Indication of doxycycline is:

Doxycycline is a second-generation, synthetic tetracycline derivative that acts as a broad-spectrum antibacterial agent. It exerts its antimicrobial effect by reversibly binding to the 30S ribosomal subunit of susceptible microorganisms, thereby inhibiting bacterial protein synthesis by blocking the attachment of aminoacyl-tRNA to the ribosomal acceptor (A) site. Doxycycline exhibits extensive antibacterial activity against a wide range of Gram-positive and Gram-negative aerobic and anaerobic bacteria, as well as atypical pathogens including Rickettsia, Chlamydia, Mycoplasma, and Spirochetes. Because of its broad antimicrobial coverage, doxycycline is clinically indicated for respiratory tract infections, rickettsial diseases, sexually transmitted infections, zoonoses, and acne vulgaris, fitting its primary categorization as a broad-spectrum antibacterial agent.

A. Doxycycline lacks reliable clinical efficacy against most anaerobic Bacteroides species, particularly Bacteroides fragilis and related strains, due to widespread resistance.
B. Asthma is an inflammatory airway disease managed with bronchodilators and anti-inflammatory corticosteroids, not antibacterial agents like doxycycline.
C. Meniere's disease is an inner ear disorder treated with dietary sodium restriction, diuretics, or anti-vertigo medications, not antibiotics.
D. Water intoxication is a condition of hyponatremia managed by fluid restriction and hypertonic saline, whereas demeclocycline—not doxycycline—is used off-label for SIADH.

1. Brunton, L. L., Hilal-Dandan, R., & Knollmann, B. C. (Eds.). (2018). Goodman & Gilman's: The Pharmacological Basis of Therapeutics (13th ed., pp. 1049–1053). McGraw-Hill Education.

2. US Food and Drug Administration (FDA). Doxycycline Hyclate Capsules Prescribing Information (Label Reference ID: 4321098).

3. Holmes, N. E., & Charles, P. G. (2009). Safety and efficacy profile of doxycycline. Therapeutics and Clinical Risk Management, 5, 403–410. https://doi.org/10.2147/tcrm.s3520