Dog Benadryl Dosage Calculator
Ask your veterinarian before giving your dog any medication. The figure below is the dose range the Merck Veterinary Manual publishes — it is what to bring to that conversation, not a decision to make from this page. Many human antihistamine products contain ingredients that are toxic to dogs.
Benadryl is the brand name for diphenhydramine, an antihistamine. How much Benadryl to give a dog is worked out from its weight: the Merck Veterinary Manual publishes a range of 2–4 mg per kilogram of body weight. Enter your dog’s weight to see what that comes to.
Published dose range
27–54 mg
per dose, every 8–12 hours — for a 30 lb dog.
About 1.1–2.2 tablets of 25 mg.
The tablet figure assumes 25 mg of diphenhydramine per tablet, the strength on the FDA label for Benadryl film-coated tablets. Check the strength printed on your own box — it may differ.
How much Benadryl can I give my dog?
The Merck Veterinary Manual’s dosage table for skin disease gives diphenhydramine at 2–4 mg/kg, by mouth or by injection, every 8–12 hours as needed. For hives specifically it describes a course of 3–5 days. We show the whole range rather than one number inside it, because the manual publishes a range and choosing a point in it would invent a precision no document supports.
Does Benadryl actually work in dogs?
A published trial says less than you would expect. In a randomized, double-blinded crossover study of 12 healthy beagles (Papich et al., Veterinary Dermatology), oral diphenhydramine at 2.2 mg/kg twice daily for 6 days failed to show an inhibitory effect on cutaneous allergic reactions — even though it reached blood concentrations considered effective in people. Oral cetirizine at 2 mg/kg, tested the same way, did prevent them.
A separate study measured how much of an oral dose actually reaches a dog’s bloodstream: 7.8%. Both findings are reasons to ask your veterinarian which antihistamine fits your dog, rather than reasons to change anything on your own. The dose above is what Merck publishes; this is what the trials measured. We show both.
Never for a cat, and not every product
The Merck Veterinary Manual states that in cats diphenhydramine should be avoided, because it may cause paradoxical excitation. That is why this calculator is for dogs only. And “Benadryl” is a brand covering many formulations: the American Kennel Club warns that some contain xylitol, alcohol, acetaminophen, all toxic to dogs, and advises against time-release capsules. Read the ingredient list on the box you own, not the name on the front.
Common questions
How much Benadryl can I give my dog?
That is a question for your veterinarian, and the number above is what to bring them. The Merck Veterinary Manual publishes a diphenhydramine dose for dogs of 2–4 mg per kilogram of body weight, every 8–12 hours as needed. Enter your dog's weight to see what that range works out to in milligrams. We publish the range Merck publishes rather than picking one number inside it, because the manual doesn't pick one either.
How much Benadryl for a 50 lb dog?
Applying the published 2–4 mg/kg to 50 lb gives 45–91 mg per dose. On the one FDA label we checked — Benadryl film-coated tablets, 25 mg of diphenhydramine and nothing else — that works out to about 1.8 to 3.6 tablets. Check the strength printed on your own box before assuming it matches, and confirm the dose with your veterinarian.
Does Benadryl actually work in dogs?
Less well than its popularity suggests, and that is worth knowing before giving any. In a randomized, double-blinded crossover study of 12 healthy beagles (Papich et al., Veterinary Dermatology), oral diphenhydramine at 2.2 mg/kg twice daily for 6 days failed to show an inhibitory effect on cutaneous allergic reactions, even though the drug reached blood levels considered effective in people. Oral cetirizine at 2 mg/kg, in the same study, did prevent them. A separate study measured oral diphenhydramine's systemic availability in dogs at only 7.8%. None of that is a reason to switch drugs on your own — it is a reason to ask your veterinarian which one fits your dog.
Can I give my cat Benadryl?
Not on the strength of this page. The Merck Veterinary Manual states that in cats diphenhydramine should be avoided, because it may cause paradoxical excitation — the opposite of the calm effect people expect. That is why this calculator is for dogs only, and why there is no cat version of it here. Ask your veterinarian what is appropriate for a cat.
Which Benadryl products are dangerous for dogs?
"Benadryl" is a brand covering many formulations, and some carry active ingredients a dog must not have. The American Kennel Club warns that some contain xylitol, alcohol, acetaminophen — all toxic to dogs — and advises against time-release capsules, which dogs absorb differently from people. Read the full ingredient list on the box you actually own rather than the name on the front, and check it with your veterinarian.
How often can the dose be repeated?
The Merck Veterinary Manual publishes every 8–12 hours as needed, and for hives it describes a course of 3–5 days. How long to continue — and whether to start at all — is a decision for your veterinarian, not one to read off a calculator.
This is a published dose range, not a recommendation to medicate. Only a veterinarian can decide whether an antihistamine is right for your dog, which product to use and for how long. It does not replace professional advice.
Verified on . Based on Antihistamine Dosages for Integumentary Disease in Animals (Merck Veterinary Manual, dosage table, accessed 2026-07-30), Urticaria (Hives, Wheals) in Animals — antihistamine course and the feline caution (Merck Veterinary Manual, accessed 2026-07-30), BENADRYL — diphenhydramine hydrochloride tablet, film coated (25 mg, single active ingredient) (DailyMed, U.S. National Library of Medicine, FDA product label, accessed 2026-07-30) and Effect of diphenhydramine and cetirizine on immediate and late-phase cutaneous allergic reactions in healthy dogs (Papich et al., Veterinary Dermatology, randomized, double-blinded crossover study, 12 dogs (abstract, PMID 31899570)). See the method.