The medical abbreviation "PO" stands for "by mouth" or "per oral". It is often used in medication orders or medical instructions to indicate that a medication should be taken by mouth, either in the form of a pill, capsule, liquid, or other oral medication.
For example, a medication order may read: "Take two tablets PO twice daily with food." This means that the patient should take two tablets of the medication by mouth, two times per day, and should take the medication with food to avoid stomach upset.