Cephalexin is a first-generation cephalosporin antibiotic and one of the most frequently prescribed antibiotics in veterinary medicine, particularly for dogs. Sold under the human brand name Keflex and the veterinary brand Rilexine, cephalexin is a workhorse antibiotic for treating skin infections (pyoderma), urinary tract infections, wound infections, and bone infections. It is generally well-tolerated in dogs and cats with mild side effects. However, cephalexin and other oral cephalosporins and penicillin-class antibiotics are fatally toxic to rabbits, guinea pigs, hamsters, and chinchillas, as they disrupt the critical gut flora in these species.
What Is Cephalexin Used For?
Cephalexin is prescribed for a wide variety of bacterial infections in dogs and cats:
- Skin infections (pyoderma) — the most common use; superficial and deep bacterial skin infections
- Urinary tract infections (UTIs) — commonly caused by susceptible gram-positive and some gram-negative bacteria
- Wound infections — surgical wounds, bite wounds, lacerations
- Bone infections (osteomyelitis) — cephalexin achieves good bone penetration
- Ear infections (otitis) — when systemic antibiotics are warranted
- Soft tissue infections — abscesses, cellulitis
- Dental infections — though clindamycin is often preferred for deeper dental/bone infections
How Does Cephalexin Work?
Cephalexin is a beta-lactam antibiotic that works by inhibiting bacterial cell wall synthesis. It binds to penicillin-binding proteins (PBPs) in the bacterial cell wall, disrupting the cross-linking of peptidoglycan chains that give the wall its structural integrity. Without a functional cell wall, bacteria cannot maintain their shape or withstand osmotic pressure, leading to cell lysis (bursting) and death.
As a first-generation cephalosporin, cephalexin is primarily effective against gram-positive bacteria (Staphylococcus, Streptococcus) and some gram-negative bacteria (E. coli, Proteus). It is bactericidal (directly kills bacteria rather than just stopping their growth).
Dosage
Your veterinarian will determine the exact dose and duration for your pet.
Dogs
| Dog’s Weight | Typical Dose Range | Frequency | Common Duration |
|---|---|---|---|
| 10 lbs (4.5 kg) | 100-150 mg | Every 8-12 hours | 7-28 days |
| 20 lbs (9 kg) | 200-300 mg | Every 8-12 hours | 7-28 days |
| 40 lbs (18 kg) | 400-550 mg | Every 8-12 hours | 7-28 days |
| 60 lbs (27 kg) | 550-800 mg | Every 8-12 hours | 7-28 days |
| 80 lbs (36 kg) | 800-1100 mg | Every 8-12 hours | 7-28 days |
Standard dose: 10-15 mg/lb (22-30 mg/kg) every 8-12 hours. Skin infections often require longer courses (14-28 days or more for deep pyoderma).
Cats
| Cat’s Weight | Typical Dose Range | Frequency | Common Duration |
|---|---|---|---|
| 8 lbs (3.6 kg) | 100-130 mg | Every 8-12 hours | 7-14 days |
| 10 lbs (4.5 kg) | 100-150 mg | Every 8-12 hours | 7-14 days |
| 12 lbs (5.4 kg) | 130-180 mg | Every 8-12 hours | 7-14 days |
Standard feline dose: 10-15 mg/lb (22-30 mg/kg) every 8-12 hours.
How to Give Cephalexin to Your Dog or Cat
- Available as capsules (250 mg, 500 mg), tablets (various strengths), and oral suspension (125 mg/5 mL, 250 mg/5 mL).
- Can be given with or without food. Giving with food may reduce GI upset.
- The oral suspension is helpful for cats and small dogs.
- Complete the full prescribed course even if your pet appears better before the medication is finished. Stopping early promotes antibiotic resistance and can lead to relapse.
- If you miss a dose, give it as soon as you remember and resume the regular schedule.
- NEVER give to rabbits, guinea pigs, hamsters, or chinchillas — oral cephalosporins cause fatal disruption of the hindgut flora, leading to enterotoxemia and death.
