Doxycycline
Doxycycline
Generic Name
Doxycycline
Mechanism
- Bacteriostatic effect achieved by binding reversibly to the 30S ribosomal subunit.
- Inhibits the attachment of amino‑acyl‑tRNA to the A‑site, blocking peptide chain elongation.
- Eliminates growth of susceptible Gram‑positive, Gram‑negative, and some atypical bacteria (e.g., *Mycoplasma*, *Chlamydia*, *Rickettsia*, *Borrelia*, *Toxoplasma*, and *Plasmodium falciparum*).
Pharmacokinetics
- Absorption: Rapid; peak plasma concentration in 1–3 h. Food reduces, but does not eliminate, absorption; most significant inhibition by dairy products and calcium‑fortified juices.
- Distribution: High tissue penetration; not highly bound to plasma proteins (<30 %). Excellent CNS, ocular, bile, and muscle distribution.
- Half‑life: 18–22 h (oral), enabling twice‑daily dosing.
- Metabolism & Excretion: Minimal hepatic metabolism. ~80 % excreted unchanged in feces (bile), ~20 % in urine.
- Drug Interactions: Chelating agents (antacids, calcium, iron, magnesium, aluminum) delay absorption; fluoroquinolones or sulfonamides may increase risk of photosensitivity.
Indications
- Bacterial infections:
- Community‑acquired pneumonia, Lyme disease (early stage), *Legionella* and *Ureaplasma*
- Skin & soft‑tissue infections, acne vulgaris, rosacea, and post‑traumatic joint infections
- Oropharyngeal and ear infections (children)
- Endocarditis prophylaxis (selected cases)
- Vector‑borne diseases:
- Malaria prophylaxis and treatment for *P. falciparum,* *P. vivax*
- Tick‑borne diseases: *Ehrlichia chaffeensis,* *Anaplasma phagocytophilum,* *Borrelia burgdorferi* (early Lyme disease)
- Other indications:
- Chlamydial cervicitis/urogenital infections
- Respiratory tract infections in cystic fibrosis
- Cystic fibrosis exacerbations, antistaphylococcal coverage
- Empirical coverage for Severe Acute Respiratory Syndrome and Middle East Respiratory Syndrome (study‑based)
Contraindications
- Contraindicated in patients < 8 years and during pregnancy (risk of teeth discoloration, enamel hypoplasia).
- Caution in lactation; minimal excretion into breast milk—use only if benefits outweigh risks.
- 819: Photosensitivity—avoid sun exposure and high‑dose ibuprofen, phenothiazines.
- Taste disturbances, esophageal ulceration—take with full glass of water, keep upright for at least 30 min.
- Nephrolithiasis & hepatic impairment: monitor renal and hepatic function; adjust dosing if necessary.
- Ocular use in children < 5 yrs: risk of vision loss; use only for severe cases with close monitoring.
Dosing
| Indication | Typical Dose & Schedule | Notes |
| Adults (bacterial infection) | 100 mg orally once or twice daily | Initiate 200 mg BID on day 1 for 1–3 days, then 100 mg BID |
| Malaria prophylaxis | 100 mg orally once daily | Start 1–2 weeks before travel, continue 4 weeks post‑arrival |
| Acne / rosacea | 20–40 mg orally once daily | Lower dose may be effective; mitigate GI upset by splitting dose |
| Children (7–12 yrs) | 2.5 mg/kg orally BID | Infections >14 days: 5 mg/kg QD |
| Intravenous | 200 mg IV every 12 h (then 100 mg every 12 h) | For serious infections; monitor serum levels if renal dysfunction |
• Oral: take with a full glass of water; avoid dairy or antacids for 2 h pre/post‑dose.
• Intramuscular: feasible in acute settings, but subcutaneous tissues can develop local irritation.
Adverse Effects
Common (≥ 10 %)
• GI upset (nausea, vomiting, dyspepsia)
• Oral mucositis, taste changes
• Rash; mild photosensitivity
Serious (≤ 5 %)
• Severe photosensitivity → sunburn or dermatitis
• Exacerbation of *Mycobacterium tuberculosis* infection
• Hepatotoxicity: elevated transaminases, cholestatic jaundice
• Esophageal ulcers, perforation (particularly in the elderly or debilitated)
• Osteo‑, and dentinogenesis suppression in developing teeth
> Tip: Avoid concurrent use of high‑dose ibuprofen and phenothiazine agents to reduce photosensitivity risk.
Monitoring
- Baseline: CBC, CMP, serum creatinine, liver enzymes.
- During therapy: liver function tests every 2–4 weeks (± malaria prophylaxis > 6 wk).
- Renal: adjust dosing if eGFR 6 mos therapy in osteoporosis).
- Skin: evaluate for photosensitivity reactions; counsel on sunscreen.
Clinical Pearls
- Food Interaction: Calcium, iron, magnesium, and antacids chelate doxycycline; wait 2 h after dairy or chew‑chews.
- Split-Dose Strategy: Splitting the daily dose (morning & evening) reduces GI upset and improves adherence.
- Drug‑Drug Synergy: Combining doxycycline with a macrolide can enhance coverage against atypical pathogens while minimizing macrolide‑induced QT prolongation.
- Optical Penetration: Doxycycline’s excellent ocular uptake is why it’s the drug of choice for prophylaxis of ocular Lyme disease and for treating chronic inflammatory ocular conditions (e.g., uveitis).
- IV vs Oral: IV doxycycline 200 mg every 12 h is used for severe infections; conversion to oral 100 mg BID can be initiated early if clinical response is observed.
- Resistance Considerations: Use only when susceptible; avoid in *Streptococcus pyogenes* infections with potential for resistance.
- Rapid Decay In Pregnancy: Doxycycline’s capability to cross the placenta makes it contraindicated; alternative agents (azithromycin, clindamycin) should be preferred.
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• Key Takeaway: Doxycycline’s broad spectrum, excellent tissue penetration, and convenient twice‑daily oral dosing make it a versatile agent—yet its narrow safety profile regarding teeth development and photosensitivity requires diligent patient education and monitoring.