Drug Dosing for Community-Acquired Pneumonia in Adults
For outpatient adults without comorbidities, use amoxicillin 1 g three times daily as first-line therapy; for those with comorbidities or recent antibiotic use, use combination therapy with amoxicillin-clavulanate (875 mg/125 mg twice daily or 2,000 mg/125 mg twice daily) plus a macrolide, or monotherapy with a respiratory fluoroquinolone (levofloxacin 750 mg daily or moxifloxacin 400 mg daily). 1
Outpatient Treatment Regimens
Healthy Adults Without Comorbidities
First-line options:
- Amoxicillin 1 g three times daily (preferred) 1
- Doxycycline 100 mg twice daily 1
- Macrolide monotherapy (azithromycin 500 mg day 1, then 250 mg daily; or clarithromycin 500 mg twice daily) only in areas where pneumococcal macrolide resistance is <25% 1
The 2019 ATS/IDSA guidelines prioritize high-dose amoxicillin as the strongest recommendation for this population, representing a shift from older guidelines that emphasized macrolides more heavily.
Adults With Comorbidities
Comorbidities include chronic heart, lung, liver, or renal disease; diabetes mellitus; alcoholism; malignancy; or asplenia.
Combination therapy (preferred):
- Amoxicillin-clavulanate 875 mg/125 mg twice daily OR 2,000 mg/125 mg twice daily
- PLUS macrolide (azithromycin 500 mg day 1, then 250 mg daily; or clarithromycin 500 mg twice daily) 1
- OR doxycycline 100 mg twice daily instead of macrolide 1
Monotherapy alternative:
- Respiratory fluoroquinolone: levofloxacin 750 mg daily, moxifloxacin 400 mg daily, or gemifloxacin 320 mg daily 1
The higher-dose amoxicillin-clavulanate formulation (2,000 mg/125 mg twice daily) provides enhanced pharmacokinetic coverage against resistant Streptococcus pneumoniae 2.
Inpatient Non-ICU Treatment
Two equally effective options:
β-lactam plus macrolide:
- Ceftriaxone 1 g IV every 24 hours OR
- Cefotaxime 1 g IV every 8 hours OR
- Ampicillin 1 g IV every 6 hours
- PLUS azithromycin 500 mg IV/PO daily OR clarithromycin 500 mg IV/PO twice daily 3
Respiratory fluoroquinolone monotherapy:
- Levofloxacin 750 mg IV/PO daily OR
- Moxifloxacin 400 mg IV/PO daily 3
For patients with risk factors for gram-negative pathogens (excluding Pseudomonas), ertapenem is an acceptable β-lactam alternative 3.
Inpatient ICU Treatment (Severe CAP)
Standard severe CAP regimen:
- β-lactam (ceftriaxone 1 g IV every 24 hours, cefotaxime 1 g IV every 8 hours, OR ampicillin-sulbactam 2 g IV every 6 hours)
- PLUS azithromycin 500 mg IV daily OR respiratory fluoroquinolone (levofloxacin 750 mg IV daily or moxifloxacin 400 mg IV daily) 3
For Pseudomonas aeruginosa risk factors:
- Antipseudomonal β-lactam (piperacillin-tazobactam 4.5 g IV every 6 hours, cefepime 2 g IV every 8 hours, imipenem 500 mg IV every 6 hours, OR meropenem 1 g IV every 8 hours)
- PLUS ciprofloxacin 400 mg IV every 8 hours OR levofloxacin 750 mg IV daily
- OR antipseudomonal β-lactam PLUS aminoglycoside (gentamicin or tobramycin 5-7 mg/kg IV daily) PLUS azithromycin or respiratory fluoroquinolone 3
For community-acquired MRSA:
- Add vancomycin 15-20 mg/kg IV every 8-12 hours (target trough 15-20 mcg/mL) OR linezolid 600 mg IV every 12 hours 3
Penicillin Allergy Alternatives
Outpatient:
- Respiratory fluoroquinolone (levofloxacin 750 mg daily or moxifloxacin 400 mg daily) 1
- Doxycycline 100 mg twice daily plus azithromycin 500 mg day 1, then 250 mg daily 1
Inpatient non-ICU:
- Respiratory fluoroquinolone monotherapy 3
Inpatient ICU:
- Respiratory fluoroquinolone plus aztreonam 2 g IV every 8 hours 3
Duration of Therapy
Minimum 5 days of treatment, with discontinuation when:
- Afebrile for 48-72 hours AND
- No more than 1 sign of clinical instability (heart rate >100 bpm, respiratory rate >24/min, systolic BP <90 mmHg, oxygen saturation <90%, inability to maintain oral intake, abnormal mental status) 3
Most uncomplicated cases require 5-7 days total. Longer duration (up to 21 days) is needed for Legionella infection 4.
Recent evidence supports 3-day treatment for patients achieving clinical stability by day 3, though this represents emerging practice not yet incorporated into major guidelines 5.
IV to Oral Transition
Switch criteria:
- Hemodynamically stable
- Clinically improving
- Able to ingest medications
- Functioning gastrointestinal tract 3
Typically occurs within the first 3 days of hospitalization. Patients can be discharged once stable on oral therapy without requiring prolonged inpatient observation 3.
Renal Dose Adjustments
Key adjustments for common agents:
- Levofloxacin: CrCl 20-49 mL/min: 750 mg loading dose, then 750 mg every 48 hours; CrCl 10-19 mL/min: 750 mg loading dose, then 500 mg every 48 hours
- Ceftriaxone: No adjustment needed (biliary excretion)
- Cefotaxime: CrCl <20 mL/min: reduce dose by 50%
- Azithromycin: No adjustment needed
- Vancomycin: Dose based on renal function and therapeutic drug monitoring
Elderly Patient Considerations
Elderly patients often have multiple comorbidities and should be treated according to the comorbidity algorithms above. The 2019 guidelines do not recommend different antibiotic choices based solely on age, but elderly patients warrant:
- Lower threshold for hospitalization
- Careful attention to renal function for dose adjustments
- Monitoring for drug interactions with existing medications
- Assessment of ability to tolerate oral medications 1
Critical Pitfalls to Avoid
- Do not use macrolide monotherapy in areas with >25% macrolide-resistant S. pneumoniae or in patients with comorbidities 1
- Do not use fluoroquinolone monotherapy for outpatients without comorbidities as first-line (reserve for specific indications) 1
- Do not delay antibiotics in hospitalized patients—first dose should be given in the emergency department 3
- Do not use inadequate β-lactam doses for resistant pneumococcus—use high-dose amoxicillin (3 g daily) or enhanced formulations 1
- Do not forget atypical coverage for hospitalized patients—combination therapy with macrolide or fluoroquinolone is essential 3