What is the typical dosage and treatment duration of doxycycline (antibiotic) for bacterial infections?

Doxycycline Dosing for Bacterial Infections

Standard Adult Dosing

For most bacterial infections in adults, doxycycline should be dosed at 200 mg on the first day (100 mg every 12 hours), followed by 100 mg daily maintenance dosing, though severe infections require 100 mg every 12 hours throughout the treatment course. 1

Loading and Maintenance Regimens

• Loading dose: 200 mg on day 1, administered as 100 mg every 12 hours 1
• Maintenance dose: 100 mg once daily for mild-to-moderate infections 1
• Severe infections: 100 mg every 12 hours for the entire treatment duration, particularly for chronic urinary tract infections 1

Treatment Duration by Infection Type

Skin and Soft Tissue Infections

• MRSA skin infections: 100 mg twice daily for 7-14 days based on clinical response 2, 3
• The 7-14 day range allows flexibility, with most uncomplicated cases resolving by day 7, while more extensive infections may require the full 14 days 4, 2

Respiratory Tract Infections

• Community-acquired respiratory infections: 100 mg twice daily for 7-10 days 5, 6
• Clinical success rates of approximately 80% can be expected in respiratory infections 5
• Particularly effective for Branhamella catarrhalis infections, including beta-lactamase producing strains 6

Sexually Transmitted Infections

• Chlamydia trachomatis (urethral, endocervical, or rectal): 100 mg twice daily for 7 days 1
• Nongonococcal urethritis: 100 mg twice daily for 7 days 1
• Uncomplicated gonorrhea: 100 mg twice daily for 7 days 1
• Acute epididymo-orchitis (gonococcal or chlamydial): 100 mg twice daily for at least 10 days 1

Syphilis (Penicillin-Allergic Patients)

• Early syphilis: 100 mg twice daily for 2 weeks 1
• Late syphilis (>1 year duration): 100 mg twice daily for 4 weeks 1

Bioterrorism-Related Infections

• Inhalational anthrax: 100 mg twice daily for 60 days (post-exposure prophylaxis and treatment) 4, 2, 1
• Cutaneous anthrax: 100 mg twice daily for 60 days due to risk of simultaneous aerosol exposure 4, 2
• Tularemia, bubonic plague, bacillary angiomatosis: 100 mg twice daily 2

Malaria Prophylaxis

• Adults: 100 mg once daily, starting 1-2 days before travel, continuing during travel, and for 4 weeks after leaving malarious area 1

Pediatric Dosing

Weight-Based Calculations

• Children >100 lb (45 kg): Use adult dosing (100 mg twice daily) 2, 1
• Children >8 years and ≤100 lb (45 kg):
  • Loading: 2 mg/lb divided into two doses on day 1 1
  • Maintenance: 1 mg/lb daily as single dose or divided twice daily 1
  • Alternative calculation: 2.2 mg/kg every 12 hours 4, 2
• Severe infections: Up to 2 mg/lb daily may be used 1

Age Restrictions

• Contraindicated in children <8 years due to permanent tooth discoloration and enamel hypoplasia risk 2, 3
• Exception: Life-threatening infections (e.g., anthrax, plague) where benefits outweigh risks 4, 2

Special Populations

Pregnancy

• Generally contraindicated but may be used for life-threatening infections when benefits outweigh risks 2, 3
• For inhalational anthrax in pregnant women, standard adult dosing (100 mg every 12 hours for 60 days) is recommended despite teratogenic concerns 4

Renal Impairment

• No dosage adjustment required for patients with renal impairment, as doxycycline does not accumulate excessively 1

Hepatic Elimination

• Approximately 75% absorbed orally and largely eliminated via hepatic and intestinal routes 5
• Contraindicated in severe liver dysfunction 5

Administration Guidelines

Timing and Food Interactions

• Administer with adequate fluids to reduce risk of esophageal irritation and ulceration 1
• May be given with food or milk if gastric irritation occurs; absorption is not markedly affected 1
• Avoid lying down for 1 hour after administration to prevent esophagitis 2

Drug Interactions

• Separate by at least 2 hours from dairy products, antacids, and supplements containing calcium, iron, magnesium, or sodium bicarbonate 2

Common Pitfalls and Caveats

Treatment Duration Errors

• Inadequate duration (<7 days) may lead to treatment failure 3
• Complete the full course even if symptoms improve before completion 3
• Streptococcal infections require 10 days of therapy 1

Resistance Patterns

• Haemophilus influenzae: Treatment unsuccessful in approximately one-third of cases 6
• Streptococcus pneumoniae: Some strains may be resistant; one case of septicemia with resistant pneumococcus reported during therapy 6
• Overall bacterial resistance to doxycycline remains low in many regions 5

Side Effects

• Gastrointestinal symptoms are most common (12.1% incidence in comparative trials) 7
• Phototoxicity is clinically important; advise sun protection 5
• Side effects are infrequent and comparable to fluoroquinolones and macrolides 5

Dosing Frequency Matters

• Exceeding recommended dosage may result in increased incidence of side effects 1
• For severe infections, twice-daily dosing (100 mg every 12 hours) is superior to once-daily maintenance dosing 1