An R with a cross-stroke indicating an abbreviation (of recipe, response, retrograde etc). In the case of a prescription, abbreviating Latinrecipe(“take this”, second-person singular imperative of recipiō); it is sometimes typeset as, or even interpreted as, a digraph Rx. Compare also Dx(“diagnose, diagnosis”) and Hx(“history”).
1624, Philip Barrough [i.e., Philip Barrow], “Of Making Bolus”, in The Method of Physick, Contaning[sic] the Cavses, Signes, and Cvres of Inward Diseases in Mans Body, from the Head to the Foote. Whereunto is Added, The Forme and Rule of Making Remedies and Medicines, which Our Physitions Commonly Vse at this Day, with the Proportion, Quantity, and Names of Each Medicine, 6th edition, book VII, London: Imprinted by Richard Field, dwelling in great Woodstreete, →OCLC, page 397:
Bolvs in Engliſh is called a morſell. It is a medicine laxatiue, in forme and faſhion it is meanely whole, and it is ſwallowed by little gobbets. […]℞Medulla Caſſiæ fiſtulæ [n]ewly drawne, ℥. j. or ʒ. x. the graines, that is, the kernels, of Barberies, ℈. ß and with Sugar roſet [sugar compounded with rose petals], make a bole.