Cephalexin
Cephalexin monohydrate
Also known as: Keflex · Ceporex · Cefalexina · Cefaseptin · Cephacare · Cephorum · Rilexine · Therios · Tsefalen · cefalexinum · 66873 · Cephalexin
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Audited v13 dosing evidence
Structured calculator evidenceSusceptible infections (extra-label)
- q8-12h
NA
Governing clinical context (3)
- ■ Therapeutic drug monitoring is not routinely done with these agents.
- ■ This medicine can be given with or without food, but gastrointestinal side effects might be prevented if given with food.
- Cephalexin tablets, chewable tablets, capsules, and powder for oral suspension should be stored at room temperature (15°C-30°C; 59°F-86°F) in tight containers. After reconstitution, the oral suspension is stable for 2 weeks when refrigerated.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
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 of action
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 → Inhibition of the final transpeptidation step of peptidoglycan synthesis → Formation of a defective, osmotically unstable cell wall (spheroplast) → Cell lysis and death.
- Like other beta-lactams, it is most effective against actively dividing bacteria.
Safety & warnings
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
Precautions
Hypersensitivity Cross-Reactivity: Use with caution in patients with documented hypersensitivity to other beta-lactam antibiotics (e.g., penicillins, carbapenems), as cross-reactivity can occur.
- Renal Impairment: Use cautiously and consider dose reduction in patients with renal dysfunction or those receiving concurrent nephrotoxic drugs.
- Critically Ill Patients: Do not use oral systemic antibiotics in patients with shock or septicemia due to delayed or diminished GI absorption.
- Laboratory Interference: May cause false-positive urine glucose tests (with cupric sulfate solutions like Clinitest) and falsely elevated serum/urine creatinine (with Jaffe reaction).
Drug interactions
Competitively blocks the tubular secretion of cephalosporins, thereby increasing serum levels and prolonging serum half-lives.
Increased risk of nephrotoxicity. Do not mix in the same syringe.
Increased risk of nephrotoxicity.
Increased risk of nephrotoxicity.
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
Pharmacokinetics
Half-life
Absorption
Rapidly and completely absorbed after oral administration. Bioavailability is approximately 75% in dogs and cats. Food has little impact on absorption. In horses, oral bioavailability is very low (approx. 5%).
Distribution
Widely distributed into most body tissues and fluids. Crosses the placenta and is distributed into maternal milk in small amounts.
Metabolism
Undergoes minimal hepatic metabolism.
Elimination
Primarily excreted unchanged in the urine via glomerular filtration and tubular secretion.
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.
Available products
Formulations
- Oral capsules
- Oral tablets
- Powder for oral suspension
- Injectable suspension (available outside the USA)
Veterinary
- None approved in the USA (injectable forms available internationally)
Human-labeled
- Cephalexin Oral Capsules: 250 mg, 333 mg, 500 mg & 750 mg (Keflex, generic)
- Cephalexin Oral Tablets: 250 mg & 500 mg (Keflex, generic)
- Cephalexin Powder for Oral Suspension: 125 mg/5mL & 250 mg/5 mL (Keflex, generic)
Regulatory status
POM-V classification
POM-V classification
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Store tablets, capsules, and powder for oral suspension at room temperature (15-30°C) in tight containers. After reconstitution, the oral suspension is stable for 2 weeks.
Client information
Cephalexin is a commonly prescribed antibiotic used to treat bacterial infections, especially skin infections (pyoderma).
- Administration: Can be given with or without food. If your pet vomits or acts sick after receiving the dose, try giving it with a small meal or treat.
- Finish the Course: It is critical to give the medication for the entire time prescribed by your veterinarian, even if your pet seems completely better. Stopping early can lead to resistant infections.
- Allergic Reactions: Though rare, allergic reactions can occur. Contact your veterinarian immediately if you notice facial swelling, hives, severe scratching, sudden diarrhea, or difficulty breathing.
- Storage: Store capsules and tablets at room temperature. If you were given a liquid suspension, follow the storage instructions on the bottle (usually stable for 14 days after mixing).
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.