From Brewer:
R in prescriptions. The ornamental part of this letter is the symbol of Jupiter , under whose special protection all medicines were placed. The letter itself (Recipe, take) and its flourish may be thus paraphrased:“Under the good auspices of Jove, the patron of medicines, take the following drugs in the proportions set down.” It has been suggested that the symbol is for Responsum Raphaelis, from the assertion of Dr. Napier and other physicians of the seventeenth century, that the angel Raphael
imparted them.