Side Effects
| Common Side Effects | Serious Side Effects (Contact Your Vet) |
|---|---|
| Vomiting | Allergic reaction (facial swelling, hives, difficulty breathing) |
| Diarrhea | Severe or bloody diarrhea |
| Decreased appetite | Anaphylaxis (rare) |
| Drooling | Persistent vomiting |
| Signs of yeast infection (skin, ears) due to bacterial die-off |
GI upset is the most common side effect and is usually mild. Animals with known allergies to penicillin-class antibiotics may cross-react to cephalosporins (though cross-reactivity is low, estimated at 1-10%).
Drug Interactions
- Aminoglycoside antibiotics (gentamicin, amikacin): Potential additive nephrotoxicity (kidney damage). Monitor kidney values if combined.
- Probenecid: Increases cephalexin blood levels by reducing renal excretion.
- NSAIDs: Theoretical increase in kidney stress when combined; ensure adequate hydration.
- No other major drug interactions at standard doses.
Contraindications
- Known allergy to cephalosporins or penicillins — cross-reactivity is possible
- FATAL to rabbits, guinea pigs, hamsters, and chinchillas — oral cephalosporins and penicillin-class antibiotics cause lethal enterotoxemia in these species. This is well-documented and absolute.
- Use with caution in animals with severe kidney disease (dose adjustment may be needed)
- Pregnant or lactating animals — generally considered safe, but consult your veterinarian
Storage
- Store at controlled room temperature, 68-77degF (20-25degC).
- Oral suspension: Refrigerate after reconstitution; discard after 14 days.
- Keep in original container.
- Keep out of reach of children and pets.
Cost & Availability
Cephalexin is a widely available generic human antibiotic at very affordable pricing.
- Approximate cost: $10-$25 per treatment course for most dogs
- Available forms: 250 mg and 500 mg capsules; tablets; oral suspension (125 mg/5 mL, 250 mg/5 mL)
- Can be filled at human pharmacies with a veterinary prescription at low cost
- Rilexine (veterinary brand) is also available through veterinary clinics
- One of the most affordable prescription antibiotics
Frequently Asked Questions
Why is my vet prescribing cephalexin for my dog’s skin infection? Cephalexin is highly effective against Staphylococcus pseudintermedius, the most common bacterial cause of skin infections in dogs. It has a long track record of safety, is affordable, and achieves good drug concentrations in the skin. For simple superficial pyoderma, it is often the first-choice antibiotic.
How long does my dog need to take cephalexin? Treatment duration depends on the infection type and severity. Simple UTIs may need 7-14 days. Superficial skin infections typically require 14-21 days. Deep skin infections or bone infections may need 4-8 weeks or longer. Always complete the full course as directed by your veterinarian.
Why is cephalexin fatal to rabbits? Rabbits, guinea pigs, hamsters, and chinchillas are hindgut fermenters whose digestive health depends on a delicate balance of gut bacteria. Oral cephalosporins (and penicillin-class antibiotics) destroy beneficial bacteria in the cecum, allowing overgrowth of toxin-producing Clostridium species. This causes fatal enterotoxemia — a rapid, often untreatable condition. This is one of the most important species-specific drug safety facts in veterinary medicine.
Can I use the fish or bird version of cephalexin for my dog? No. While the active ingredient may be the same, fish and bird antibiotics are not quality-controlled for use in mammals, may contain different fillers or contaminants, and their use in dogs and cats is not recommended by any veterinary organization. Always use medications dispensed through a veterinary prescription.
Sources & References
- Merck Veterinary Manual - Cephalosporins
- VCA Hospitals - Cephalexin
- PetMD - Cephalexin for Dogs
- Merck Veterinary Manual - Antibiotic-Associated Enterotoxemia in Rabbits
- Plumb’s Veterinary Drug Handbook - Cephalexin
Verification Notes
Cephalexin’s description as a “First-generation cephalosporin. Very commonly prescribed for dogs. Generally safe with mild GI side effects” is confirmed by the research corpus. The fatal toxicity to rabbits and small herbivores is verified by the fact-check report (Claim C20, rated TRUE): “Oral amoxicillin and several antibiotic classes can cause fatal dysbiosis/enterotoxemia in rabbits.” This extends to all oral beta-lactam antibiotics including cephalosporins.