "recipe," which means "Take, thou." In the days before manufactured drugs, apothecaries (who were also doctors) would write out a formula for medications. They would mix up and compound ingredients to make drugs or remedies. Not until well into the 19th century was the distinction between the apothecary/pharmacist as a compounder of medicines and the physician as a therapist generally accepted. According to the Pharmaceutical Handbook (nineteenth edition, 1980), the Latin abbreviation Rx is completed by some statement such as "fiat mistura," which means "let a mixture be made," sometimes abbreviated to f. m. or ft. mist. or fait mist.). Pharmacists required a knowledge of compounding until recently. In 1920, 80% of prescriptions were compounds mixed in the shop. In the 1940s the number of prescriptions requiring compounding had declined to 26%, and then to 1% or less in 1971.

Other abbreviations with "x"s are used by medical people too; sx = signs and symptoms, tx = treatment or transplant, hx = history, and dx = diagnosis. But Rx isn't just a normal R and x. It's a symbol (not available in the ASCII list) of an italic R with a leg that hangs down below the line with an X line through it. This brings me to theory number two, from the book Devils, Drugs, and Doctors, written in 1931: "Rx is not, as is frequently supposed, an abbreviation of a Latin word meaning recipe or compound, but is an invocation to Jupiter, a prayer for his aid to make the treatment effective...sometimes in old medical manuscripts all the R's occurring in the text were crossed." In other words, the Rx symbol was a corruption of the ancient symbol for the Roman god Jupiter. If you're an astrology fan, you know this symbol which has a very similar crossed leg at the bottom right.

Another theory goes, the Egyptian god Horus was the "father of pharmacy." The Medieval Latin word "pharmacia," a medicine, comes from the Greek word "pharmakeia," use of drugs, from "pharmakon," drug or remedy. The real history of pharmacy begins with the Chinese (the great Chinese herbal compilation "Ben cao" was attributed to the emperor Shennong in 2700 BC) and the Egyptians. The Ebers papyrus, circa 1550 BC, listed 700 drugs and 800 compounds, and is thought to be a copy of the even more ancient books of Thoth (3000 BC). One source I saw suggested that that there is a connection between the word "pharmacy" and the Egyptian term ph-ar-maki ("bestower of security"), "which the God Thoth, patron of physicians, conferred as approbation on a ferryman who had managed a safe crossing." Whoa. Hey Jupiter, scoot over for Horus.

The Greek tradition is considered the beginning point of European pharmacy, but it drew on Egyptian and Asian sources. "Horus was the son of two of the main gods in Egyptian mythology, Isis and Osiris. Horus had an evil uncle (Seth) who murdered Osiris, the father of Horus. Horus did battle with Seth to avenge his father's murder. During the fight, Seth plucked out Horus' left eye and tore it apart. Thoth (god of wisdom and magic) found the eye, pieced it together and added some magic. He returned the eye to Horus, who in turn gave it to his murdered father Osiris, thereby bringing him back to life.

"The Eye of Horus (or 'udjat') became a powerful symbol in ancient Egypt. It was worn as an amulet to ensure good health and ward off sickness. The Eye of Horus is depicted as a human eye and eyebrow, decorated with the markings seen under the eyes of falcons since Horus had the head of a falcon. The right eye represented the sun and the left eye the moon.... The left eye is the origin of the pharmacist's symbol for prescription, 'Rx' [my emphasis].

Sounds good to me. Just for laughs, ask your local pharmacist if s/he knows about the origin of Rx.