Often pictured with a mortar and pestle, the initials "Rx" have a significant meaning on a prescription label. But the exact meaning and even whether "Rx" are initials or symbols are debated in medical circles to this day. There are, however, some common interpretations that are generally supported by most pharmacists and doctors.
In ancient times, apothecaries mixed medications by hand. "Rx" symbolized the Latin word for recipe, "recipere." When written, the Latin term was abbreviated and became standardized to "Rx." In common terms, it meant that the patient was to take the medication as prescribed and mixed by the apothecary.
Symbol, Not Letters, to a Appeal to a God
It's interesting to note that some interpret the "Rx" as not simply an R with a lowerase x. It's actually a full symbol, according to the StraightDope.com. The R loops down and curls below the line with the x slashing through the end. When interpreted in this way, the "Rx" symbolizes a call to Jupiter, the king of the Roman gods, for his aid in ensuring that the prescription would help the patient.
Another interpretation, discussed on Medicine.net, states that "Rx" simply stands in place of the words "to take." This supports the idea that "Rx" is an abbreviation for "recipe." However, this requires the "Rx" to be the heading of the prescription and followed by abbreviations that identify the specifics of the medication's dosage and timing.