RX, n. — Green’s Dictionary of Slang

RX n.

[the Rx notation placed on prescriptions, meaning recipe, i.e. of the medical preparation]

(US drugs) a prescription.

1959		W. Burroughs Naked Lunch (1968) 102: Let’s stop over and make him for an RX.
1988		W. Burroughs Foreword in Black You Can’t Win (2000) 10: The old Mexican druggist who filled a morphine Rx after ten shits had snarled it back at me.