"to write down as a direction," mid-15c., from Latin praescribere "write beforehand" (see prescription). Related: Prescribed; prescribing. Medical sense is from 1580s, probably a back formation from prescription.
prescribe pre·scribe (prĭ-skrīb')
v. pre·scribed, pre·scrib·ing, pre·scribes
To give directions, either orally or in writing, for the preparation and administration of a remedy to be used in the treatment of a disease.