Doxycycline

Tetracyclines

ATC code: J01AA02 (Doxycycline)

Mechanism of action

Binds the 30S ribosomal subunit and blocks aminoacyl-tRNA attachment to the A-site, inhibiting bacterial protein synthesis. Bacteriostatic. Broad spectrum: gram-positive and gram-negative organisms, atypical pathogens – Mycoplasma, Chlamydia, Legionella, and also Rickettsia, Borrelia burgdorferi, Treponema pallidum. Compared to tetracycline, it has better bioavailability and a longer half-life of 18-22 hours.

Indications

Acne vulgaris

First line

First-line systemic antibiotic for moderate-to-severe acne per AAD 2024 guidelines. Dose is 100 mg once or twice daily for 3 months. Sub-antimicrobial dose of 40 mg modified-release (not registered in Russia) reduces inflammation without antibiotic pressure. Always combine with a topical retinoid and/or benzoyl peroxide to prevent resistance.

Chlamydial infection

First line

First-line for uncomplicated chlamydial infection alongside azithromycin. Doxycycline 100 mg twice daily for 7 days. Recent meta-analyses suggest a slight advantage over single-dose azithromycin in eradication rates. In men with chlamydial proctitis, doxycycline is preferred.

Community-acquired pneumonia

First line

Alternative to macrolides for atypical community-acquired pneumonia. Active against Mycoplasma pneumoniae, Chlamydophila pneumoniae, Legionella. Per IDSA/ATS 2019, doxycycline is an outpatient CAP monotherapy option in patients without comorbidities when a macrolide is not suitable. Dose is 100 mg twice daily.

Lyme disease (borreliosis)

First line

Drug of choice for Lyme disease per IDSA/AAN/ACR 2020. For early localized disease (erythema migrans), prescribe 100 mg twice daily for 10-21 days. For early disseminated disease without CNS involvement, the same regimen for 14-21 days. Doxycycline is the only oral antibiotic effective in neuroborreliosis without meningitis.

Practical notes

Timing and administration

Take with food, swallowed with a full glass of water. Remain upright for at least 30 minutes after the dose – doxycycline causes esophageal erosions on mucosal contact. Taking the evening dose right before bed is a common cause of esophagitis. Avoid milk: although dairy affects doxycycline less than tetracycline, spacing is still advisable.

Special situations

Photosensitivity is the main practical side effect. Patients should avoid direct sunlight and use SPF 50+. Risk is higher in summer months. Antacids, iron, calcium, and magnesium chelate doxycycline and reduce absorption – separate dosing by 2-3 hours. Contraindicated in children under 8 years due to teeth discoloration.

Safety

Contraindications

Age under 8 years (teeth staining and bone growth impairment)
Pregnancy (FDA category D)
Hypersensitivity to tetracyclines
Severe hepatic impairment

Serious adverse effects

Benign intracranial hypertension (pseudotumor cerebri) – rare
Clostridioides difficile-associated diarrhea
Hepatotoxicity (at high doses)
Teeth discoloration in children – irreversible

Common adverse effects

Nausea, diarrhea
Photosensitivity
Esophagitis, esophageal ulceration (with improper administration)
Vaginal candidiasis

PregnancyFDA D

FDA category D. Tetracyclines cross the placenta and accumulate in fetal bones and teeth, causing discoloration and growth impairment. Contraindicated in all trimesters.

Breastfeeding

Passes into breast milk in small amounts. Calcium in milk binds doxycycline, reducing its bioavailability to the infant. Per LactMed, short-term use is acceptable, but prolonged courses during breastfeeding are not recommended.