What is the typical dosage of Doxycycline (tetracycline antibiotic) for an adult patient with a bacterial infection, considering their medical history, including potential allergies, kidney or liver disease, and pregnancy status?

Doxycycline Dosing for Adult Bacterial Infections

Standard Adult Dosing

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

Loading and Maintenance Regimens

• Loading dose: 200 mg on day 1, administered as 100 mg every 12 hours 1
• Maintenance dose: 100 mg daily for mild-to-moderate infections 1
• Severe infections: 100 mg every 12 hours (particularly chronic urinary tract infections) 1
• Maximum daily dose: Do not exceed 200 mg per 24 hours, as higher doses increase side effects without additional efficacy 2

Infection-Specific Dosing

Respiratory Infections

• Community-acquired pneumonia: 100 mg twice daily, with some experts recommending 200 mg as the first dose to achieve therapeutic levels more rapidly 3, 2

Skin and Soft Tissue Infections

• MRSA/MSSA infections: 100 mg twice daily for 7-14 days based on clinical response 3, 2, 4
• Septic prepatellar bursitis: 100 mg twice daily for mild-to-moderate cases (alternative to beta-lactams) 3

Sexually Transmitted Infections

• Chlamydia trachomatis (urethral/endocervical/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: 100 mg twice daily for at least 10 days 1
• Early syphilis (penicillin allergy): 100 mg twice daily for 2 weeks 1
• Late syphilis (>1 year duration): 100 mg twice daily for 4 weeks 1

Tickborne and Rickettsial Diseases

• Human granulocytic anaplasmosis: 100 mg twice daily for 10 days (extend to 10-14 days if concurrent Lyme disease suspected) 3, 4
• Rickettsial diseases: 100 mg twice daily, continuing at least 3 days after fever subsides, typically 5-7 days minimum 3, 2
• Lyme disease treatment: 100 mg twice daily for 10-21 days 4
• Lyme disease prophylaxis: Single 200 mg dose within 72 hours of tick removal 3, 4

Bioterrorism Agents

• Inhalational anthrax: 100 mg twice daily for 60 days 2, 1
• Cutaneous anthrax: 100 mg twice daily for 60 days 2
• Tularemia and bubonic plague: 100 mg twice daily 2

Other Infections

• Q fever (acute): 100 mg twice daily for 14 days 4
• Q fever (chronic): 100 mg twice daily plus hydroxychloroquine 200 mg three times daily for ≥18 months 3, 4
• Malaria prophylaxis: 100 mg daily, starting 1-2 days before travel and continuing 4 weeks after leaving malarious area 1

Special Population Considerations

Renal Impairment

• No dose adjustment required: Doxycycline does not accumulate excessively in renal impairment and is not significantly removed by hemodialysis 1, 5

Hepatic Impairment

• Severe hepatic dysfunction is a contraindication 6
• Stable hepatitis B carriers: A single 200 mg dose appears safe in inactive HBV carriers with normal liver enzymes and no cirrhosis 3, 4
• Use with extreme caution in severe hepatic impairment, as doxycycline undergoes hepatic elimination 5

Pregnancy

• Generally contraindicated: Alternative agents should be considered except for life-threatening infections where benefits outweigh risks 3, 2, 4
• Life-threatening rickettsial diseases: May be used when no alternatives exist, as emerging evidence suggests doxycycline's safety profile differs significantly from older tetracyclines 7
• Recent systematic reviews found no correlation between doxycycline use and teratogenic effects during pregnancy 7

Allergies

• Doxycycline serves as an alternative for patients with beta-lactam allergies in many infections 3
• Cross-reactivity with other tetracyclines should be considered 6

Administration Guidelines

Optimal Absorption and Tolerability

• Administer with adequate fluids to reduce esophageal irritation and ulceration risk 1
• Avoid lying down for 1 hour after dosing to prevent esophagitis 3, 2
• May be taken with food or milk if gastric irritation occurs; absorption is not significantly affected 1, 5

Drug Interactions

• Separate dosing by at least 2 hours from dairy products, antacids, and supplements containing calcium, iron, magnesium, or sodium bicarbonate 3, 2
• These agents chelate doxycycline and reduce absorption 5

Critical Clinical Pitfalls

Treatment Delays

• Do not delay treatment in suspected rickettsial diseases while awaiting laboratory confirmation—delay can lead to severe disease and death 3, 2
• Fever should subside within 24-48 hours; if no improvement, consider alternative diagnoses 3, 2

Photosensitivity

• Advise patients to avoid sun exposure due to significant photosensitivity risk 3, 4
• This is one of the most important adverse effects to counsel patients about 6

Gastrointestinal Effects

• Gastrointestinal side effects are common but can be minimized with proper administration technique 6
• Taking with food reduces gastric irritation without compromising efficacy 1

Streptococcal Infections

• Continue therapy for 10 days in streptococcal infections to prevent rheumatic fever 1

Route of Administration Selection

• Oral therapy: Appropriate for outpatients with early-stage disease who can tolerate oral intake 2
• Intravenous therapy: Indicated for severely ill hospitalized patients, particularly those who are vomiting or obtunded 2
• Oral bioavailability is 75% with rapid and virtually complete absorption; serum concentrations following oral and IV administration are comparable 6, 5