and refers to the administration of prescribed medication as the situation calls for it.
or "PRN", pro re nata refers to the administration of prescribed medication whose timing is left to the patient (in the case of patient-controlled analgesia), nurse, or caregiver, as opposed to medication that is taken according to a fixed (primarily daily) schedule (a.k.a.
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" differentiates pro re nata dosages (which are common) from ad libitum dosages (which are not common).
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