Subantimicrobial-dose doxycycline in periodontics has which dose and purpose?

• A. 100 mg twice daily; antibiotic therapy.
• B. 20 mg twice daily; host-modulation reducing collagenase activity.
• C. 40 mg once daily; no effect on collagenase.
• D. 10 mg once daily; analgesic effect.

Answer: (B)

Subantimicrobial-dose doxycycline tests host-modulation rather than antibiotic action. At 20 mg taken twice daily, it reduces the activity of host enzymes called matrix metalloproteinases, especially collagenases, which are responsible for breaking down collagen in periodontal tissues. By dialing down these enzymes, tissue destruction in periodontal disease is minimized while leaving bacterial counts largely unaffected, so it serves as an adjunct to mechanical therapy rather than a regular antibiotic treatment.

This dosing and purpose fit because the goal is to modulate the host response, not to act as an antimicrobial. The other options don’t fit: a high-dose regimen would act as antibiotics; a regimen like 40 mg once daily isn’t the established subantimicrobial approach and would not emphasize collagenase inhibition; and doxycycline isn’t used for analgesia.