5 Best Antihistamines for Dog Allergies in [2021]

Antihistamines are drugs used to counter allergic reactions in both humans and animals. The best antihistamines for dog allergies work by blocking histamine which stimulates cells in body tissues to react as a result of the body’s contact with the allergen. These allergic reactions are the body’s way of expelling the allergen in a bid to protect itself.

The body however has to be exposed to the allergen prior to it to be able to build antibodies against a specific allergen (antigen). Antibodies have memory and therefore with subsequent exposure to an allergen, it is able to produce antibodies specific to it.

Histamine causes adverse immune reactions. Antihistamines for dogs allergies help to relieve discomfort and symptoms of allergies which otherwise reduce their quality of life and may at times lead to damage of body tissues. Antihistamines work best when given prior to exposure to an allergen preventing an immune reaction.

Common Allergies in Dogs

An allergy is an immune reaction to a foreign substance in the body that affects organs in the body. Dogs suffer from allergies which at times are mistaken for intolerances. Intolerance mostly applies to food and is less life-threatening compared to allergies, and does not elicit an immune reaction.

Common signs of allergies include:

  • Itching (face, ear, feet/paws, anal area)
  • Runny nose
  • Watery eyes
  • Swelling
  • Ear infections
  • Diarrhea and vomiting
  • Hair loss/ hot spots

Allergies can be caused by food ingredients, environmental factors, parasites, and in some instances genetic predisposition.

Food allergies

These include:

  • Dairy: Some dogs are intolerant to lactose found in dairy products as they lack the enzyme lactase which helps with digestion. Intolerant dogs usually suffer from gas, diarrhea, and vomiting. An allergy presents in the form of constant itching.
  • Wheat
  • Beef
  • Soy
  • Eggs
  • Beef
  • chicken
  • Flea allergy dermatitis: This is an immune reaction resulting from contact with fleas saliva on the dog’s skin. This causes itchiness, redness of the skin, and loss of hair. Some dogs are more sensitive to FAD compared to others where one flea bite can cause a reaction lasting a few weeks. Fleas control and frequent grooming help to keep this allergy at bay.

Environmental elements

This includes indoor and outdoor allergens e.g. dust mites, mold, paint, chemicals, pollen, dander, and dust among others.

How Do Antihistamines Help?

When the body is exposed to allergens an immune reaction ensues to protect the body against the ‘invader’.  Once the body is sensitized, blood cells called B cells produce plasma cells which in turn produce antibodies called IgE specific to that allergen. These antibodies in turn attach themselves on the surface of cells called mast cells which are produced during allergic reactions.

The mast cells are in turn activated causing them to release inflammatory substances i.e. histamine and leukotrienes. Histamines once released attached to various receptors found on body cells namely H1, H2, H3, and H4.

H1 receptors are found on epithelial cells, the brain, and smooth muscles and are largely involved in allergic reactions.

Histamine causes dilation of blood vessels through the epithelial cells causing an influx of blood which is often seen as redness on the skin and swelling. On the brain, it stimulates nerve endings causing increased sensitivity and itchiness. It also causes contraction of the smooth muscles in the bronchioles in the lungs which often leads to coughing and difficulty in breathing.

H2 receptors are found in the stomach and once activated the parietal cells are stimulated to produce an increased amount of hydrochloric acid. Other inflammatory responses include e.g. swelling, sneezing, tearing, rash, diarrhea, vomiting, etc.

These histamine effects are geared to help the body to expel the allergens though in prolonged satiations can lead to damage of body tissues. Histamine when released too fast into the bloodstream before inactivation by the enzyme known as histaminases results in a full-blown allergic reaction and anaphylaxis which can be deadly.

Antihistamines work by blocking the histamine receptor sites found in body cells, therefore, preventing histamine from binding into the receptor sites. This prevents the activation of the tissue cells and an allergic reaction from occurring. Antihistamines work best when taken hours before exposure to an allergen to allow them time to occupy the majority of the H1 receptor sites.

This prevents the occurrence of a full-blown allergic reaction. Antihistamines can be grouped into 1st and 2nd generation antihistamines. First-generation antihistamines were the first to be discovered with the majority being able to cross the blood-brain barrier which in turn leads to drowsiness and sedation.

Second-generation antihistamines are preferred as they do not cause sedation and affect only specific H1 receptor sites in body tissues.

