Final answer:
The abbreviation prn in medicine stands for 'pro re nata', meaning 'as needed' and is used to instruct patients to take medication only when necessary.
Step-by-step explanation:
The medical abbreviation prn stands for pro re nata, which is a Latin phrase that translates to 'as needed'.
It is commonly used in prescriptions and medical orders to indicate that a medication should be taken only when necessary, rather than on a fixed schedule.
For example, a doctor may prescribe a pain reliever with the instruction to take it prn for pain, meaning the patient should only use the medication when they are experiencing pain.
Generally abbreviated to "P.R.N." 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. "scheduled dosage").
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" differentiates pro re nata dosages (which are common) from ad libitum dosages (which are not common).