A4993-E3 is a code appearing on a medical drug shown in "Not in Portland". The readable parts of the label are:
TEST SAMPLE A4993-E3
MULTIPLE DOSE VIAL
FOR INJECTION ONLY
In addition, the first dose used in the episode has "Week 6 Batch 7" hand-written on a white taped section of the label.
Juliet was stealing this drug from ex-husband Edmund Burke's laboratory, where she was working. She was injecting her sister Rachel with it to cure Rachel's infertility; Rachel later tells Juliet that the drug worked because Rachel is pregnant.
Rx is an abbreviation for the Latin word "recipe", which = "take" (imperative), used in the pharmaceutical industry for medications and drugs. In this context "Rx only" is taken to mean "prescription only".