What does PRN mean on a prescription?
PRN means as needed. A medicine prescribed PRN is taken only when you have the symptom it treats, up to the maximum frequency on the label — not on a fixed schedule.
- Means:
- as needed
- From:
- pro re nata
- Type:
- Pharmacy instruction
- Also written:
- prn, p.r.n., PRN
What it means in practice
PRN is Latin for pro re nata, roughly “as the situation arises”. It converts a frequency into a ceiling rather than a schedule: “PO TID PRN” means you may take up to three doses a day, but none if you have no symptoms. A PRN prescription normally also names the trigger, such as PRN pain or PRN nausea.
Example on a prescription
Ondansetron 4 mg PO Q8H PRN nausea
Take one 4 mg ondansetron tablet by mouth when you feel nauseated, waiting at least eight hours between doses.
Commonly confused with
- STATSTAT means give it immediately, once. PRN means take it only if and when needed.
- QDQD is a fixed daily dose. Adding PRN would make that dose optional.
Worth knowing
The frequency next to PRN is a minimum gap, not a target. Taking PRN doses closer together than the label allows is a common cause of accidental overdose, particularly with paracetamol and ibuprofen.
Frequently asked questions
Do I have to take a PRN medicine every day?
No. PRN means as needed — if you have no symptoms, you take none. The frequency on the label is the maximum, not a requirement.
What does PO TID PRN mean?
By mouth, as needed, up to three times a day. Take it only when the symptom is present and leave a gap between doses.
What does PRN pain mean?
Take it as needed for pain. The trigger after PRN tells you what the medicine is for — PRN nausea, PRN fever, PRN anxiety.
Related abbreviations
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This page explains prescription notation for general information. It is not medical advice and does not replace your prescriber or pharmacist. If a label is unclear or differs from what you were told, check with your pharmacy before taking the medicine.