Which abbreviation is used for "as needed" medication dosing?

The abbreviation "prn" stands for "pro re nata," which is a Latin term meaning "as needed." This is commonly used in medical and pharmacy settings to indicate that a medication should be administered only when necessary, rather than on a scheduled basis.

For example, a prescription might indicate that a patient should take a pain reliever "prn," meaning they should take it only when they experience pain, rather than at set times each day. This approach allows for personalized patient care, ensuring that medication is taken only when it will be most beneficial to the patient.

The other abbreviations listed serve different purposes: "qod" means "every other day," "bid" means "twice a day," and "ac" means "before meals." These indicate specific dosing schedules and do not convey the flexibility of administering medication as needed, which is what makes "prn" the correct choice for this question.