Cephalexin Dosage for Cats Calculator (mg/kg)

Cephalexin is a first-generation cephalosporin, a beta-lactam that binds penicillin-binding proteins and blocks the cross-linking step of bacterial cell wall synthesis so that growing organisms lyse. Its spectrum is strongest against gram-positive cocci, notably Staphylococcus species including Staphylococcus felis, a common feline skin and urinary isolate, along with streptococci and some Proteus and Escherichia coli. Like other beta-lactams it kills in a time-dependent fashion: efficacy tracks the proportion of the dosing interval during which drug concentration stays above the minimum inhibitory concentration, which is why holding to a twice or three times daily schedule matters more than pushing a single large dose. Feline dose ranges in the literature generally span 15 to 30 mg per kilogram every 12 hours, with deep infections such as osteomyelitis moved to the upper end and to eight-hour intervals. Courses run from roughly 10 days for simple soft tissue infections to six weeks for bone. There is no cephalexin product licensed for cats in the United States, so feline use is extra-label and rests on pharmacokinetic studies and clinical experience rather than an approved label. Practical constraints follow from that. Capsules come in 250 and 500 mg strengths far too large to divide accurately for a 4 kg cat, so compounded suspensions are common, and many cats salivate or refuse the liquid because it is bitter. Gastrointestinal upset is the usual adverse effect and is reduced by giving the dose with food. Cats with a documented cephalosporin or penicillin reaction should not receive it, and prescribing repeatedly without culture and susceptibility testing drives resistance.

The cephalosporins began with a public health observation rather than a laboratory screen. In 1945 Giuseppe Brotzu, a professor of hygiene and later rector at the University of Cagliari in Sardinia, noticed that seawater near a sewage outfall appeared to self-purify, and isolated from it a mould then called Cephalosporium acremonium whose culture filtrates inhibited a range of bacteria including Salmonella typhi. Unable to develop the finding locally, Brotzu sent the culture to Britain in 1948, where Edward Abraham and Guy Newton at Oxford isolated cephalosporin C and determined its structure, revealing a beta-lactam nucleus resistant to the penicillinase enzymes that were already eroding penicillin. Semisynthetic derivatives followed through the 1960s. Cephalexin, an orally absorbed member of the first generation, was developed by Eli Lilly and reached the market around 1969 under the name Keflex; its oral bioavailability made it a mainstay of outpatient treatment for skin and urinary infections. Veterinary adoption followed in the 1970s and 1980s, initially with human-labelled product used off-label. In the United States that practice was placed on a firm legal footing by the Animal Medicinal Drug Use Clarification Act of 1994, which permits veterinarians to prescribe human drugs extra-label under defined conditions. Cats have remained on the extra-label side of that line, so feline dose ranges derive from pharmacokinetic studies and formulary consensus, particularly the entries in Plumb's Veterinary Drug Handbook, rather than from a licensed feline indication.