Final answer:
PO Ampicillin is rarely used because it is less stable and less effective compared to other oral penicillins such as amoxicillin and penicillin V.
Step-by-step explanation:
PO Ampicillin is rarely used because it is less stable and less effective compared to other oral penicillins such as amoxicillin and penicillin V. Ampicillin cannot be administered orally because it is unstable and gets converted to an inactive derivative in the acidic pH of the stomach. On the other hand, amoxicillin and penicillin V are acid stable and more commonly used for oral administration.