Final answer:
PRN prescriptions generally expire after one year or on the date assigned by the prescriber, although this can vary especially for controlled substances.
Step-by-step explanation:
Prescriptions that are authorized by the prescriber to be dispensed as needed (i.e. PRN, which stands for pro re nata in Latin, meaning 'as the situation demands') typically expire after one year or on the date assigned by the prescriber. This is a general rule, but the specific expiration period can vary depending on the medication and state regulations. For controlled substances, the expiration could be sooner, as many states have stricter guidelines.
It's essential for patients to check their prescriptions and consult with their pharmacists or healthcare providers to ensure they understand the expiration of their PRN medications. After the expiration date, the medication should not be used without a new prescription.