Safe Use of Antihistamines in Dogs

Antihistamines are not registered as veterinary drugs and should, therefore, be used under the guidance of a registered veterinarian. They should however not be administered in pregnant and lactating dogs, as well as those suffering from heart failure, glaucoma, bladder problems, prostate enlargement, and liver failure among others.

Dogs under treatment with tranquilizers, heparin, warfarin, amitraz, and epinephrine should not be administered antihistamines. Side effects such as dry mouth excess salivation, diarrhea, increased heart rate, drowsiness, and vomiting, etc. are common and mostly occur within the first hour after administration.

The most common and safest antihistamines in dogs contain the following active ingredients: Clemastine, Chlorpheniramine, cetirizine, hydroxyzine, and diphenhydramine.

Best Antihistamines for Dog Allergies

1. Chlorpheniramine 4mg

Chlorpheniramine 4mg Tabs

9 new from $3.89
Free shipping
as of 5 October 2021 23:27


  • Anti-Allergy
  • An oral antihistamine used to control itching for Humans and Pets
  • No Bullet points has been entered by primary contributor.
  • No Bullet points has been entered by primary contributor.
  • No Bullet points has been entered by primary contributor.

These are Chlorpheniramine 4mgs, 100 tablets used for relieving allergic reactions.

Key Features

  • Each tablet contains 4mgs of chlorpheniramine maleate as the active ingredient
  • Used for both pets and humans
  • Contains 100 tablets
  • Contains in 3.1-inch 7cm bottle
  • It helps to relieve sneezing, itchiness, watery eyes, runny nose, etc.

Chlorpheniramine maleate is an antihistamine drug that blocks the action of a neurotransmitter known as acetylcholine (anticholinergic) in the brain.

Acetylcholine on release at nerve junctions causes tearing, vomiting, runny nose, contraction of the muscles of the bronchioles in the lungs, urination, and diarrhea among others common in allergic reactions.

Chlorpheniramine comes in to block these effects, therefore, relieving runny nose, sneezing, vomiting, diarrhea, itchiness, etc. Each tablet contains 4mgs given at a dosage of 0.2 to 0.8 mg/kg every 8 to 12 hours. It can be given together with food.

It is however contraindicated in pregnant dogs, dogs that are allergic to the drug as well dogs with on acetaminophen and metoclopramide.


  • Can be administered in food
  • Takes effects within 15 to 30 minutes lasting between 8 to 12 hours.
  • Helps to alleviate watery eyes, itchiness, vomiting, diarrhea among others
  • Can be used in both human and pets
  • Does not affect the heart rate or stomach secretion of hydrochloric acid which is known to cause gastric ulcers


  • Bitter to taste
  • Can cause mild sedation
  • Contraindicated in pregnant and nursing dogs
  • Can cause adverse side effects such as dry mouth, urine retention, loss of appetite
  • It should not be used in animals with kidney or liver problems, heart disease, and bladder obstruction.

Chlorpheniramine is a suitable drug due to its quick action and long residual effects. It also has mild side effects.

2. ValuMeds

ValuMeds Allergy Medicine

2 new from $8.99
Free shipping
as of 5 October 2021 23:27


  • Breathe a Little Easier – A powerful antihistamine, our formula helps control coughs and runny noses for clearer, healthier breathing.
  • Clear, Healthy Eyes – Reduce the dry, itchy eyes and redness caused by pollen, dander, and seasonal allergies for true clarity.
  • Relieve Common Cold Symptoms – This anti-allergy medicine for kids and adults also helps reduce sneezing and itchy noses and throats.
  • Rest Better – Diphenhydramine HCl can cause drowsiness which can help your body relax for a better night’s rest. Avoid driving after use.
  • Use as Directed – These HCl antihistamine tablets should always be used as directed. Please read package for more details on proper use and dosage.

ValuMeds is an antihistamine drug-containing Diphenhydramine HCl as the active ingredient.

Key Features

  • Each tablet contains 25mgs of Diphenhydramine Hydrochloride.
  • Each packet contains 600 tablets
  • Tablets are pink in color
  • It provides relief against runny nose, watery eyes, itching, swelling, etc.

Diphenhydramine HCl blocks histamine from attaching to both H1 receptor cells as well as inhibiting the neurotransmitter acetylcholine released on nerve junctions. It, therefore, prevents cholinergic and histamine effects common in allergic reactions such as watery eyes, swelling, vomiting, nausea, motion sickness, and diarrhea among others.

