Overview
Doxycycline is a broad-spectrum tetracycline-class antibiotic effective against a wide range of gram-positive, gram-negative, and atypical bacteria, as well as certain rickettsial and spirochetal infections. It is commonly used for respiratory infections, sexually transmitted diseases, skin conditions like acne, and as prophylaxis for anthrax exposure. The drug works by inhibiting bacterial protein synthesis and is available in oral formulations with good absorption even when taken with food.
Mechanism of Action
Doxycycline inhibits bacterial protein synthesis by binding to the 30S ribosomal subunit, preventing the attachment of aminoacyl-tRNA to the mRNA-ribosome complex. This bacteriostatic action is effective against a broad spectrum of microorganisms including gram-positive and gram-negative bacteria, atypical pathogens, and some protozoa.
Indications
- Rocky Mountain spotted fever, typhus, Q fever, and other rickettsial infections
- Respiratory tract infections caused by Mycoplasma pneumoniae and Haemophilus influenzae
- Sexually transmitted infections including chlamydia, gonorrhea, syphilis, and lymphogranuloma venereum
- Skin and soft tissue infections including acne vulgaris
- Anthrax prophylaxis (post-exposure)
- Plague, tularemia, brucellosis, and other zoonotic infections
- Traveler's diarrhea and cholera
- Malaria prophylaxis in certain regions
Common Doses
- 50 mg
- 100 mg
- 150 mg
- 200 mg
Dosage
Adults: 200 mg on day 1 (as 100 mg every 12 hours or 50 mg every 6 hours), then 100 mg daily as maintenance. For severe infections: 100 mg every 12 hours. Pediatric: 2.2 mg/kg every 12 hours for severe infections; 4.4 mg/kg divided twice on day 1, then 2.2 mg/kg daily for less severe disease in children >8 years and <45 kg. Duration varies by indication from 7-60 days.
Contraindications
- Hypersensitivity to any tetracycline antibiotic
- Children under 8 years (except for life-threatening conditions like anthrax or Rocky Mountain spotted fever when benefits outweigh risks)
Side Effects
- Gastrointestinal: nausea, vomiting, diarrhea, esophagitis, esophageal ulceration (especially if taken at bedtime without adequate fluids)
- Dermatologic: photosensitivity, maculopapular rash, Stevens-Johnson syndrome, toxic epidermal necrolysis
- Hypersensitivity: urticaria, angioedema, anaphylaxis, serum sickness
- Hematologic: hemolytic anemia, thrombocytopenia, neutropenia
- Renal: increased BUN (dose-related)
- Neurologic: intracranial hypertension with visual disturbances
- Hepatic: hepatotoxicity (rare)
Interactions
- Antacids containing aluminum, calcium, magnesium, or iron salts: decreased absorption of doxycycline
- Warfarin: increased anticoagulant effect requiring INR monitoring
- Oral contraceptives: reduced efficacy of birth control pills
- Penicillin: antagonistic effect when used concurrently
- Isotretinoin: increased risk of pseudotumor cerebri
- Methoxyflurane: increased nephrotoxicity risk
Counseling Points
- Take with full glass of water while sitting or standing to prevent esophageal irritation and ulceration
- Avoid taking at bedtime or immediately before lying down
- Use sunscreen and protective clothing due to photosensitivity risk
- Complete full course of therapy even if symptoms improve
- Take with food or milk if gastrointestinal upset occurs (absorption not significantly affected)
- Do not take with antacids, iron supplements, or dairy products within 2-3 hours of dose
- Report severe diarrhea, headache with visual changes, or skin rash immediately