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In this Article I will discuss the usage of Benadryl (Diphenhydramine) in dogs, Benadryl is an over-the-counter antihistamine – used to relieve allergic reactions to environmental allergens, insect bites or stings, and specific vaccination reactions.

While Benadryl can be a safe and effective medication for dogs when used properly, an overdose can be fatal and cause severe neurological and cardiovascular effects.

I have often heard the Question If Benadry can kill a Dog from several pet owners. I know, it’s a ridiculous question but I will giv an detailed answer in this article.

Can to much Benadryl Really Kill a Dog?

Yes, Benadryl can cause life-threatening consequences or even death for Dogs – when given in high doses, i.e. above 500mg in 24 hours. Side effects of an overdose may include, diarrhea, vomiting, nausea, sedation, lethargy, loss of appetite, dry mouth, dizziness, and even death (in extra high doses), etc.

What exactly is Benadryl?

Benadryl (Diphenhydramine) is a first-generation-antihistamine (It was one of the first substances of this class discovered). Benadryl can cross the blood-brain barrier very well. The active substance occupies the docking receptors for the body’s histamine so that hisatmin can no longer act inflammatory.

In the brain, the endogenous messenger substance histamine is involved, among other things, in the regulation of the wake-sleep rhythm. If this regulation is disturbed by antihistamines, fatigue is a side effect of Benadryl.

Second-generation antihistamines can hardly cross the blood-brain barrier – which means that fatigue does not occur.

Diphenhydramine also has an inhibitory effect on the vomiting center in the brainstem – thereby suppressing nausea and vomiting. Acetylcholine receptors and sodium channels (ion channels) are also blocked by diphenhydramine!

In general, the endogenous messenger histamine plays an essential role in defense reactions against invading germs and toxins. In the event of an infection or the ingestion of toxins, the immune system causes the so-called mast cells to release stored histamine and other messenger substances.

This results in increased blood flow to the affected tissue. In addition, more white blood cells are attracted, and the blood vessels become more permeable. Last but not least, the tissue swells so that the immune cells can migrate more easily: An inflammatory reaction occurs.

In some cases – this protective reaction of the immune system is undesirable, especially when it manifests as an allergic reaction to pollen, food or insect bites. This can then be suppressed explicitly with antihistamines such as Benadryl (Diphenhydramine).

Allergy Symptoms in Dogs

Why Benadryl Can be Fatal for a Dog

Although antihistamines such as Benadryl (Diphenhydramine) can be used in in Dogs – when given a secure and a correct dosage – an overdose of Benadryl can have fatal consequences for Dogs and even cause the death of a dog!

If You Overdose Benadryl it can be lethal to dogs – because Diphenhydramine can cause severe neurological and also cardiovascular effects like: Hyperexcitability, Palpitations, Seizures, Convulsions, also Respiratory Failure.

Always be extra careful about Dosage when you want/need to give your dog an antihistamine like Benadryl.

I must also clearly warn here that many Overdoses of Antihistamines, not only Benadryl (diphenhydramine), are serious or even fatal and difficult to treat.


Benadryl Dosage for Dogs

The general dosage guideline for dogs is 1 mg of Benadryl per pound of body weight. However – You must always consult your Vet – as several factors may alter this dosage recommendation, and a high overdose of Benadryl may even kill a Dog.

The average dosage in one tablet is 25 mg, so a 25-pound dog should be given one tablet. Consider: You will always need to check the dosage on the package. Because not every Product is the same and you should always Double Check!

Stick to the pill form, as the liquid form may contain alcohol, which is toxic to dogs.

For small dogs, you may use the children’s formula, which contains the less active ingredient and is easier to measure. The children’s formula of liquid Benadryl does not contain alcohol.

Benadryl Children

You can give your dog Benadryl every eight to twelve hours or about two to three times a day. If you use the product frequently, it may lose effectiveness over time.

In most cases, it is better to give medication before exposure to allergens. Follow your Vets guidelines, and stick to them for the duration of your veterinarian’s advice, even if symptoms seem to disappear.

How long does it take for Benadryl to work in dogs?

This medication takes effect quickly in about 1 to 2 hours, and improvement in clinical signs should follow.

Possible Side Effects and Risks of Benadryl

Benadryl can make a dog very sleepy, cause dry mouth or urinary retention, and possibly cause gastrointestinal upset such as Constipation, Diarrhea and/or vomiting.

Benadryl Overdose

Benadryl is historically very safe, but as with anything, too much can be harmful. If your dog is given too much Benadryl, he may be extraordinarily lethargic or very agitated. Occasionally, dry mouth, respiratory depression, seizures, coma, or even a Dog’s Death can occur if a large enough amount of Benadryl are  given.