It also causes mild sedation. It has no effect on pregnant bitches but should be restricted while lactating. It is also contraindicated in pets suffering from heart disease, glaucoma, therefore, best used under the guidance of a veterinarian.

It is given at a dose of 1mg per pound 2 to 3 times a day. It also comes with its fair share of side effects such as dry mouth, urinary retention, hypersalivation, increased heart rate, etc. It also induces sedation therefore also prescribed in dogs with anxiety.


  • Offer fast and effective relief
  • Also prescribed in humans. It has a long margin of safety
  • It helps relieve allergic reactions such as itchiness, swelling, sneezing, redness, watery eyes, and a runny nose.
  • Helps to alleviate motion sickness
  • Treats anxiety in dogs as it induces sedation
  • Can be dispensed in food.
  • It is affordable
  • Effective in the treatment of mast cell tumors
  • Helps in the treatment of heartworms


  • Should be given under the guidance of a veterinarian
  • Not prescribed in lactating bitches

Diphenhydramine hydrochloride offers valuable benefits apart from relieving allergic reactions, therefore, giving you value for your money.

3. Zyrtec

Zyrtec Allergy 24 Hour Protection

5 new from $25.25
as of 5 October 2021 23:27


BindingHealth and Beauty
LabelJohnson & Johnson
ManufacturerJohnson & Johnson
Part Number30312547204386
Product GroupBeauty
PublisherJohnson & Johnson
StudioJohnson & Johnson
TitleZyrtec Allergy 24 Hour Protection 10mg Tablets-45 count (Quantity of 1)

Zyrtec is an antihistamine drug used to relieve allergies with cetirizine hydrochloride as the active ingredient.

Key Features

  • Contains 45 tablets
  • Each tablet contains 10 mg of cetirizine hydrochloride as the active ingredient
  • Helps to relieve allergy symptoms such as a runny nose, itchiness, watery eyes, swelling, and rash among others
  • Used to treat both indoor and outdoor allergies

Cetrizine is an antihistamine. It attaches itself to H1 receptor cells in body tissues preventing their activation by histamine during an allergic reaction.

It is a second-generation antihistamine meaning it does not cross the blood-brain barrier therefore does not cause drowsiness or sedation. It may cause side effects such as dry mouth, nausea, vomiting, and diarrhea, etc.

It should not, however, be prescribed in dogs that are hypersensitive, pregnant, and lactating bitches as well as dogs with liver or kidney failure. Cetirizine should be given at a dose of 1miligram per kilogram once daily under the guidance of a veterinarian.


  • Can be dispensed in food
  • Does not cause drowsiness, increased heart rate and secretion of stomach acid
  • Given only once a day
  • It is affordable
  • Used in the treatment of allergic dermatitis and canine atopy
  • They come in the form of chewable grape-flavored tablets making it easy to administer to pets.


  • Cetirizine can cause adverse side effects especially if overdosed. Symptoms include drying of the mouth, vomiting, diarrhea, excess salivation, constipation, and urinary retention among others.
  • Not indicated for pregnant and lactating bitches, dogs with kidney and liver problems as well as those that are hypersensitive to cetirizine.

Cetrizine is a second-generation med that does not cause drowsiness, unlike 1st generation antihistamines. Zyrtec is effective in treating atopy and allergic dermatitis in dogs.

4. Claritin

Claritin 24 Hour Non-Drowsy Allergy Tablets

26 new from $26.90
as of 5 October 2021 23:27


  • Powerful, 24-hour, non-drowsy indoor and outdoor allergy relief
  • Doctor Recommended Non-Drowsy Oral Allergy Brand
  • Provides relief of symptoms that can be triggered by over 200 different allergens from pets, pollen, dust and mold. Claritin is an antihistamine so it hinders the cascade triggered by allergens.

Claritin contains antihistamine containing loratadine as the active ingredient.

Key Features

  • Contains loratadine as the active ingredient
  • Each tablets contains 10mgs of loratadine
  • Each packet contains 70 tablets
  • Treats both indoor and outdoor allergies
  • Relieves symptoms such as watery eyes. Runny nose. Sneezing, coughing and itchiness among others
  • Does not cause drowsiness or sedation
  • Provides 24-hour relief

Loratadine works by blocking histamine from attaching to H1 receptor cells. It also causes anticholinergic effects. Collectively they ease allergic symptoms such as the runny nose, itchiness, and sneezing, watery eyes, etc.

