Piriton

Indications

• Allergic rhinitis (hay fever)
• Allergic conjunctivitis
• Urticaria (hives)
• Pruritus (itching)
• Common cold symptoms

Dosage

Children: For children aged 6 to 12 years, the recommended dose is 2 mg every 4 to 6 hours, not to exceed 12 mg in 24 hours. For children aged 2 to 6 years, consult the BNF for Children for specific dosing guidance.

Adults: The usual adult dose for allergic conditions is 4 mg every 4 to 6 hours, not to exceed 24 mg in 24 hours.

Mechanism of action

Chlorpheniramine binds to the histamine H1 receptor, blocking the action of endogenous histamine. This prevents histamine from producing its effects, such as vasodilation, increased vascular permeability, and bronchoconstriction. By competing with histamine for H1-receptor sites, chlorpheniramine provides temporary relief from allergy symptoms, including urticaria and pruritus.

Pharmacodynamics

In allergic reactions, allergens induce cross-linking of surface IgE antibodies on mast cells and basophils, leading to degranulation and histamine release. Chlorpheniramine, as an H1 antagonist, competes with histamine at the receptor sites on effector cells in various tissues, including the gastrointestinal tract, blood vessels, and respiratory tract. This action results in the alleviation of allergic symptoms, such as pruritus and nasal congestion.

Pharmacokinetics

Chlorpheniramine is absorbed rapidly after oral administration, with peak plasma concentrations typically occurring within a few hours. It is extensively metabolized in the liver, primarily by cytochrome P450 enzymes. The drug has a relatively long half-life, allowing for once or twice daily dosing. It is excreted in urine, mainly as metabolites.

Adverse effects

• Drowsiness
• Dry mouth
• Dizziness
• Nausea
• Constipation
• Blurred vision

Interactions

• Alcohol may increase the sedative effects of chlorpheniramine.
• Other central nervous system depressants may enhance drowsiness.
• MAO inhibitors can increase the effects of chlorpheniramine.

Precautions

• Use with caution in patients with asthma or chronic obstructive pulmonary disease (COPD).
• Patients with glaucoma or urinary retention should be monitored closely.
• Caution in patients with cardiovascular disease.

Pregnancy

Chlorpheniramine is classified as Category B. Animal studies have not shown any harm, but there are no well-controlled studies in pregnant women.

Breast-feeding

Chlorpheniramine is excreted in breast milk. Caution is advised when administered to nursing mothers.

Storage

Store at room temperature, away from light and moisture. Keep out of reach of children.

Formulations

• Tablets
• Syrup

Mechanism of action

Chlorpheniramine binds to the histamine H1 receptor. This blocks the action of endogenous histamine, which subsequently leads to temporary relief of the negative symptoms brought on by histamine. Antihistamines used in the treatment of allergy act by competing with histamine for H1-receptor sites on effector cells. They thereby prevent, but do not reverse, responses mediated by histamine alone. Antihistamines antagonize, in varying degrees, most of the pharmacological effects of histamine, including urticaria and pruritus. Also, the anticholinergic actions of most antihistamines provide a drying effect on the nasal mucosa. /Antihistamines/ H1 antagonists inhibit most responses of smooth muscle to histamine. Antagonism of the constrictor action of histamine on respiratory smooth muscle is easily shown in vivo and in vitro. /Histamine Antagonists: H1 Antagonists/ H1 antagonists strongly block the action of histamine that results in increased permeability and formation of edema and wheal. /Histamine Antagonists: H1 Antagonists/ Within the vascular tree, the H1 antagonists inhibit both the vasoconstrictor effects of histamine and, to a degree, the more rapid vasodilator effects that are mediated by H1 receptors on endothelial cells. Residual vasodilatation reflects the involvement of H2 receptors on smooth muscle and can be suppressed only by the concurrent administration of an H2 antagonist. Effects of the histamine antagonists on histamine induced changes in systemic blood pressure parallel these vascular effects. /Histamine Antagonists: H1 Antagonists/ Many of the H1 antagonists tend to inhibit responses to acetylcholine that are mediated by muscarinic receptors. These atropine like actions are sufficiently prominent in some of the drugs to be manifest during clinical usage ... . /Histamine Antagonists: H1 Antagonists/

Pharmacodynamics

In allergic reactions an allergen interacts with and cross-links surface IgE antibodies on mast cells and basophils. Once the mast cell-antibody-antigen complex is formed, a complex series of events occurs that eventually leads to cell-degranulation and the release of histamine (and other chemical mediators) from the mast cell or basophil. Once released, histamine can react with local or widespread tissues through histamine receptors. Histamine, acting on H₁-receptors, produces pruritis, vasodilatation, hypotension, flushing, headache, tachycardia, and bronchoconstriction. Histamine also increases vascular permeability and potentiates pain. Chlorpheniramine, is a histamine H1 antagonist (or more correctly, an inverse histamine agonist) of the alkylamine class. It competes with histamine for the normal H₁-receptor sites on effector cells of the gastrointestinal tract, blood vessels and respiratory tract. It provides effective, temporary relief of sneezing, watery and itchy eyes, and runny nose due to hay fever and other upper respiratory allergies.