Oral Antibiotic Options for Group B Streptococcus Infections
For susceptible Group B Streptococcus (GBS) infections in otherwise healthy adults, oral amoxicillin 500 mg twice daily for 10 days is the first-line treatment, with cephalexin 500 mg twice daily for 10 days as the preferred alternative for patients with non-immediate penicillin allergy. 1, 2
First-Line Oral Treatment
Amoxicillin remains the drug of choice because all GBS isolates worldwide remain uniformly susceptible to penicillin and amoxicillin, with zero documented resistance after decades of use. 3, 4, 5, 6 The FDA-approved indication specifically includes GBS (β-hemolytic Streptococcus species) for upper respiratory, genitourinary, skin, and lower respiratory tract infections. 1
- Dosing: Amoxicillin 500 mg orally twice daily for 10 days in adults 7, 8
- Rationale: Provides high and sustained serum concentrations, excellent tissue penetration, narrow spectrum activity, and proven efficacy 9, 2
- Complete the full 10-day course even if symptoms resolve earlier, as shorter courses increase treatment failure rates 2, 8
Alternatives for Penicillin-Allergic Patients
Non-Immediate (Delayed) Penicillin Allergy
First-generation cephalosporins are preferred with only 0.1% cross-reactivity risk in patients with delayed, non-anaphylactic reactions (mild rash occurring >1 hour after exposure). 2
- Cephalexin: 500 mg orally twice daily for 10 days 2, 8
- Cefadroxil: 1 gram orally once daily for 10 days 2, 8
Immediate/Anaphylactic Penicillin Allergy
All β-lactam antibiotics must be avoided due to up to 10% cross-reactivity risk in patients with immediate reactions (anaphylaxis, angioedema, urticaria within 1 hour). 2
Clindamycin is the preferred non-β-lactam alternative:
- Dosing: 300 mg orally three times daily for 10 days 2, 8
- Resistance: Only 3.4–9% resistance among GBS isolates, significantly lower than macrolides 3, 4, 5
- Efficacy: Superior eradication rates compared to macrolides, particularly effective in treatment failures 2
Macrolides are less preferred alternatives due to higher resistance:
- Erythromycin resistance: 7.4–24% depending on geographic region and time period 3, 4, 5, 6
- Clindamycin resistance: 3.4–12% with geographic variation (California 12%, Florida 2.1%) 5, 6
- Azithromycin: 500 mg once daily for 5 days (only antibiotic requiring <10 days due to prolonged tissue half-life) 2, 7
- Clarithromycin: 250 mg twice daily for 10 days 2, 8
Critical Resistance Patterns
Penicillin/amoxicillin resistance: 0% – all GBS isolates remain uniformly susceptible to penicillin, ampicillin, cefazolin, cefotaxime, and vancomycin 3, 4, 5, 6
Macrolide resistance is increasing: Erythromycin resistance rose from 4.7% in 1997 to 12% in 1999 in some regions, with continued upward trends 4, 3
Geographic variation matters: California shows 32% erythromycin resistance versus 8.5% in Florida, so local resistance patterns should guide macrolide selection 6
Common Pitfalls to Avoid
- Never use trimethoprim-sulfamethoxazole (Bactrim) – it fails to eradicate GBS in 20–25% of cases and is absolutely contraindicated 2, 7
- Do not shorten the antibiotic course below 10 days (except azithromycin's 5-day regimen) – this dramatically increases treatment failure rates 2, 8, 7
- Do not prescribe cephalosporins to patients with immediate/anaphylactic penicillin reactions due to 10% cross-reactivity risk 2
- Do not assume macrolides are safe alternatives – resistance rates of 7–24% make them less reliable than clindamycin for penicillin-allergic patients 3, 4, 5, 6
When Oral Therapy May Be Insufficient
For severe invasive GBS infections (bacteremia, meningitis, endocarditis, osteomyelitis), intravenous penicillin G remains the drug of choice and oral therapy is inadequate. 10, 9 Penicillin G treatment was identified as a protective factor against mortality in severe GBS bacteremia. 10