CHLORPHENIRAMINE

Chlorpheniramine is used in dogs, cats, cattle, horses, sheep, goats, and other animals for allergic reactions, itching, eczema, dermatitis, insect bites, tail eczema, laminitis, and in bloat.

Chlorpheniramine is an alkylamine derivative antihistamine (first-generation H1 histamine blocker). Other commonly used antihistamines in animals include pheniramine, diphenhydramine, cetirizine, levocetirizine, meclizine, and hydroxyzine.

Chlorpheniramine is the active antihistamine, while chlorpheniramine maleate is its stable, soluble salt form used in medications.

CATEGORY

• Anti-Histamine
• H1-Blocker

MECHANISM OF ACTION

Similar to other antihistamines, it acts by blocking the H1 receptor and suppressing inflammatory reactions caused by histamine.

CLINICAL USES

• Control pruritus and skin inflammation in dogs and cats
• Used in allergic reactions
• Used for itching, eczema, dermatitis, insect bite, tail eczema in horses
• Reduce inflammation of the hooves in cattle
• Used in anaphylactic shock
• Used for laminitis & bloat in cattle
• Having mild antiemetic effect

SIDE EFFECTS

• Sedation is the most common side effect of chlorpheniramine (Due to inhibition of Histamine-N-Methyltransferase.)
• Dry mouth and decreased GI secretions (atropine like effect)

CONTRAINDICATIONS

Antimuscarinic effects (atropine-like effects) are common so do not use in conditions for which anticholinergic drugs may be contraindicated.

FORMULATIONS

Injectables:

10mg/mL injections are available for chlorpheniramine.

Tablets:

2 mg, 4 mg,  8 mg tablets are available.

Syrup/Suspensions:

2 mg/5mL PO syrups are available.

Other formulations:

Nil.

STORAGE

Store in a tightly sealed container, protected from light, and at room temperature. Protect from freezing. Stability of compounded formulations has not been evaluated.

DOSE RATE

Dose rates for various animals are listed below. Adjust the dose rates according to the pathophysiology of each case.

• Dog: 0.5-2 mg/kg q12hrs
• Cat: 0.5-2 mg/kg q12hrs
• Cattle: 0.25-0.5 mg/kg q12hrs
• Horse: 0.25-0.5 mg/kg q12hrs
• Sheep: 0.25-0.5 mg/kg q12hrs
• Goat: 0.25-0.5 mg/kg q12hrs

ADMINISTRATION

Chlorpheniramine is typically administered via intravenous (IV), intramuscular (IM), subcutaneous (SC), and per oral (PO) routes. It is essential to check the label for specific preparation instructions since certain formulations may be designated for specific routes of administration.

BIOAVAILABILITY

A bioavailability of 25% to 50% can be observed for chlorpheniramine when administered orally.

ELIMINATION HALF LIFE

The elimination half-life of chlorpheniramine for animals ranges between 4 and 40 hours.

METABOLISM

Metabolism of chlorpheniramine takes place in the liver.

EXCRETION

Excretion of chlorpheniramine takes place in the kidneys.

TOXICITY

The estimated lethal dose of chlorpheniramine is 25 to 50 mg/kg body weight. Symptoms and signs of toxicity include sedation, paradoxical excitation of the CNS, toxic psychosis, convulsions, apnea, anticholinergic effects, dystonic reactions, and cardiovascular collapse, including arrhythmias.

ANTIDOTE

There is no specific antidote for the treatment of chlorpheniramine overdose. However, cholinergic drugs (e.g., physostigmine) may be an option if a patient is experiencing atropine-like effects.

NOTES

For anaphylactic shock, order of the best suitable treatment is:

• Mild cases: Chlorpheniramine
• Moderate cases: Dexamethasone
• Severe cases: Adrenaline