Doxycycline (chlamydia / PID)

Overview

First-line for chlamydia (restored over azithromycin for better rectal efficacy) and a backbone of PID, LGV and penicillin-allergic syphilis.

Mechanism

Tetracycline — binds the 30S ribosomal subunit → blocks aminoacyl-tRNA binding → inhibits protein synthesis (bacteriostatic). Active against intracellular Chlamydia trachomatis.

Indications

• Chlamydia (100 mg BD 7 days)
• PID (with ceftriaxone + metronidazole, 14 days)
• Lymphogranuloma venereum (21 days)
• Early syphilis if penicillin-allergic & non-pregnant

Adverse effects

Cautions & contraindications

Interactions

• Absorption ↓ by calcium/iron/antacids/dairy
• May enhance warfarin

Monitoring & kinetics

Test of cure (rectal/pregnancy); partner notification; re-test at 3 months

Oral. Azithromycin 1 g then 500 mg OD 2 days = second-line / pregnancy chlamydia option.