Signs of overdose:

  • Sedation or drowsiness
  • Dry mouth
  • Increased urination
  • Increased heart rate
  • Vomiting
  • Diarrhea
  • Loss of appetite
  • Disorientation
  • Muscle tremors
  • Seizures
  • Coma
  • In extreme Overdosage of Benadryl has the potential to kill a dog

    If you see any of the above effects in your pet after using Benadryl (antihistamine) medication you must contact your pet’s doctor.

    How much Benadryl is too much for a dog?

    Lethal Dose of Benadryl for Dogs

    Benadryl may cause death to Dogs when it will be given in extremely high doses like 500mg a day or even more than that.

    High doses of Benadryl (diphenhydramine) may cause sleepiness, sedation, and even death. Extreme dose works as a nervous system broker and shuts up the brain with shock. Hence, cause instant death (within a few seconds or minutes).

    Usage of Benadryl in Dogs

    What can Benadryl be used for?

    Benadryl can be a great solution for dogs with allergies or those who are traveling. Here are some specific situations where Benadryl may be used:

    • When a dog is stung or bitten by an insect.
    • When a dog is bitten by a venomous reptile or insect.
    • When a dog is itchy or scratchy due to environmental or food allergies.
    • When a dog is traveling and may suffer from motion sickness.
    • When a dog needs something to make it a little sleepy so it doesn’t get impatient, bored, or bark when it needs to be calm and quiet.
    • When pretreatment with an antihistamine is needed before a vaccination that is known to cause an allergic reaction in that dog.

    Even if your Dog DOES NOT have any known Allergies – it is still a good idea to have some Benadryl on the hand in the case of an emergency. Perhaps your dog is bitten by a spider or stung by a bee, giving your Dog a proper Dosage of Benadryl can prevent or mitigate an allergic reaction.

    This is especially important for brachycephalic breeds such as English Bulldogs and Pugs, who already have naturally compromised airways without the additional airway constriction that can cause an allergic reaction.

    What should Benadryl NOT be used for?

    There are some situations where Benadryl may not be the best option for your dog. Here are some reasons when you may want to avoid using Benadryl:

    • If your dog is very stressed or anxious and you want to make him sleepy. His anxiety and stress need to be addressed and treated rather than just sedating him with Benadryl.
    • If your dog has heart disease, high blood pressure, or glaucoma.
    • If your dog is taking certain medications, you should discuss the safety of Benadryl with your veterinarian before administering it.
    • As a sole remedy after a poisonous bite. Your dog needs immediate veterinary attention.

    As always, your veterinarian is the best source of advice and information for your dog’s health. If you are considering giving Benadryl to your dog, PLEASE make sure to check with your Vet first!

    Is Benadryl Approved for Dogs?

    Benadryl is an over-the-counter antihistamine that relieves allergic reactions to environmental allergens, insect bites or stings, and specific vaccination reactions.

    Although Benadryl it a drug for humans- and Not FDA Approved For Dogs and other animals – you can safely give it to your dog when you use the correct dosage and consult with you VET before.

    Some pet parents also give it to their dogs as a sedative, especially during noisy events like New Year’s Eve and Fourth of July celebrations.

    Benadryl is a relatively safe and effective Dog Medication when used according to a veterinarian’s instructions. However –  as with any medication – you should continuously monitor your dog closely after administration of Benadryl – to ensure no side effects will occur.

    Benadryl is the Brand Name, not the name of the drug that treats allergies; the active ingredient is Diphenhydramine! Always use Benadryl in the correct dosage based on your dog’s weight, overdosing Benadryl can harm and even kill your dog.

    Many other brands produce similar products, and There are other drugs under the brand Benadryl that you should avoid with caution.


    Benadryl (Diphenhydramine) can be used in dogs as an over-the-counter antihistamine to relieve allergic reactions to environmental allergens, insect bites or stings, and specific vaccination reactions.

    It is VERY IMPORTANT to be cautious with the dosage as an overdose of Benadryl can be lethal to dogs, cause severe neurological and cardiovascular effects or even kill a dog. Best onsult with a VET before administering any medication to your dog – continuously monitor them for any side effects.

    Be aware that Benadryl is a brand name and there are many other brands that produce similar medications! 

    Read also: Can I Give My Dog Benadryl with Acepromazine? – Quick Answer

    Doctor Xeeshan

    Doctor Xeeshan

    I am Doctor Xeeshan, located in Lahore, Punjab, Pakistan. In this blog, I am providing authentic information about dog breeds, diseases, medications, etc.


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