Where did the symbol Rx originate in the writing of prescriptions?

It is an abbreviation for the Latin word "recipere" or "recipe," which means "Take, thou."

The symbol Rx is derived from the major lines in the symbol of the Eye of Horus. Horus was an Egyptian god, the god of Nekhen, a village in Egypt, and god of the sky, of light, and of goodness. He was the son of Isis, the nature goddess, and Osiris, the god of the underworld. Osiris was murdered by his evil brother Seth, the god of darkness and evil. Horus sought to avenge his father’s death by challenging his uncle Seth to a fight. Seth cut out Horus’s eye, but Thoth, a god associated with wisdom and compassion, magically restored the eye. Horus did defeat Seth, finally. Horus’s eye, also called the wadjet eye, became a symbol for health. The Egyptians considered it a symbol of good and restored health.
The symbol was passed along through the ages. As William Osler wrote in 1910, “In a cursive form it is found in mediaeval translations of the works of Ptolemy the astrologer, as the sign of the planet Jupiter. As such it was placed upon horoscopes and upon formula containing drugs made for administration to the body, so that the harmful properties of these drugs might be removed under the influence of the lucky planet.”
There is another theory of Rx’s origin. In that version, Rx is an abbreviation for the Latin word recipere, which means “take” or “take thus.” Long ago, this would not have been a direction to a patient but to a pharmacist, preceding the physician’s “recipe” for preparing a medication.
That may be, but the shape of the symbol is a strong argument in favor of the Eye of Horus as its origin.
If you look closely at the major lines of the eye of Horus, you can see the elements of the symbol Rx.

Also connected to the Egyptian god Horus and the symbol for him - the famous eye hieroglyphic. This is purely conjecture but seems to carry some weight historically as Horus was known as a healer (some calling him the 'father of pharmacy'!).

Alan Macpherson