What is the preferred treatment between Penicillin (Penicillin V potassium) VK and Amoxicillin for Group B streptococcus (strep B)?

Penicillin VK vs Amoxicillin for Group B Streptococcus

For Group B Streptococcus (GBS) infections, penicillin is the preferred agent over amoxicillin due to its narrower spectrum of activity, although both are equally effective. 1

Clinical Context Matters

The choice between penicillin VK and amoxicillin depends on the specific clinical scenario:

For Intrapartum Prophylaxis (Preventing Neonatal GBS)

• Intravenous penicillin G is the first-line agent, not oral penicillin VK, with a regimen of 5 million units IV initially, then 2.5 million units IV every 4 hours until delivery 2, 1
• Ampicillin (not amoxicillin) is the acceptable alternative for intrapartum prophylaxis: 2 g IV initially, then 1 g IV every 4 hours until delivery, though it is considered second-line due to broader spectrum 2, 1
• The narrow spectrum of penicillin G makes it superior for prophylaxis because it minimizes disruption of maternal and neonatal flora 2, 1

For Active GBS Infections (Sepsis, Meningitis, Soft Tissue)

• High-dose IV penicillin G (2-4 million units every 4-6 hours) is the drug of choice for serious invasive GBS disease 1, 3
• Higher doses are required because GBS has somewhat higher minimum inhibitory concentrations compared to Group A Streptococcus 3
• No documented penicillin resistance exists in GBS anywhere in the world 4, 5

For Oral Therapy (If Applicable)

• Neither penicillin VK nor amoxicillin is typically used for GBS because most GBS infections requiring treatment are serious enough to warrant IV therapy 1, 3
• If oral therapy were considered for minor infections, amoxicillin would have better bioavailability than penicillin VK, though this is not a standard recommendation for GBS 6

Important Distinctions from Group A Streptococcus

Do not confuse GBS treatment with strep throat (Group A Streptococcus), where penicillin VK and amoxicillin are both acceptable first-line oral agents 2, 6. For GBS:

• The infections are typically more severe (neonatal sepsis, meningitis, bacteremia) 3
• IV therapy is standard, not oral 1
• The clinical context is usually perinatal or in immunocompromised/elderly adults 3, 7

Key Pitfalls to Avoid

• Do not use amoxicillin-clavulanate (Augmentin) for GBS - the clavulanate adds no benefit since GBS does not produce beta-lactamase, and it unnecessarily broadens the spectrum 1
• Do not rely on erythromycin or clindamycin without susceptibility testing - resistance rates have increased to 16-21% for erythromycin and 4-9% for clindamycin 8, 4
• Do not use gentamicin monotherapy - while combination therapy with penicillin plus gentamicin accelerates bacterial killing, all GBS strains show high-level gentamicin resistance when used alone 4, 5

Penicillin-Allergic Patients

For patients with penicillin allergy:

• Non-anaphylactic allergy: Use cefazolin 2 g IV initially, then 1 g IV every 8 hours 2
• High-risk anaphylaxis: Use clindamycin or erythromycin only if susceptibility testing confirms sensitivity; otherwise use vancomycin 1 g IV every 12 hours 2, 1