Penicillins and acid stability — Which penicillin is sufficiently stable in gastric acid to be suitable for oral administration in routine clinical use?

Introduction / Context:
Acid stability determines which penicillins can be given orally. Acid-labile agents are degraded in the stomach and require parenteral dosing, whereas acid-stable agents achieve adequate serum levels by mouth.

Given Data / Assumptions:

• Methicillin and carbenicillin are historically acid-labile.
• Penicillin G is acid-labile and therefore not reliably effective orally.
• Cloxacillin is an isoxazolyl penicillin designed to be acid-stable and β-lactamase–resistant.

Concept / Approach:
Identify the penicillin designed for oral administration due to both acid stability and β-lactamase resistance against staphylococcal penicillinase. Among the options, cloxacillin satisfies these criteria and is commonly used orally for skin/soft-tissue staphylococcal infections.

Step-by-Step Solution:

Verification / Alternative check:
Drug monographs list oral cloxacillin with reliable bioavailability; Penicillin V (not listed) is another classic acid-stable penicillin, but among the given options cloxacillin is the correct choice.

Why Other Options Are Wrong:

• Methicillin: degraded by gastric acid; largely obsolete clinically.
• Carbenicillin: acid-labile; antipseudomonal spectrum; parenteral historically.
• Penicillin G: acid-labile; best given parenterally.

Common Pitfalls:
Confusing cloxacillin with oxacillin/flucloxacillin; all are acid-stable, but only cloxacillin appears in the choices.

Final Answer:
Cloxacillin