What symbol must be displayed on the label of a prescription drug?

The correct answer is that the label of a prescription drug must prominently display the symbol "Rx." This symbol has long been recognized as shorthand for "prescription" in the pharmaceutical context, indicating that the medication has been prescribed by a licensed healthcare provider and is intended for use for a specific patient.

Its presence on a label helps to clearly identify that the drug is not available for general sale over the counter but is instead regulated and intended for use under the supervision of a healthcare professional. The "Rx" symbol denotes not only the nature of the medication but also clarifies its legal status, ensuring that patients and pharmacists understand it relates to a prescription-only medication.

Other symbols such as "Pr," "Med," or "Ph" do not have the same universal recognition and association with prescribed medications. "Pr" could suggest the term "prescription" but is not commonly used nor as widely understood as "Rx." "Med" is too vague and does not specifically indicate a medication's prescription status. "Ph" may refer to pharmacy as a field but does not serve to indicate that a drug is prescribed or the conditions under which it should be dispensed. Thus, "Rx" remains the definitive and legally accepted symbol for prescription medications.