Cephalexin for Cats Dosage Calculator
Cephalexin for Cats Dosage Calculator
Enter your cat's current body weight
Select the type of infection being treated
Select the severity of the infection
Select the cephalexin formulation prescribed
Kittens and seniors receive a reduced dose
Reduced kidney function requires a 30% dose reduction
Number of days for the full antibiotic course
Enter Your Cat's Details
Fill in your cat's weight, infection type, severity, and formulation to calculate the recommended cephalexin dose, frequency, and full treatment course.
Estimate antibiotic dose by weight, indication, severity, and formulation
Cephalexin is one of the most commonly prescribed antibiotics for cats in veterinary practice. As a first-generation cephalosporin antibiotic, it works by interfering with bacterial cell wall synthesis, making it particularly effective against gram-positive organisms such as Staphylococcus and Streptococcus species — the primary culprits in feline skin infections, urinary tract infections, and wound infections. Veterinarians have relied on cephalexin for decades because of its broad safety profile, oral availability, and excellent tissue penetration. Determining the correct dose for your cat is not a straightforward one-size-fits-all calculation. Unlike many human medications, cat dosing is highly dependent on body weight, the specific infection being treated, its severity, the individual cat's age, and underlying organ function — particularly kidney health. A 2 kg kitten with a mild skin infection will need a very different dose than a 6 kg senior cat with a severe bone infection and reduced kidney function. Getting the dose right matters: too little cephalexin risks under-treatment and antibiotic resistance; too much can cause unnecessary side effects or toxicity. This calculator brings together the clinical dose rate ranges published in veterinary references including the Merck Veterinary Manual, and adjusts for indication-specific dosing, severity tiers, age group modifiers, and kidney function — all in one tool. The results are expressed not just in milligrams, but in practical terms: how many capsules to give, how many millilitres of suspension to draw up, how often to give each dose, and how much medication to purchase for the entire treatment course. Cephalexin is available in several formulations, each suited to different cats and household circumstances. Standard human-grade capsules (250 mg and 500 mg) are the most widely available, but the doses for cats are often fractions of a capsule, which creates precision challenges at home. Oral suspensions at 125 mg/5 mL (25 mg/mL) or 250 mg/5 mL (50 mg/mL) are easier to dose accurately and are generally better tolerated by cats. For very small cats — typically under 3 kg — even the 25 mg/mL suspension may require volumes so small that a compounded formulation from a veterinary compounding pharmacy may be recommended to ensure accuracy. Dosing frequency is another critical variable. Cephalexin has a relatively short half-life in cats, which means it must be given multiple times daily to maintain therapeutic blood levels. For mild infections, twice-daily (every 12 hours, or BID) dosing is typically sufficient. Moderate infections may require three times daily (every 8 hours, or TID), while severe or deep infections like osteomyelitis may need dosing every 6 hours (QID). Missing doses or stopping treatment early — even when the cat appears to have recovered — is one of the most common reasons antibiotic treatment fails, and is also a significant driver of antibiotic resistance. Treatment duration varies substantially by infection type. Simple UTIs may resolve in 7–14 days. Skin and soft-tissue infections typically require at least 21 days, and severe pyoderma or deep skin infections can require 8–12 weeks of continuous treatment. Bone infections (osteomyelitis) often require 28 days or longer. Understanding the full course commitment upfront helps pet owners plan medication supply and budget appropriately. Cephalexin is generally a safe antibiotic for cats, but it is not without side effects or contraindications. The most common side effect is gastrointestinal upset — vomiting, diarrhea, or decreased appetite — which often resolves if the medication is given with a small amount of food. Cats with known hypersensitivity to cephalosporins or penicillins should not receive cephalexin due to potential cross-reactivity. Cats with significant kidney disease require dose reductions, as cephalexin is primarily renally excreted; failure to reduce the dose in these patients can lead to drug accumulation and toxicity. This tool is designed to help cat owners understand their prescribed treatment plan, ask informed questions at the vet, and avoid dosing errors at home. It is not a replacement for a veterinary diagnosis or prescription — every result generated by this calculator should be reviewed with a licensed veterinarian before any medication is administered.
