Cephalexin (Keflex) Does NOT Achieve Therapeutic Brain/CSF Concentrations
Cephalexin should not be used for central nervous system infections because it produces little to no cerebrospinal fluid levels and does not penetrate the blood-brain barrier adequately 1.
Key Evidence on CNS Penetration
Poor CSF Penetration
The FDA label for Keflex 2 does not list any CNS indications, and clinical pharmacology studies demonstrate that cephalexin:
- Is rapidly absorbed orally with peak serum levels of 9-32 μg/mL depending on dose
- Is excreted >90% unchanged in urine
- Does not achieve therapeutic concentrations in CSF
Clinical Guidance Against CNS Use
A comprehensive review explicitly warns: "caution should be exercised to exclude meningitis when treating patients with respiratory tract infections since the majority of these compounds, including cephalexin, produce little or no cerebrospinal fluid levels" 1.
Appropriate Antibiotics for CNS Infections
When CNS penetration is required (meningitis, brain abscess), guidelines recommend 3:
First-Line Agents with Good CSF Penetration:
- Third-generation cephalosporins: Ceftriaxone 2g IV q12h or cefotaxime 2g IV q6h
- Vancomycin: 15-20 mg/kg IV q12h (though CSF penetration is only 1-5%)
- Rifampin: 600 mg q12h (22% CSF penetration, bactericidal concentrations achievable)
Alternative Agents:
- Linezolid: 600 mg IV/PO q12h (excellent 66% CSF penetration) 4
- TMP-SMX: 5 mg/kg/dose IV q8-12h (13-63% CSF penetration) 4
- Chloramphenicol: 25 mg/kg IV q6h (for severe penicillin allergy)
Important Caveat
One experimental study 5 found higher CSF levels after intranasal administration compared to IV/oral routes, suggesting a direct olfactory pathway. However, this was an animal pharmacokinetic study with no clinical validation and is not applicable to standard oral/IV administration in clinical practice.
Bottom Line
Cephalexin is excellent for skin/soft tissue infections, UTIs, and non-CNS respiratory infections 6, 7, 8, but must never be relied upon for brain or CSF infections. Always use agents with documented CSF penetration for suspected or confirmed CNS infections.