Pro re nata

Pro re nata is a Latin phrase that literally means "for the thing born". It is commonly used in medicine to mean "as needed" or "as the situation arises." It is generally used as the acronym PRN to refer to dosage of prescribed medication that is not scheduled; instead administration is left to the caregiver or the patient's prerogative. PRN administration of medication is not meant to imply and should never allow for exceeding a prescribed daily regimen.

Common PRN medications
Most often PRN medications are analgesics such as paracetamol (Tylenol), which is known as acetaminophen in the US, or hydrocodone (Vicodin), laxatives such as coloxyl, sleeping aids (sedatives) such as zolpidem (Ambien) or lorazepam (Ativan), and antiemetics such as ondansetron or dimenhydrinate (Gravol).

It should be noted none of the medications above are considered maintenance. PRN is often added to the prescribed directions for medication used to treat symptoms (as above: pain/fever, constipation, insomnia, anxiety, nausea/vomiting), but generally never for a maintenance drug.