Cephalexin
Cephalexin is a widely used **first-generation cephalosporin** antibiotic. It is highly effective against Gram-positive bacteria, particularly **Staphylococcus pseudintermedius** and **Staphylococcus aureus**, making it a cornerstone treatment for veterinary dermatology, especially canine pyoderma. * **Spectrum of Activity**: Excellent coverage against most Gram-positive pathogens (Streptococci, Staphylococci) and variable coverage against some Gram-negative pathogens (*E. coli*, *Klebsiella*, *Proteus mirabilis*). * **Resistance**: Generally ineffective against Enterococci, *Pseudomonas* spp., Methicillin-resistant Staphylococci (MRSA/MRSP), and *Bacteroides fragilis*. * **Clinical Pearls**: Cephalexin is a **time-dependent** antibiotic, meaning its efficacy depends on the amount of time the drug concentration remains above the Minimum Inhibitory Concentration (MIC) at the site of infection. It is well-tolerated orally in dogs and cats, though it has very poor oral bioavailability in horses.
Mechanism: Cephalexin is a **bactericidal** antibiotic that disrupts bacterial cell wall synthesis. * **Mechanism**: It binds to specific enzymes known as **penicillin-binding proteins (PBPs)** (e.g., transpeptidases, carboxypeptidases) located on the inner surface of the bacterial cell membrane. * **Pathway**: Binding to PBPs โ Inhibition of the final transpeptidation step of peptidoglycan synthesis โ Formation of a defective, osmotically unstable cell wall (spheroplast) โ **Cell lysis and death**. * Like other beta-lactams, it is most effective against actively dividing bacteria.
Dosing by species
- For superficial and deep pyoderma ยท 22-33 mg/kg PO two to three times daily ยท PO ยท q8-12h ยท 5-7 days beyond resolution
- For recurrent pyoderma ยท 22 mg/kg PO q12h (use at q8h for deep pyoderma) ยท PO ยท q8-12h ยท 5-7 days beyond resolution
- For superficial canine pyoderma ยท 30-40 mg/kg PO once daily (q24h) for 4 weeks. ยท PO ยท q24h ยท 4 weeks
- For pyoderma ยท 22-35 mg/kg PO q12h or 22 mg/kg PO q8h ยท PO ยท q8-12h ยท 5-7 days beyond resolution
- For respiratory infections ยท 20-40 mg/kg PO q8h ยท PO ยท q8h ยท 5-7 days beyond resolution
- For soft tissue infections ยท 30-50 mg/kg PO q12h ยท PO ยท q12h ยท 5-7 days beyond resolution
- For systemic infections ยท 25-60 mg/kg PO q8h ยท PO ยท q8h ยท 5-7 days beyond resolution
- For orthopedic infections ยท 22-30 mg/kg PO q6-8h for 28 days ยท PO ยท q6-8h ยท 28 days
- For pyometra/metritis ยท 10-30 mg/kg PO q8-12h ยท PO ยท q8-12h
- For UTI ยท 30-40 mg/kg PO q8h. ยท PO ยท q8h ยท 7-10 days for acute; up to 4-8 weeks for chronic/pyelonephritis
- For neonates ยท 10-30 mg/kg PO twice daily-three times daily ยท PO ยท q8-12h ยท Weak neonates should be given via stomach tube
Routes of administration
Contraindications
- Known hypersensitivity to cephalosporins
- Patients with septicemia, shock, or grave illness (oral absorption is unreliable; parenteral routes preferred)
Adverse effects
- Gastrointestinal upset (anorexia, vomiting, diarrhea)
- Hypersensitivity reactions (rashes, fever, eosinophilia, anaphylaxis)
- Salivation, tachypnea, and excitability (reported in dogs)
- Emesis and fever (reported in cats)
- Nephrotoxicity (rare)
- Toxic epidermal necrolysis (rarely reported in cats)
- Antibiotic-associated diarrhea or gut flora alteration
Drug interactions
- Probenecid ยท Competitively blocks the tubular secretion of cephalosporins, thereby increasing serum levels and prolonging serum half-lives.
- Aminoglycosides ยท Increased risk of nephrotoxicity. Do not mix in the same syringe. ยท major
- Amphotericin B ยท Increased risk of nephrotoxicity. ยท major
- Loop diuretics (e.g., furosemide) ยท Increased risk of nephrotoxicity. ยท moderate
Monitoring
- Clinical efficacy (resolution of clinical signs and negative cultures)
- Renal function (BUN, creatinine, urinalysis) in patients with pre-existing renal disease or those on concurrent nephrotoxic drugs
Overdose
Acute oral cephalosporin overdoses are unlikely to cause significant problems other than **gastrointestinal distress** (vomiting, diarrhea, anorexia). * **Severe Overdose**: Extremely high doses or very prolonged use has been associated with neurotoxicity, neutropenia, agranulocytosis, thrombocytopenia, hepatitis, interstitial nephritis, and tubular necrosis. * **Treatment**: Treatment is generally supportive and symptomatic. Emptying the gut may be considered for massive recent ingestions.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.