Final answer:
The abbreviation for 'as needed' is PRN, a Latin-derived term used in medical prescriptions indicating that the medication should be taken only when required.
Step-by-step explanation:
The abbreviation for the phrase “as needed” in the context of medication is PRN. PRN stands for the Latin phrase “pro re nata,” which translates to “as the situation arises” or “as necessary.” It is commonly used in medical prescriptions to indicate that a medication should be taken only when the patient feels it is required, often to control symptoms that do not occur continuously or predictably, such as pain or nausea.