What is the recommended dosage and treatment duration of Doxycycline (tetracycline antibiotic) 100 mg for an adult patient with normal renal function and no known allergies to tetracyclines?

Doxycycline 100 mg Dosing for Adults

For most infections in adults with normal renal function, doxycycline is dosed as 200 mg on day 1 (given as 100 mg every 12 hours), followed by 100 mg daily or 100 mg twice daily depending on infection severity and type. 1

Standard Dosing Algorithm

Loading and Maintenance Doses

• Day 1: Administer 200 mg total (100 mg every 12 hours) as the loading dose 1
• Day 2 onwards: Continue with 100 mg daily for mild-to-moderate infections, or 100 mg every 12 hours for severe infections 1
• The twice-daily dosing achieves therapeutic serum activity that persists for 24 hours 1

Infection-Specific Dosing

Tick-borne and Rickettsial Diseases:

• 100 mg twice daily is the standard dose for Rocky Mountain spotted fever, ehrlichiosis, and anaplasmosis 2
• Continue for at least 3 days after fever subsides, with a minimum total course of 5-7 days 2
• For human granulocytic anaplasmosis (HGA), extend treatment to 10-14 days if concurrent Lyme disease is suspected 2, 3
• Fever should resolve within 24-48 hours; failure to respond suggests an alternative diagnosis 2, 3

Lyme Disease:

• Early localized/disseminated disease: 100 mg twice daily for 14 days (range 10-21 days) 4
• Neurological manifestations: 100 mg twice daily for 14-28 days 4
• Lyme arthritis: 100 mg twice daily for 28 days 4
• Post-exposure prophylaxis: Single 200 mg dose within 72 hours of tick removal 3, 4

Sexually Transmitted Infections:

• Uncomplicated chlamydial infections: 100 mg twice daily for 7 days 1
• Nongonococcal urethritis: 100 mg twice daily for 7 days 1
• Early syphilis (penicillin allergy): 100 mg twice daily for 2 weeks 2, 1
• Late syphilis (>1 year duration): 100 mg twice daily for 4 weeks 1
• STI post-exposure prophylaxis: Single 200 mg dose within 72 hours, not exceeding 200 mg per 24 hours 3

Skin and Soft Tissue Infections:

• MRSA skin infections: 100 mg twice daily for 7-14 days based on clinical response 3
• Acne vulgaris: 200 mg loading dose on day 1, then 100 mg daily as maintenance (not as monotherapy) 3

Respiratory Infections:

• Community-acquired pneumonia: 100 mg twice daily, with some experts recommending 200 mg as the first dose for rapid serum levels 3
• Acute bronchitis: 100 mg daily after 200 mg loading dose 5

Other Infections:

• Acute Q fever: 100 mg twice daily for 14 days 3
• Chronic Q fever: 100 mg twice daily plus hydroxychloroquine 200 mg three times daily for ≥18 months 3
• Inhalational anthrax (post-exposure): 100 mg twice daily for 60 days 1

Administration Guidelines

Timing and Food Interactions

• Take with 8 ounces of fluid to reduce esophageal irritation risk 4, 1
• Remain upright for 1 hour after administration to prevent esophagitis 3
• May take with food or milk if gastric irritation occurs; absorption is not significantly affected 1, 6
• Separate by at least 2 hours from dairy products, antacids, calcium, iron, magnesium, or sodium bicarbonate supplements 3

Route Selection

• Oral route is preferred for outpatients and those tolerating oral intake 2, 1
• Intravenous route is indicated for hospitalized patients, those unable to take oral medications, or severely ill patients 2
• Both routes provide similar bioavailability 7

Critical Clinical Considerations

When to Initiate Treatment

• Start immediately when tick-borne or rickettsial disease is suspected based on clinical presentation; do not delay for laboratory confirmation 2
• Delayed treatment in rickettsial diseases can lead to severe complications and death 2, 3

Monitoring Response

• Expect defervescence within 24-48 hours of treatment initiation 2, 3, 4
• Lack of fever reduction by 48 hours strongly suggests an alternative diagnosis or treatment failure 2, 3
• Maintain close follow-up for outpatients to ensure appropriate response 2

Common Pitfalls and Contraindications

• Photosensitivity: Advise patients to avoid sun exposure; risk is dose-dependent 3, 4, 6
• Esophageal ulceration: Prevented by adequate fluid intake and remaining upright 3, 1
• Pregnancy: Generally contraindicated; use only for life-threatening infections 3, 4
• Lactation: Avoid use; choose alternative agents 4
• Drug interactions: Potentiates anticoagulants; avoid concurrent retinoids 6

Renal Impairment

• No dose adjustment required in renal insufficiency, as doxycycline undergoes compensatory gastrointestinal excretion rather than renal accumulation 1, 8, 9

Pediatric Considerations (>8 years)

• Weight <100 lbs (45 kg): 2.2 mg/kg twice daily 2, 4
• Weight ≥100 lbs: Use adult dose of 100 mg twice daily 4, 1
• Short-term use carries minimal risk of dental staining; benefits outweigh risks for serious infections 2, 7