First-generation antihistamine · For Cats

Chlor-Trimeton dosage reference for cats (Chlorpheniramine)

Veterinary references give this medication as a fixed amount per cat, given every 8-12 hours, not as a per-kilogram dose, so this guide shows that reference instead of a weight-based calculator. Confirm the product and amount with a veterinarian.

Onset: 30-60 minutes
Duration: 8-12 hours
Class: First-generation antihistamine

Weight-based calculator estimate from the references listed on this page. This page has not been independently DVM-reviewed — see our methodology .

This information is for educational purposes only. Always consult your veterinarian before giving medication to your pet. Do not use this calculator for emergencies, overdoses, pregnancy, very young animals, very old animals, pets with liver/kidney disease, or pets taking multiple medications.

Sources and review process

Each medication page lists the references recorded for its data. The calculator math and editorial presentation are maintained by PetDosageChart; a page is independently DVM-reviewed only when a named reviewer byline appears. See our methodology and limitations.

Chlorpheniramine (often known by brand names such as Chlor-Trimeton) is a first-generation antihistamine used extra-label in cats for allergies, itching, allergic skin disease, and feline eosinophilic granuloma complex. Understanding the dose, administration, and potential side effects of Chlorpheniramine is critical for your cat’s safety and well-being.

No weight-based calculator on this page: Veterinary references give chlorpheniramine for cats as a fixed amount per cat, not a per-kilogram dose. The Merck Veterinary Manual lists 1-2 mg per cat by mouth every 8-12 hours (one-quarter to one-half of a plain 4 mg tablet); its skin-disease table lists up to 2-4 mg per cat every 12 hours as needed. Give only the amount your veterinarian prescribes, and ask before using it in kittens or very small cats.

What the source establishes

Lists chlorpheniramine for cats at 1–2 mg per cat by mouth every 8–12 hours, a flat amount per cat rather than mg/kg. It also notes that mild signs (depression, GI upset) can occur at less than 1 mg/kg.

This toxicology table gives a therapeutic reference, not an allergy treatment plan. It is why this page shows a per-cat reference instead of a weight-based calculator.

Merck Veterinary Manual: First-Generation Antihistamines (table) — Chlorpheniramine row; from Toxicoses in Animals From Human Cold and Allergy Medications.

Lists chlorpheniramine for cats with skin disease at 2–4 mg per cat by mouth, IM or SC, every 12 hours as needed, again as a per-cat amount.

This dermatology table uses a higher range than the toxicology table. Your veterinarian decides which amount fits your cat.

Merck Veterinary Manual: Antihistamine Dosages for Integumentary Disease in Animals (table) — Chlorpheniramine row; cats.

Chlorpheniramine Reference Amount for Cats

Cat weight	Reference amount (every 8-12 hours)
Any weight	1-2 mg per cat (one-quarter to one-half of a plain 4 mg tablet)

Veterinary references list this amount per cat, not per pound. Give only the amount and schedule your veterinarian prescribes.

Use plain chlorpheniramine only. Combination cold and allergy products can contain decongestants such as pseudoephedrine or phenylephrine, which are dangerous for cats.

By Weight

Chlor-Trimeton reference by cat weight

Choose a common weight to open its optional reference page. Return to the main guide for the per-cat reference amount.

Common-weight pages are convenience views of the same educational reference, not separate medical recommendations.

How to Give Chlorpheniramine to Your Cat

The Merck Veterinary Manual lists 1-2 mg per cat (one-quarter to one-half of a 4 mg tablet) every 8-12 hours; this is a flat amount per cat, not a per-pound dose. Give only the amount your veterinarian prescribes. The small tablet pieces can be hidden in a pill pocket, soft treat, or small amount of wet food. Chlorpheniramine is one of the better-tolerated antihistamines for cats compared to diphenhydramine (Benadryl), which is difficult to dose and poorly accepted by most cats. Use plain chlorpheniramine without added decongestants.

It is generally recommended to give Chlorpheniramine with food to help reduce stomach upset. It may interact with CNS depressants, anticholinergic drugs, and MAO inhibitors.

Side Effects and Safety

Common side effects include drowsiness, dry mouth, decreased appetite, and mild GI upset. More serious side effects can occur, such as urinary retention, severe sedation, paradoxical excitation, and seizures (overdose).

Cautions · 12

When Chlor-Trimeton should be used with caution or avoided

Some cats should not take Chlor-Trimeton, or need a different dose and closer monitoring. Tell your veterinarian about your pet's full health history and every other medication or supplement they take before giving Chlor-Trimeton.

urinary obstruction
glaucoma
severe cardiac disease
pregnancy

Interactions · 13

Chlor-Trimeton drug interactions

Chlor-Trimeton can interact with other drugs. This list is not complete — always give your veterinarian a full list of your cat's medications and supplements.

