The “R” in “Rx” stands for the Latin word “recipe,” meaning “take,” and the first doctor to use “Rx” used it as a verb with the same meaning, “Rx two aspirin” being equivalent to today’s “Take two aspirin.” (The word “recipe” had had the same function from the 13th through the 17th centuries.) Those two letters were a 19th-century take on a 16th-century symbol, the letter R with a line through its slanted leg—the line signaling that the “R” is functioning as an abbreviation. It wasn’t till the early 20th century that “Rx” came to be used as the noun we know today. As for the noun “recipe,” it followed the same trajectory, referring to a medical prescription for about 100 years before it developed its connection with cooking in the early 17th century.

Merriam- Webster (source)