First-Line Antibiotic Regimens for Common Infections in Healthy Adults
For uncomplicated UTI in women, prescribe nitrofurantoin 5 days, trimethoprim-sulfamethoxazole 3 days, or fosfomycin single dose; for community-acquired pneumonia, use a minimum 5-day course targeting typical pathogens with clinical stability criteria; and for uncomplicated cellulitis, use penicillin, amoxicillin, or cephalexin targeting streptococci and methicillin-sensitive staph aureus.
Uncomplicated Urinary Tract Infection
The American College of Physicians 2021 guideline provides clear first-line options 1:
- Nitrofurantoin for 5 days
- Trimethoprim-sulfamethoxazole (TMP-SMX) for 3 days
- Fosfomycin as a single dose
Avoid fluoroquinolones as empiric therapy due to high propensity for adverse effects; reserve them only for patients with documented resistant organisms 1. This recommendation is reinforced by 2024 evidence confirming these same agents remain appropriate first-line choices 2.
Key Caveats:
- TMP-SMX should only be used if local resistance rates are <20%
- Fluoroquinolones, while effective in 3-day regimens, carry unacceptable risk-benefit ratios for empiric use
- In men with UTI symptoms, always obtain urine culture and use 7-day courses of the same agents 2
Community-Acquired Pneumonia
Prescribe antibiotics for a minimum of 5 days, but only continue beyond this if the patient has NOT achieved clinical stability 1. Clinical stability is defined as:
- Resolution of vital sign abnormalities
- Ability to eat
- Normal mentation
- No fever for 48 hours
The evidence strongly supports this approach: a 2018 meta-analysis of 21 studies (19 RCTs) demonstrated that short-course treatment (≤6 days) showed lower mortality (risk ratio 0.52) and fewer serious adverse events (risk ratio 0.73) compared to longer courses 1. A subsequent multicenter RCT successfully limited treatment to 5 days in 70% of patients without compromising outcomes 1.
Critical Pitfall:
Despite this evidence, clinicians commonly over-prescribe at discharge—93% of excess antibiotic days occur at discharge, with each additional day carrying a 5% increased risk for adverse events without benefit 1.
Uncomplicated Cellulitis of Lower Extremity
Use oral antibiotics targeting β-hemolytic streptococci and methicillin-sensitive Staphylococcus aureus 3:
- Penicillin
- Amoxicillin
- Cephalexin
Do NOT provide empiric MRSA coverage for non-purulent cellulitis, even with rising community-acquired MRSA rates 3. The majority of uncomplicated cellulitis cases are caused by streptococci or methicillin-sensitive staph, making broader coverage unnecessary and potentially harmful from an antimicrobial stewardship perspective.
Diagnostic Consideration:
Cellulitis is a clinical diagnosis based on acute onset of redness, warmth, swelling, tenderness, and pain. Be vigilant for mimickers including venous stasis dermatitis, contact dermatitis, eczema, and lymphedema, which do not require antibiotics 3.
Algorithmic Approach Summary
- UTI: Start nitrofurantoin 5 days (or TMP-SMX 3 days if local resistance <20%, or fosfomycin single dose)
- CAP: Choose appropriate agent for typical pathogens, treat minimum 5 days, stop when clinically stable for 48 hours
- Cellulitis: Use penicillin, amoxicillin, or cephalexin; avoid MRSA coverage unless purulent
This approach prioritizes shorter courses with narrower spectrums, which the evidence demonstrates reduces mortality, adverse events, and antimicrobial resistance without compromising clinical outcomes.