Cat Benadryl Dosage Calculator (mg by Weight)

Diphenhydramine is a first-generation ethanolamine antihistamine that acts as an inverse agonist at the H1 receptor, blunting the itch, oedema and wheal produced when mast cells release histamine. It is lipophilic enough to cross the blood-brain barrier, which produces the sedation it is often used for, and it also has anticholinergic and vestibular-suppressant activity that underlies its use against motion sickness. Feline dosing is entirely extra-label and is scaled to body weight: the commonly cited figure is 1 mg per pound, equivalent to roughly 2 mg/kg, given by mouth every 8 to 12 hours, within a working range of about 0.5 to 2 mg per pound and a ceiling near 4 mg per pound per day. Oral onset is about 30 to 60 minutes with effects lasting 8 to 12 hours. The arithmetic exposes a practical problem. Children's elixir at 12.5 mg per 5 mL is 2.5 mg/mL, so a 10 lb cat needs 4 mL, while the same dose from a 25 mg tablet is 0.4 of a tablet, a fraction no pill cutter delivers reliably; compounded preparations solve this. Combination cold products are the main hazard, because acetaminophen is lethal to cats at very low doses given their limited glucuronidation capacity, and pseudoephedrine, phenylephrine, alcohol and xylitol are all unsafe. Cats with glaucoma, hyperthyroidism, cardiac disease, seizure history or urinary obstruction should not receive it without veterinary clearance, paradoxical excitement is not rare in cats, and antihistamines control feline allergic skin disease far less reliably than they do in dogs. This is reference information, not veterinary advice.

Diphenhydramine was synthesised in 1943 by George Rieveschl, a chemistry professor at the University of Cincinnati who was working on compounds related to smooth-muscle relaxants rather than on allergy. The class itself had been opened a few years earlier at the Pasteur Institute in Paris, where Daniel Bovet and Anne-Marie Staub identified the first compounds that antagonised histamine in 1937; Bovet received the Nobel Prize in Physiology or Medicine in 1957 for that line of work. Rieveschl's compound was licensed to Parke-Davis, marketed as Benadryl and approved by the United States Food and Drug Administration in 1946 as the first prescription antihistamine, moving to over-the-counter status decades later. No diphenhydramine product has ever been approved for cats, so feline use rests on the extra-label prescribing framework created in the United States by the Animal Medicinal Drug Use Clarification Act of 1994, which allows veterinarians to use human-approved drugs in animals under a valid client-patient relationship. The weight-based doses in circulation come from veterinary pharmacology references, above all Plumb's Veterinary Drug Handbook, first published in 1991 and revised through many editions, together with accumulated clinical experience. The strict warning against combination products grew out of clinical reports of feline acetaminophen poisoning from the 1970s onward, which established that cats lack the hepatic glucuronidation capacity that protects other species.