Understanding Cephalexin Dosing for Cats
What Is Cephalexin?
Cephalexin (brand names Keflex, Rilexine) is a first-generation cephalosporin antibiotic that kills bacteria by disrupting their cell wall synthesis. It is bactericidal — meaning it kills bacteria outright rather than just inhibiting growth. Cephalexin is particularly effective against gram-positive bacteria such as Staphylococcus intermedius and Streptococcus species, which are the most common causes of feline skin infections, wound infections, and urinary tract infections. In cats, it is available as human-label capsules (250 mg and 500 mg), oral suspensions (125 mg/5 mL and 250 mg/5 mL), and compounded preparations in various strengths and flavours. Veterinarians may also prescribe chewable tablets marketed for dogs, though these are often too high a dose for small cats and compounded alternatives are usually preferred.
How Is the Dose Calculated?
The fundamental formula is straightforward: Single dose (mg) = Cat weight (kg) × Dose rate (mg/kg). The dose rate varies by infection type and severity — ranging from 10 mg/kg for mild general infections up to 35 mg/kg for severe skin or bone infections. This base rate is then multiplied by age and kidney modifiers: kittens and seniors receive 85% of the adult rate (×0.85), and cats with reduced kidney function receive 70% of the standard rate (×0.70). Both adjustments can apply simultaneously if appropriate. The resulting dose in mg is then divided by the formulation concentration to give a practical amount — capsule count for solid formulations, or millilitres for liquids. Daily dose equals the single dose multiplied by doses per day (2 for BID, 3 for TID, 4 for QID), and total course mg is daily dose multiplied by treatment duration in days.
Why Does Accurate Dosing Matter?
Under-dosing cephalexin is one of the most common drivers of treatment failure and antibiotic resistance. When antibiotic blood levels fall below the minimum inhibitory concentration (MIC) for the target bacteria, some organisms survive and can develop resistance — making them harder to treat not just in this patient but potentially in others through transmission. Conversely, over-dosing — particularly in cats with reduced kidney function — can cause drug accumulation and toxicity, manifesting as worsening gastrointestinal signs, neurological effects, or kidney damage. The therapeutic window for cephalexin in cats is generally 10–35 mg/kg per dose, with safety margins depending on the individual patient's health status. Accurate weight-based dosing, appropriate frequency, and completing the full prescribed course are all essential to successful antibiotic therapy.
Limitations and Important Caveats
This calculator provides reference estimates based on published veterinary dosing guidelines and should never replace a veterinary examination or prescription. Actual doses prescribed by your veterinarian may differ based on culture and sensitivity results showing the specific antibiotic susceptibility of the infecting organism, the cat's complete medical history and concurrent medications, the anatomical location and depth of the infection, clinical response during treatment, and regional antibiotic resistance patterns. Cephalexin may not be the appropriate antibiotic for all feline infections — gram-negative infections, anaerobic bacteria, and certain resistant strains require different antibiotics. Additionally, cats with penicillin or cephalosporin allergies must not receive cephalexin. The dose adjustments for kittens, seniors, and cats with kidney disease in this tool are generalised estimates; your veterinarian may apply different adjustments based on bloodwork and clinical assessment.
How to Use This Calculator
Enter Your Cat's Weight
Type your cat's current body weight and select the unit (kg or lb). Use a recent weight from a vet visit for accuracy — even small weight differences affect the dose meaningfully, especially for smaller cats.
Select Infection Type, Severity, and Age
Choose the type of infection being treated and how severe it is. Then select your cat's age group (kitten, adult, or senior) and kidney function status. These fields automatically adjust the dose rate up or down using published veterinary modifiers.
Choose the Formulation
Select the cephalexin formulation your veterinarian has prescribed — capsule, oral suspension, chewable tablet, or oral paste. The calculator converts the mg dose into practical units (capsules, half-tablets, or millilitres) so you can measure accurately at home.
