How this instrument works
Cephalexin is a first-generation cephalosporin antibiotic sometimes used in cats for skin and soft-tissue infections. Unlike the dog version of this drug — where the product Rilexine carries a full FDA approval and label dose — there is no FDA-approved cephalexin product for cats at all. Every use in a cat is what veterinary medicine calls off-label or extra-label use: a licensed veterinarian prescribing a human- or dog-labeled product, at their own clinical judgment, because no feline-specific approved alternative exists.
That distinction matters for what this calculator can and can't tell you. Merck's Veterinary Manual lists a general cephalosporin dosing range for cats of 15–35 mg/kg, and that is the range this tool multiplies against your cat's weight — but it is reference information from a general dosing table, not an FDA-reviewed label dose the way the dog Rilexine figure is. A veterinarian prescribing cephalexin for a cat is making a clinical judgment call, not reading it off an approved package insert.
This tool provides a reference range based on published veterinary sources. It is not a substitute for professional veterinary advice. Always consult your veterinarian before giving any medication, and contact your vet or an animal poison control hotline — for example the ASPCA Animal Poison Control Center, (888) 426-4435 — immediately if you suspect a toxic ingestion. Off-label use is a routine, legal, and often necessary part of veterinary medicine — it simply means the prescribing decision rests more heavily on your veterinarian's judgment, which is one more reason not to dose without them.
- Weigh your cat on a kitchen or pet scale, or use a recent veterinary weight, in kilograms.
- If you only know pounds, divide by 2.2 to convert to kilograms before entering the weight.
- Read the low and high milligram outputs — the range this tool computes from 15 and 35 mg/kg.
- Remember this range is off-label reference information, not an FDA-approved feline label dose.
- Confirm the exact dose, product, and course length with your veterinarian before giving anything to your cat.
Worked example — a 4.5 kg cat
A 4.5 kg cat (roughly 10 lb): low end = 4.5 × 15 = 67.5 mg, high end = 4.5 × 35 = 157.5 mg. That range comes from Merck's general dosages-of-cephalosporins table — the same table used for the dog calculator's general range, but with a lower feline-specific ceiling of 35 mg/kg rather than 45.
A smaller 3 kg cat works out to 45–105 mg, and a larger 6 kg cat reaches 90–210 mg. Whatever the weight, remember every figure here is off-label reference information for your veterinarian to weigh against the specific infection, not a stand-in for their prescription.
Questions
Is cephalexin approved for use in cats?
No. There is no FDA-approved cephalexin product for cats — every feline use is off-label, extra-label veterinary use, prescribed at a veterinarian's clinical judgment. This differs from dogs, which have the FDA-approved product Rilexine with its own specific label dose. This tool provides a reference range based on published veterinary sources; it is not a substitute for professional veterinary advice.
Is off-label use of a drug unsafe?
Not inherently — off-label, extra-label drug use is a routine and legal part of veterinary medicine, common whenever no approved product exists for a given species or condition. What it does mean is that the dosing decision rests more heavily on your veterinarian's clinical judgment rather than an FDA-reviewed label, so their involvement matters even more, not less.
Can I use human cephalexin prescribed for a family member?
Never give a cat medication prescribed for a person without your veterinarian's explicit instruction, even if the drug name matches. Formulations, concentrations, and inactive ingredients can differ, and self-medicating a cat this way risks both under- and over-dosing. Always consult your veterinarian before giving any medication.
What if my cat has a known drug allergy?
Tell your veterinarian before starting any antibiotic. Cats with a known allergy to cephalexin or to the broader penicillin or cephalosporin class of antibiotics should not receive this drug. Contact your vet or an animal poison control hotline immediately if you see facial swelling, hives, vomiting, or breathing trouble after a dose.
Should I stop giving the antibiotic once my cat seems better?
No, not without checking with your veterinarian first. Stopping an antibiotic course early, even when symptoms visibly improve, risks incomplete treatment of the infection and can contribute to antibiotic resistance. Follow the full course length your veterinarian prescribes.
References
- Merck Veterinary Manual — Table: Dosages of Cephalosporins
- Merck Veterinary Manual — Cephalosporins and Cephamycins Use in Animals
- FDA/DailyMed — Rilexine (cephalexin) Chewable Tablets label
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.