CNS depressants

anticholinergic drugs

MAO inhibitors

Reference Limits · 14

When this reference is not enough

A reference table cannot decide whether a product or treatment is appropriate for an individual animal. Do not rely on it in these situations — talk to your veterinarian instead:

In an emergency, or after accidental ingestion or a suspected overdose
For pregnant or nursing pets
For very young or very old cats
For pets with liver disease, kidney disease, or another chronic illness
For pets taking several medications at once
When you are not sure of your cat's current, accurate weight
For combination products that contain additional active ingredients

Overdose · 15

Suspected overdose: what to do

If you think your cat had too much Chlor-Trimeton, or swallowed any medication by accident, act right away. Do not wait for symptoms to appear.

Call your veterinarian or the nearest emergency animal hospital.
ASPCA Animal Poison Control Center: (888) 426-4435
Pet Poison Helpline: (855) 764-7661

A consultation fee may apply to the poison hotlines. Have the medication packaging ready, and note how much your pet may have taken and when. Do not try to make your pet vomit unless a veterinarian tells you to.

When to Call the Vet

If your cat experiences any of the following, contact your veterinarian or an emergency clinic immediately:

Difficulty urinating or no urine output (urinary retention)
Extreme sedation or inability to rouse
Agitation or hyperactivity (paradoxical reaction)
Seizures or tremors
No improvement in allergy symptoms after 2 weeks

Frequently Asked Questions

Frequently Asked · 07

Questions about this medication

Is chlorpheniramine better than Benadryl for cats?

Many veterinarians prefer chlorpheniramine over diphenhydramine (Benadryl) for cats because it is better tolerated, easier to dose (smaller tablet), and has fewer side effects in felines. Benadryl has a very bitter taste that causes drooling and foaming in cats, making administration difficult. Chlorpheniramine is generally considered one of the best antihistamine choices for cats.

How much chlorpheniramine can I give my cat?

The Merck Veterinary Manual lists 1-2 mg per cat (not per kilogram), which is one-quarter to one-half of a plain 4 mg tablet, given every 8-12 hours. Its skin-disease antihistamine table lists a higher 2-4 mg per cat every 12 hours as needed. Because references give a flat amount per cat, this page does not offer a weight-based calculator. Ask your vet which amount fits your cat, especially for kittens or very small cats.

Do antihistamines really work for cat allergies?

Antihistamines work for approximately 40-70% of cats with mild to moderate allergies. They are most effective when started early in an allergy flare. For cats that do not respond to antihistamines alone, they can be combined with other treatments like omega-3 fatty acids, steroids, or immunotherapy for better results.

Can I use the extended-release form of chlorpheniramine for my cat?

No, do not use extended-release or sustained-release formulations for cats. These are designed for humans and can release the drug too unpredictably in a cat's smaller GI tract. Use only the immediate-release 4 mg tablets, which can be safely split.

How long can a cat take chlorpheniramine?

Chlorpheniramine is safe for long-term use in cats for ongoing allergy management. Many cats take it seasonally or year-round with minimal side effects. Drowsiness usually diminishes after the first few days of use as the cat develops tolerance.

Sources · 09

Sources & references

The structured medication record behind this page names the references below. The links open the publisher or official resource; they do not turn this page into a prescription for an individual animal.

Merck Veterinary Manual: First-Generation Antihistamines (table)
Chlorpheniramine row; from Toxicoses in Animals From Human Cold and Allergy Medications. Supports the specific claim described above; it is not a review of the entire page.
Merck Veterinary Manual: Antihistamine Dosages for Integumentary Disease in Animals (table)
Chlorpheniramine row; cats. Supports the specific claim described above; it is not a review of the entire page.
Plumb's Veterinary Drug Handbook — listed in this medication's source record.
Merck Veterinary Manual — listed in this medication's source record.
DailyMed label search for Chlorpheniramine — check current product and formulation labels; some veterinary uses may be off-label.

Specific citations above support the identified claims. Other source names remain record-level references; further claim review is still needed. Read our methodology and limitations.

Transparency · 11

How this page was made

This page's structured medication record includes Plumb's Veterinary Drug Handbook among its source names. This medication is presented with its per-animal reference amount instead of a weight-based calculator. A source record is not the same as an individualized prescription or an independent clinical review.

Sources cited: Plumb's Veterinary Drug Handbook
Merck Veterinary Manual
Calculator formula: No weight-based calculator is shown because veterinary references give this medication as a fixed amount per animal rather than per kilogram. Use the exact amount directed by the veterinarian.
Editorial owner: Prepared and maintained by the PetDosageChart editorial team, a software-led publisher and not a veterinary practice.
DVM review status: Not independently DVM-reviewed. A named reviewer and review date appear here only after a confirmed review.

More on our methodology and editorial process — how we source, calculate, and review cats dosage data.

Safety · 10

More resources

Pet Medication Safety Hub — What's safe, what's dangerous, and emergency signs
How we calculate dosages — Our sources, math, and review process
Printable emergency checklist — One-page reference for your fridge or first aid kit