Pro re nata is a Latin phrase meaning in the circumstances or as the circumstance arises (literally "for the affair born"). It is commonly used in medicine to mean as needed or as the situation arises. It is generally abbreviated to p.r.n. or PRN in reference to dosage of prescribed medication that is not scheduled; instead, the decision of when to administer the drug is left to the nurse, caregiver, or patient (such as in patient-controlled analgesia). Pro re nata does not imply that the patient may take as much of the medicine as desired, but rather that the medicine may be taken in the prescribed dosage if needed. Such administration of medication is not meant to imply, and should never allow for, exceeding a maximum daily dosage. This aspect of "only if needed, and only up to some maximum" makes pro re nata dosages (which are common) differentiable from ad libitum dosages (which are not common).
Other uses
Pro re nata is sometimes used for blood tests. A physician will write a p.r.n. order for blood work. The patient may then use that order when needed.
The initials p.r.n. may also refer to casual, or "as needed", employment.