praescriptio

Praescriptio in Latin means a writing before, a preface, a rule or direction written in advance, from prae- (before) + scribere (to write). In Roman law, praescriptio was a preliminary instruction to the judge — the rule applied before the case was heard. The medical meaning — an instruction written by a physician to a pharmacist — emerged in medieval Latin. The doctor writes before the pharmacist acts. The word preserves the chain of command.

The prescription symbol ℞ comes from the Latin recipe, meaning 'take' — the imperative form of recipere (to take, to receive). Medieval prescriptions began with the word recipe: 'Take these ingredients in these proportions.' The abbreviation ℞ evolved from the letter R with a stroke through its leg. The symbol is not an R with an x. It is a Latin command: take this.

Prescription medicines — drugs available only by doctor's order — were formalized in the twentieth century. The 1951 Durham-Humphrey Amendment in the United States established the legal distinction between prescription drugs and over-the-counter drugs. Before this, the distinction was informal. The amendment codified the authority relationship embedded in the word: some medicines require a written order from an authorized person.

In legal English, 'prescription' has a different meaning — the acquisition of a right through long, uninterrupted use (prescriptive easement, statute of limitations by prescription). This preserves the original Latin sense of a rule established in advance by time and custom. The medical and legal meanings diverged from the same root: something determined before the current action.