Does Keflex (Cephalexin) Treat Group B Streptococcus?
Yes, cephalexin has activity against Group B Streptococcus and can be used as an alternative agent in penicillin-allergic patients with non-immediate (non-anaphylactic) reactions, though it is not first-line therapy for most Group B Strep infections.
Clinical Context and Appropriate Use
Cephalexin is explicitly recommended for patients with non-immediate penicillin allergies who require coverage for streptococcal infections, including Group B Streptococcus 1, 2. The cross-reactivity risk with first-generation cephalosporins like cephalexin is only 0.1% in patients with delayed, non-severe penicillin reactions 2.
When Cephalexin Is Appropriate for Group B Strep:
Skin and soft tissue infections: Group B Streptococcus can cause cellulitis, particularly in patients with previous gynecologic cancer treated with surgery and radiation, or in lower extremity infections 1. Cephalexin is suitable for oral therapy in these cases at 500 mg twice daily for adults 1, 3.
Penicillin-allergic patients (non-anaphylactic): For patients who cannot receive penicillin but have only delayed reactions (rash occurring >1 hour after exposure), cephalexin provides effective streptococcal coverage 1, 2.
Continuation therapy: After initial parenteral treatment for more severe infections, cephalexin can be used for step-down oral therapy 4.
Critical Limitations and Contraindications
When Cephalexin Should NOT Be Used:
Immediate/anaphylactic penicillin allergy: Patients with a history of anaphylaxis, angioedema, respiratory distress, or urticaria occurring within 1 hour of penicillin exposure have up to a 10% cross-reactivity risk with all cephalosporins, including cephalexin 1, 2. These patients require clindamycin or vancomycin instead 2.
Intrapartum prophylaxis for Group B Strep: While first-generation cephalosporins are used in this setting, cefazolin (not cephalexin) is the recommended agent for intrapartum antibiotic prophylaxis in pregnant women with non-anaphylactic penicillin allergy 2. Cephalexin is not appropriate for this indication.
Viridans group streptococci with high resistance: Cephalexin showed 96% resistance among viridans group streptococci in one study, making it the least active cephalosporin tested 1. While this data primarily applies to viridans streptococci (not Group B), it highlights that cephalexin is not universally effective against all streptococcal species.
Dosing and Duration
For adults with Group B Streptococcal skin/soft tissue infections:
- Cephalexin 500 mg orally every 12 hours 2
- Duration: 5-10 days depending on infection severity (uncomplicated cellulitis can be treated for 5 days) 1
For children:
- 20 mg/kg per dose twice daily (maximum 500 mg per dose) 2
Preferred Alternatives
Penicillin or amoxicillin remains first-line for Group B Streptococcal infections in non-allergic patients due to proven efficacy, narrow spectrum, and lack of resistance 2.
For severe invasive Group B Strep infections (bacteremia, meningitis, endocarditis):
- Parenteral therapy is required: Penicillin G or ampicillin IV, NOT oral cephalexin 1
- Cephalexin does not achieve adequate CSF levels and should never be used for meningitis 4
Common Pitfalls to Avoid
- Do not use cephalexin for patients with immediate penicillin hypersensitivity due to the 10% cross-reactivity risk 1, 2
- Do not use cephalexin for intrapartum GBS prophylaxis—cefazolin is the appropriate first-generation cephalosporin for this indication 2
- Do not rely on cephalexin for serious invasive infections—parenteral beta-lactams are required 1
- Do not assume all streptococcal species respond equally—resistance patterns vary significantly 1