Which antibiotics can be prescribed to an adult patient?

Antibiotics for Adult Patients

For adult patients, the choice of antibiotic depends entirely on the suspected or confirmed infection site and severity—there is no single "anti-antibiotic" or universal antibiotic for all conditions.

Infection-Specific Antibiotic Selection

Community-Acquired Pneumonia (Outpatient, No Risk Factors)

For adults >40 years or with underlying disease: Amoxicillin 3 g/day orally is the first-line choice 1. This targets the most common pathogen, Streptococcus pneumoniae.

For adults <40 years without underlying disease in an epidemic context, oral macrolides (azithromycin 500 mg daily for 3 days, or clarithromycin 250-500 mg twice daily) are preferred for atypical bacterial coverage 1, 2.

Telithromycin represents an alternative to both regimens 1.

Hospitalized Pneumonia

For medical ward patients:

• Second-generation cephalosporins (cefuroxime 750-1500 mg IV every 8 hours) 2
• Third-generation cephalosporins (ceftriaxone 1 g IV every 24 hours or cefotaxime 1 g IV every 8 hours) 2
• Macrolides (erythromycin 1 g IV every 8 hours, azithromycin, or clarithromycin) 2

For ICU patients, combine a second or third-generation cephalosporin with either a fluoroquinolone (ciprofloxacin or ofloxacin) or macrolide 2.

Complicated Intra-Abdominal Infections

First-line options include 3:

• Piperacillin-tazobactam 3.375 g IV every 6 hours
• Carbapenems: Ertapenem 1 g IV every 24 hours, meropenem 1 g IV every 8 hours, or imipenem 500 mg every 6 hours
• Cephalosporins with metronidazole: Cefepime 2 g every 8-12 hours, cefotaxime 1-2 g every 6-8 hours, or ceftriaxone 1-2 g every 12-24 hours, plus metronidazole 500 mg every 8-12 hours

Duration: 4-7 days unless source control is difficult 3.

Urinary Tract Infections

Based on prescribing patterns, fluoroquinolones (ciprofloxacin, levofloxacin) and amoxicillin-clavulanate are commonly used 4, 5, 6, though specific guidelines should guide selection based on local resistance patterns.

Skin and Soft Tissue Infections

For non-purulent infections (likely streptococcal): Benzylpenicillin, phenoxymethylpenicillin, clindamycin, nafcillin, cefazolin, or cefalexin 7.

For purulent infections (likely S. aureus): Dicloxacillin, cefazolin, clindamycin, cefalexin, doxycycline, or trimethoprim-sulfamethoxazole 7.

For suspected MRSA: Vancomycin, linezolid, clindamycin, daptomycin, or doxycycline 7.

Respiratory Tract Infections (Non-Pneumonia)

Acute bronchitis in healthy adults: Antibiotics should NOT be prescribed 1. The clinical course is spontaneously favorable, and antibiotic therapy has not shown benefit in clinical trials.

For COPD exacerbations with increased sputum purulence, volume, and dyspnea:

• Amoxicillin 500-1000 mg every 8 hours orally
• Amoxicillin-clavulanate 1 g every 8 hours orally
• Macrolides (azithromycin or clarithromycin)
• Fluoroquinolones (ciprofloxacin 500 mg every 12 hours or ofloxacin 400 mg every 12 hours)
• Cefuroxime axetil 750 mg every 12 hours orally 2

Treat for at least 7 days 2.

Critical Prescribing Principles

Common Pitfalls to Avoid

1. Do not prescribe antibiotics for acute bronchitis in healthy adults—this is the most common inappropriate indication 1, 6.

2. Avoid unnecessarily broad-spectrum agents. In ambulatory care, 61% of prescriptions are for broad-spectrum antibiotics, often inappropriately 6. Match the spectrum to the likely pathogen.

3. Do not combine aminopenicillin with beta-lactamase inhibitors for pneumococcal pneumonia—there is no justification 1.

4. Avoid first-generation oral cephalosporins, tetracyclines, and trimethoprim-sulfamethoxazole for pneumonia due to inadequate activity against penicillin-resistant S. pneumoniae 1.

Special Populations

Elderly patients require dose adjustments based on renal and hepatic function due to altered pharmacokinetics 8. Monitor for drug-drug interactions, which are common in this population.

Renal impairment: Adjust dosing intervals for renally cleared antibiotics (aminoglycosides, vancomycin, fluoroquinolones, ethambutol, pyrazinamide) 9. Aminoglycosides should be dosed 12-15 mg/kg two to three times weekly rather than daily in patients with creatinine clearance <30 mL/min 9.

Monitoring and De-escalation

Assess clinical response within 48-72 hours 1. Do not change therapy within the first 72 hours unless the patient worsens.

Use culture results to narrow therapy when available, especially in high-severity or healthcare-associated infections 3, 10.

Shorter durations (7-8 days) are appropriate for uncomplicated infections with good clinical response 10.

Most Commonly Prescribed Antibiotics in Practice

The most frequently prescribed antibiotics in US ambulatory care are 11, 6:

1. Azithromycin (macrolide)
2. Amoxicillin (penicillin)
3. Amoxicillin-clavulanate (beta-lactam/beta-lactamase inhibitor)
4. Ciprofloxacin (fluoroquinolone)
5. Cefalexin (first-generation cephalosporin)

However, prescribing patterns do not always align with guidelines—over 25% of prescriptions are for conditions where antibiotics are rarely indicated 6, and inappropriate prescribing remains widespread 4, 5.