Final answer:
The abbreviation p.r.n. on a prescription means 'as needed,' directing the patient to take the medication only when necessary, based on their symptoms.
Step-by-step explanation:
The abbreviation p.r.n. on a prescription stands for "pro re nata," which is a Latin term that translates to "as needed" in English. This direction indicates that the patient should take the medication only when it is necessary, based on their symptoms, rather than at scheduled times. For instance, pain relief medication might be prescribed p.r.n. so that the patient can use it when they are experiencing pain rather than taking it continuously.