A patient with localised persistent periodontitis is ALLERGIC to tetracycline. Which locally delivered adjunct is MOST appropriate?

Correct answer: B. A chlorhexidine gluconate chip

• A. DOXYCYCLINE is likewise a tetracycline and is contraindicated for the same reason.
• B. CORRECT. The CHLORHEXIDINE CHIP is a biodegradable gelatin matrix placed into the pocket, releasing chlorhexidine over 7–10 days. It is a bisbiguanide antiseptic with no structural relationship to the tetracyclines, so it is entirely safe in a tetracycline-allergic patient and is the appropriate local adjunct here.
• C. Subantimicrobial-dose doxycycline, used systemically as a collagenase inhibitor rather than an antibiotic, is still doxycycline and is still contraindicated in a tetracycline-allergic patient.
• D. MINOCYCLINE IS a tetracycline. It is absolutely contraindicated — cross-reactivity within the tetracycline class is complete, since the allergy is to the shared four-ring structure.

Note: A class allergy applies to every member of the class. The tetracyclines are tetracycline, doxycycline, minocycline, demeclocycline and tigecycline — all share the four-ring core. And whichever adjunct is chosen, it is only an ADJUNCT: mechanical debridement remains the foundation of periodontal therapy.