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If your dog is itchy, swollen or reacting to something, Benadryl is usually the first thing owners reach for. I dose it at 2 to 4 mg per kilogram of body weight, every 8 to 12 hours. Before any of that, check the package. Diphenhydramine should be the only active ingredient on the label, and a lot of boxes on the shelf have more than that in them.
Key Takeaways
Go straight to a vet if: your dog has facial or throat swelling with laboured breathing, has collapsed, has pale gums, or started vomiting suddenly after a sting, vaccine or new medication. Benadryl works too slowly and too weakly for anaphylaxis. If we're closed please contact the Emergency Veterinary Clinic of Waterloo Region. Otherwise book online or call us at (519) 896-0532.
Benadryl, also known as diphenhydramine HCl, is a first-generation antihistamine. It blocks the histamine that drives itching, swelling, sneezing and the rest of the allergic picture.
It also crosses the blood-brain barrier, which is why it makes dogs drowsy and why it has some use against nausea.
One thing worth knowing before you start. Diphenhydramine isn't approved for veterinary use, so when your vet recommends it, that's an off-label call based on clinical experience and published dosing references. That's routine in veterinary medicine and nothing to worry about. It's also why I'd rather you confirmed the dose with your own vet than treat any chart on the internet, including mine, as the final word.
I dose Benadryl by weight, at 2 to 4 mg per kilogram, which is roughly 0.9 to 1.8 mg per pound. Most people find "about 1 mg per pound" the easiest version to hold onto for the low end. The Merck Veterinary Manual lists the same range, given every 8 to 12 hours as needed.1
A few worked examples at the 4 mg/kg ceiling:
Start at the lower end if the signs are mild. If your dog is having an active allergic reaction, the higher end of the range is more useful.
For dogs over 80 lbs, do the arithmetic from body weight rather than reading off the end of the chart. Divide pounds by 2.2 to get kilograms, then multiply by 2 to 4. Check that number with us before you give it, because at that size a rounding error gets big quickly.
| Dog's weight | Low end (2 mg/kg) | High end (4 mg/kg) |
|---|---|---|
| 5 to 10 lbs | 4.5 to 9 mg | 9 to 18 mg |
| 10 to 20 lbs | 9 to 18 mg | 18 to 36 mg |
| 20 to 30 lbs | 18 to 27 mg | 36 to 54 mg |
| 30 to 40 lbs | 27 to 36 mg | 54 to 72 mg |
| 40 to 50 lbs | 36 to 45 mg | 72 to 90 mg |
| 50 to 60 lbs | 45 to 54 mg | 90 to 108 mg |
| 60 to 70 lbs | 54 to 63 mg | 108 to 127 mg |
| 70 to 80 lbs | 63 to 72 mg | 127 to 145 mg |
| 80 to 100 lbs | 72 to 90 mg | 145 to 181 mg |
Each cell spans the whole weight band, so a dog at the bottom of a row takes the lower number and a dog at the top takes the higher one. A 25 lb dog sits mid-row, which puts the low end near 22 mg and the ceiling near 45 mg.
Standard Benadryl tablets are 25 mg, so the arithmetic rarely lands on a whole tablet. Children's liquid at 12.5 mg per 5 ml is easier to measure accurately for small dogs. Ask us which form suits your dog before you start splitting anything.
Dogs under 5 lbs aren't on this chart on purpose. At that size the margin between a useful dose and too much is small enough that I want to work it out with you directly rather than have you round off a tablet. Call us on (519) 896-0532 and we'll give you a number for your dog.
Treat the chart as a reference point, not a replacement for your vet's advice. Doses can be repeated every 8 to 12 hours as needed.1
Sedation varies a lot between dogs. Some show none at all, others go flat out on the same milligrams per kilo. It's generally dose-dependent, so the higher you go in the range, the more of it you should expect.
Treat the chart as a reference point, not a replacement for your vet's advice. Doses can be repeated every 8 hours as needed.
Sedation varies a lot between dogs. Some show none at all, others go flat out on the same milligrams per kilo. It's generally dose-dependent, so the higher you go in the range, the more of it you should expect.
Something I've noticed over the years is that Benadryl works better when it's started early. Give it before the itch is in full swing, not once the dog is already inflamed and chewing at themselves.
That turned out to be the published consensus rather than just my impression. The International Committee on Allergic Diseases of Animals reviewed the evidence and found type-1 antihistamines "are not likely to be beneficial after a flare of AD (Atopic dermatitis, or seasonal allergies) has occurred." For best effect they should be given before a flare starts, ideally on a continuous daily basis.3
The same guidelines are honest about the ceiling. Oral antihistamines offer a small and limited benefit in some dogs, and they're most useful in mild cases. Part of what owners read as improvement may simply be the sedation.3
So if your dog has seasonal allergies you can see coming, starting Benadryl ahead of the season is a reasonable use of it. If you're waiting until they're miserable and then reaching for the box, expect less. Most dogs with ongoing allergic skin disease need a proper allergy workup rather than antihistamines alone.
Check the active ingredients before every dose. Merck reports pseudoephedrine causing clinical signs in dogs at 5 to 6 mg/kg and deaths at 10 to 12 mg/kg, with agitation, racing heart, high blood pressure, tremors and dangerously high body temperature.2 Diphenhydramine should be the only active ingredient on the label. If your dog has already swallowed a combination product, call the ASPCA Animal Poison Control Center on (888) 426-4435 now rather than waiting to see what happens.4
At normal doses most dogs tolerate Benadryl well. What you might see:
This is the part most dosing articles skip, and it's why I'd rather you worked from a calculated dose than a guess.
Overdose looks different from ordinary side effects. The Merck Veterinary Manual lists first-generation antihistamine toxicity as lethargy, sedation, agitation, rapid heart rate, vomiting, low blood pressure, dilated pupils, tremors and potentially seizures, usually appearing within 30 to 60 minutes of ingestion.2
So a dog who is tremoring, agitated, disoriented or seizing after a dose isn't having a side effect you should wait out. Seizures belong in this list, not the one above it.
If you think your dog has had too much, call for advice straight away:
Have the package and your dog's weight in front of you when you call. The milligram strength and roughly how much went down changes the advice considerably.
Yes, within limits. It can take the edge off symptoms, particularly when started early. Published guidelines describe the benefit as small and limited, and most dogs with ongoing allergies need additional therapy to actually get comfortable.
Dose at 2 to 4 mg per kilogram of body weight, which is about 0.9 to 1.8 mg per pound. Confirm the number with your vet, since your dog's other conditions and medications can change what's appropriate.
The common ones are drowsiness, dry mouth, constricted pupils and digestive upset such as diarrhea or vomiting. Tremors, agitation and seizures point to overdose rather than ordinary side effects, and those warrant a call to your vet or a poison control line.
Every 8 to 12 hours, so two to three times per day. Speak with your veterinarian about dosing specific to your dog.
Call the ASPCA Animal Poison Control Center at (888) 426-4435, the Pet Poison Helpline at (855) 764-7661, or your vet. Have the package and your dog's weight ready. Signs typically show up within 30 to 60 minutes, so don't wait to see what happens.
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