Review Results and Export
Check the dose per administration, daily dose, frequency, safety range status, and full treatment course totals. Use the weight-band reference table for context and export a CSV summary to share with your vet or keep as a reference during the treatment course.
Frequently Asked Questions
What dose of cephalexin is typically prescribed for cats?
The standard cephalexin dose for cats ranges from 10 to 35 mg/kg per administration, depending on the infection type and severity. For mild infections, veterinarians typically start at the lower end of 10–15 mg/kg. Skin and soft-tissue infections often require 22–35 mg/kg, while UTIs are commonly treated at 12–25 mg/kg. Dosing frequency ranges from twice daily (every 12 hours) for mild cases to every 6–8 hours for moderate-to-severe infections. Your veterinarian will determine the specific dose based on your cat's weight, health history, and the infection being treated. Never adjust the dose without veterinary guidance, as under-dosing risks treatment failure and antibiotic resistance.
Can I use human cephalexin capsules for my cat?
Yes, human-label cephalexin capsules (250 mg and 500 mg) are commonly prescribed for cats, often off-label. However, because many cats weigh 3–5 kg, the required dose is frequently a fraction of a standard capsule — sometimes as little as half or a quarter of a 250 mg capsule — which is difficult to measure accurately at home. For small cats or precise dosing situations, veterinarians often recommend a compounding pharmacy to prepare cephalexin in a suspension or a lower-strength capsule tailored to the cat's exact dose. This calculator flags when a compounding pharmacy formulation may be advisable based on the calculated dose versus capsule size.
How long does my cat need to take cephalexin?
Treatment duration depends on the infection type. Simple urinary tract infections typically resolve in 7–14 days. Wound infections and respiratory infections usually require 7–14 days as well. Skin and soft-tissue infections (pyoderma) often need at least 21 days of treatment, and severe or deep pyoderma can require 8–12 weeks. Bone infections (osteomyelitis) require the longest courses — typically 28 days or more. A critical rule for all antibiotic therapy: always complete the full prescribed course even if the cat seems better before the end. Stopping early allows partially resistant bacteria to survive and can cause relapse with a harder-to-treat infection.
Why does kidney function affect the cephalexin dose?
Cephalexin is primarily eliminated from the body through the kidneys. In cats with reduced kidney function (chronic kidney disease or acute kidney injury), the drug is cleared more slowly than normal, leading to higher drug concentrations in the blood over time. If the standard dose is given to a cat with impaired kidneys, cephalexin can accumulate to toxic levels, potentially worsening kidney function and causing neurological side effects. To prevent this, veterinarians typically reduce the dose by approximately 30% — the same adjustment applied by this calculator when 'Reduced' kidney function is selected. Cats with kidney disease should always have their antibiotic therapy monitored closely by a veterinarian.
What are the most common side effects of cephalexin in cats?
The most common side effect of cephalexin in cats is gastrointestinal upset, which can include vomiting, diarrhea, decreased appetite, and drooling. These effects typically occur shortly after dosing and often improve if the medication is given with a small amount of food. Less commonly, cats may experience lethargy, increased activity, or fever. Serious allergic reactions — while rare — can include facial swelling, hives, or difficulty breathing, and require immediate veterinary attention. Cats with a history of allergic reactions to cephalosporins or penicillin antibiotics must not receive cephalexin due to potential cross-reactivity. Always report any unusual symptoms to your veterinarian promptly.
Does cephalexin dose change for kittens or senior cats?
Yes, both kittens (under 1 year) and senior cats (over 7 years) typically receive a reduced dose compared to healthy adult cats. Kittens have immature organ systems, particularly the liver and kidneys, which means they metabolise and eliminate drugs differently than adults. Senior cats commonly have declining organ reserve — especially kidney function — even if bloodwork appears normal, and may be more susceptible to side effects. This calculator applies a 0.85 (85%) multiplier for both kittens and seniors as a conservative starting adjustment. Your veterinarian may apply a different modifier based on specific bloodwork results, body condition score, and clinical judgment. Age-adjusted dosing helps balance therapeutic effectiveness with safety in these more vulnerable life stages.