Antihistamines For Cats



Symptoms of allergies   Dosage of antihistamines   Diagnosis of allergies   Antihistamine trials   Other ways to reduce allergies in cats




antihistamines for cats

Antihistamines are a class of drug which block the effects of histamine, a chemical produced by the mast cells and basophils in response to a real (bacteria/virus/fungus) or perceived (allergen such as dust) invasion.

Histamine binds to receptors (docking stations) on certain cells (H1), which triggers several responses such as dilation of the blood vessels, which causes fluid to leak into nearby tissue (runny nose, watery eyes for example) as well as inducing itchiness and smooth muscle contraction, which can lead to breathing difficulty due to bronchodilation.  Antihistamines work by competing with histamine to bind to the receptors on the cells. Interestingly, allergies in humans more commonly produce sneezing, runny nose and watery eyes whereas cats more commonly develop skin disorders such as itching and rashes.

Antihistamines may be first generation or second generation. First generation often cause drowsiness, whereas second generation don’t. Second generation are less likely to cause drowsiness, are longer acting but more expensive to purchase.

What are the symptoms of allergies?

This can vary depending on the underlying cause but it is common for allergies in cats to affect the skin. Depending on the cause, symptoms may be seasonal or non-seasonal. Common symptoms may include:

  • Itching and scratching
  • Urticaria (rash)
  • Crusty lesions on the back, close to the base of the tail (milinary dermatitis)
  • Overgrooming
  • Hair loss
  • Vomiting and/or diarrhea (with food allergies)

How are allergies diagnosed in cats?

This may depend on the type of allergy your veterinarian thinks your cat may have. There are four types of allergy to affect cats:

  • Insect – The most common type of allergies in cats are insect, predominantly fleas, however, any biting or stinging insect has the potential to cause allergic reactions in cats.
  • Inhalant – The second most common type of allergy to affect cats. Responsible allergens include pollens, cat litter, smoke, dust mites and moulds.
  • Food – The most common food allergies in cats are to beef, fish, chicken, eggs and wheat.
  • Contact – The least common of the four groups of allergies to affect cats. This may be due to contact with an irritant (such as a chemical) or with an allergen including plants, cat litter, topical medications, shampoos etc.

Food trials: If your veterinarian suspects your cat has a food allergy, a food trial will be carried out. The cat is switched to a novel protein (such as duck, or lamb) to see if symptoms improve. If they do, your cat will then be put back on his regular diet to see if symptoms return.

Intradermal allergy testing: This may be necessary for other types of allergy to determine the cause. Your cat will be given light sedation and an area of skin will be shaved on your cat’s belly, and a number of common allergens are injected into the skin. Small red hives may appear where specific allergen has been injected.

What is dosage for antihistamines?

When purchasing antihistamines, make sure they contain no other ingredient and never mix medications unless your veterinarian has said it is safe to do so.




Dosage is based on a 5kg or 11-pound cat. It is always recommended that you check with your veterinarian before administering any medications to your cat, particularly if your cat has any medical conditions, pregnant or lactating or is a kitten.

A number of the antihistamines listed below are for ‘off-label’ use in cats although they are commonly used. Below is a quick guide, followed by more detailed information on each antihistamine.

Product Dosage Frequency
Atarax (Hydroxyine) 10 mg tablet 5 mg (1/2 tablet) 2-3 times a day
Benadryl (Diphenhydramine) liquid 10 mg Twice a day
Claratyne (Loratadine) 10 mg tablet 5 mg (1/2 tablet) Twice a day
Iramine (Chlorpheniramine) 4 mg tablet 2 mg (1/2 tablet) 2-3 times a day
Periactin (Cyproheptadine) 4 mg tablet 2 mg (1/2 tablet) 2-3 times a day
Phenergan (Promethazine)10 mg tablet 5 mg (1/2 tablet) Twice a day
Polaramine (Dexchlorpheniramine) 6 mg tablet 6 mg (1 tablet) Twice a day
Zyrtec (Cetirizine) 10 mg tablet 5 mg (1/2 tablet) Once a day

Atarax (Hydroxyine) – 10 mg tablet

Dosage: 1/2 tablet 2 to 3 times a day.

Uses: Allergy, urticaria.

Side effects: Drowsiness, increased thirst.

Contraindications: Not safe for use in pregnancy, do not use in cats with urinary or gastrointestinal obstruction or heart disease.