It acts specifically on peripheral H1 receptors and not those found in the brain therefore not causing drowsiness and sedation. It reaches peak blood levels in about 2 to 3 hours. It is broken down by the liver therefore not indicated in dogs suffering from liver failure.

Care should be taken when purchasing to avoid loratadine containing pseudoephedrine as it is harmful to dogs. It should be given under the guidance of a veterinarian, especially in pregnant or lactating bitches.

It should also not be given concurrently with other antihistamines. Liquid versions of the same drug should also be avoided as they contain alcohol which is toxic to dogs. The recommended dose is 0.2 mg per pound once daily. The duration of use depends on the severity of the allergy.


  • Does not cause drowsiness
  • It is affordable
  • Can be administered in food
  • Effective for both indoor and outdoor allergies
  • It is also used in the treatment of mast cell tumors


  • Contraindicated in dogs with liver and kidney failure

Claritin helps in preventing allergic reactions by blocking histamine receptors. It’s doesn’t cause drowsiness or sedation.

5. Clemastine


as of 5 October 2021 23:27


  • 100 Tablets
  • By Sandoz
  • 12 Hour Allergy Relief

This antihistamine medication contains clemastine fumarate as the active ingredient.

Key Features

  • Contains 100 tablets each containing 1.34 milligrams of clemastine fumarate.
  • It helps in the relief of allergic reactions such as itchiness, runny nose, swelling, watery eyes, rash, redness, etc.
  • Provides 12-hour relief from allergic symptoms

Clemastine works by blocking H1 receptors found in body tissues from being activated by histamine preventing its effects on the body. It also blocks the neurotransmitter acetylcholine providing relief from allergic symptoms such as watery eyes, runny nose, diarrhea, and vomiting among others.

It also crosses the blood-brain barrier causing drowsiness and sedation. It is however contraindicated in animals with heart disease, glaucoma, prostate, and bladder infections. It safe for use in pregnant bitches but traces of the drug have been found milk, therefore, contraindicated in lactating bitches.

The recommended dose is 0.5 milligrams per kilogram given every 12 hours. Common side effects include hyperactivity, dry mouth, redness of the skin, and sedation. It is not a registered veterinary drug therefore should be administered under the guidance of a veterinarian.


  • Can be bought over the counter
  • Provides long-lasting relief
  • Can be used on both cats and dogs
  • Has lesser sedative effects compared to other antihistamines
  • It is affordable (generic versions)
  • It is easy to administer especially in food
  • Suitable for treatment of allergic dermatitis and canine atopy


  • Contraindicated in lactating dogs
  • Causes side effects such as dry mouth, diarrhea etc
  • Contraindicated in animals suffering from bladder infections, heart diseases, enlarged prostate and glaucoma among others
  • It should not be administered in working dogs as it causes drowsiness and sedation.

Clemastine is an effective antihistamine providing extended relief from allergic reactions.

Bottom Line

Antihistamines provide relief from allergic reactions by blocking H1 receptor cells found in body tissue. Others also inhibit the production of acetylcholine which helps to relieve allergic symptoms such as runny nose, vomiting, diarrhea etc. Histamine causes adverse body reactions such as itchiness, redness of the skin, swelling, watery eyes, running nose among others.

Most antihistamines are not registered as veterinary drugs therefore should be used under the guidance of a veterinarian. Most veterinarians prescribe drugs with the following active ingredients: Clemastine, Chlorpheniramine, cetirizine, hydroxyzine, and diphenhydramine among others.

Care should be taken when administering the drug to pregnant and lactating bitches. It is also contraindicated on dogs suffering from heart failure, glaucoma, liver failure, and bladder problems. Antihistamines should not be administered concurrently with tranquilizers.

The duration of use is determined by the severity of the allergy. In the event of adverse side, effects the administration should be discontinued and a veterinarian contacted. Read our comprehensive guide on Dog Medication here. 

Razvan is a dog trainer and veterinary technician with over 10 years of experience working with dog daycares, animal shelters, vet clinics and private dog training. Razvan is one of the lead publishers on Dog Struggles covering the latest product reviews and dog health advice.

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