Which antibiotic is prescribed for necrotising ulcerative gingivitis when systemic therapy is indicated?

Correct answer: A. Metronidazole

• A. Correct. Metronidazole targets the anaerobic flora — fusiform bacteria and spirochaetes — driving the necrotising process, and the clinical response is usually rapid.
• B. Azithromycin has useful activity in periodontal disease but is not the drug of choice for anaerobic necrotising infection.
• C. Doxycycline is used in periodontal therapy for its anticollagenase effect at subantimicrobial dose, not for acute necrotising disease.
• D. Cephalexin covers gram-positive aerobes and offers little against strict anaerobes.

Note: Antibiotics are adjunctive. Debridement, oxygenating rinses, pain control and addressing the predisposing factors — smoking, stress, immunosuppression — do the real work. Warn patients about the disulfiram-like reaction with alcohol.