Benadryl (Diphenhydramine) – Liquid (also comes in injectable form and can be compounded into a transdermal paste)

Dosage: 10mg twice a day.

Uses: Allergies, urticaria, motion sickness, may be used to help dry up nasal secretions in cats with the flu.

Side effects: Drowsiness, urinary retention, dry mouth and in some cases gastrointestinal disturbances. These tend to wear off in time.

Contraindications: Should be used with caution in cats who have hyperthyroidism, high blood pressure or heart disease. Do not give Benadryl to pregnant or lactating cats unless your veterinarian has told you to do so.

Claratyne/Claritin (Loratadine) – 10mg tablet (do not use the liquid as it contains propylene glycol)

Dosage: 1/2 tablet twice a day.

Uses: Allergies, urticaria.

Side effects: Nausea, urinary retention, gastrointestinal disturbances and in some cases drowsiness may develop although this is less common with second generation antihistamines.

Contraindications: Check with your veterinarian if your cat is pregnant or lactating, suffers from asthma, liver or kidney disease.

Iramine (Chlorpheniramine) – Script required – 4 mg tablet

Dosage: 1/2 tablet two or three times a day.

Uses: Allergies, urticaria, milinary dermatitis and may be used to help dry up nasal secretions in cats with the flu.

Side effects: Drowsiness, hyperexcitability, dry mouth, loss of appetite, diarrhea, vomiting, urinary retention.

Contraindications: Should not be used in cats with glaucoma, urinary or gastrointestinal obstruction, high blood pressure, hyperthyroidism and heart disease.

Periactin (Cyproheptadine) – 4 mg tablet

Dosage: 1/2 tablet two or three times a day.

Uses: Antihistamine, appetite stimulant.

Side effects: Drowsiness, dry mouth, hyperexcitability, increased appetite, increased thirst and increased urination.

Contraindications: Should not be used in cats with severe heart disease or urinary obstruction. Check with your veterinarian if your cat is pregnant or lactating.

Phenergan (Promethazine) – 10 mg tablet

Dosage: 1/2 tablet twice a day.

Uses: Allergies, anti-emetic (nausea), motion sickness, urticaria, eosinophilic granuloma complex.

Side effects: Drowsiness.

Contraindications: Check with your veterinarian if your cat suffers from asthma or other breathing disorders, glaucoma, seizures, high blood pressure, heart disease, liver disease, hypocalcemia is pregnant or lactating.

Polaramine (Dexchlorpheniramine) – 6 mg tablet

Dosage: 1 tablet twice a day.

Uses: Allergies, urticaria.

Side effects: Not available.

Contraindications: Not available.

Zyrtec (Cetirizine) – 10 mg tablet

Dosage: 1/2 tablet once a day

Uses: Allergies, urticaria.

Side effects: Vomiting, hypersalivation (drooling).

Contraindications: Consult your veterinarian if your cat has liver or kidney disease. Should not be used on pregnant cats.

Antihistamine trials:

Your veterinarian may recommend a trial, to determine the most effective antihistamine for your cat. Initially your cat may need cortisone and antihistamines, however, once symptoms improve, cortisone may be stopped, while antihistamines are continued. While these don’t cure allergies, the side effects are less severe than those of cortisone. Not all antihistamines will work with your cat or will produce undesirable side effects, so a trial will help to determine the best type for your cat.

Your cat will be put on an antihistamine for ten days to see how he responds, if there is no improvement in symptoms, then he will be moved on to the next type of antihistamine, this will be repeated until a suitable type can be found.

Other ways to reduce allergies in cats:

Steroids are very effective for controlling itching and may be prescribed to relieve symptoms while the allergy is being brought under control. Injectable steroids may be used initially, these can last between 2-6 months. After that, your cat may be given oral steroids, long-term use of steroids can have side effects, so your cat will either be weaned off if possible or kept on the lowest dose possible.

Use a reliable flea control programme too. See here for a list of flea control products for cats.

Omega-3 fatty acids are essential fatty acids (EFA’s) which must be consumed as cats are unable to synthesise them. There are a number of omega-3 fatty acids, including ?-linolenic acid  (ALA), docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). These fatty acids have been shown to help reduce the effects of histamine and reduce inflammation in cats suffering from allergies. Not all cats will respond to omega-3 fatty acids, but they can be of help, especially when used with antihistamines. 

Anti-itch shampoos and rinses may be recommended to help relieve itching. Look for products containing oatmeal.




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