Overview
Chlorpheniramine, often recognized in many over-the-counter cold and allergy preparations (commonly known as ‘yellow pills’ in some regions), is a classic first-generation alkylamine antihistamine. It is primarily utilized for the temporary, symptomatic relief of allergic conditions such as allergic rhinitis (hay fever), urticaria (hives), and the common cold, effectively managing symptoms like sneezing, itchy and watery eyes, and runny nose.

Mechanism of Action
During an allergic reaction, the body releases histamine, a chemical that binds to specific H1 receptors, triggering inflammation, itching, and increased mucus production. Chlorpheniramine works as a competitive antagonist at these H1 histamine receptors. By blocking histamine from binding to its receptors, it effectively prevents and reverses the physiological effects of histamine, thereby alleviating the allergic symptoms.

Dosage and Administration
For adults and children over 12 years old, the typical oral dosage is 4 mg every 4 to 6 hours, not exceeding 24 mg in a 24-hour period. It is also available in extended-release formulations. Because it can cause significant drowsiness, it is often preferred for nighttime use to aid sleep while relieving symptoms.

Side Effects
The most prominent and common side effect of chlorpheniramine, typical of first-generation antihistamines, is central nervous system depression leading to drowsiness, sedation, and impaired cognitive or motor function. Other common anticholinergic side effects include dry mouth, dry eyes, blurred vision, constipation, and urinary retention.

Contraindications and Precautions
It should be used with caution in patients with conditions that can be exacerbated by its anticholinergic effects, such as narrow-angle glaucoma, symptomatic prostatic hypertrophy (enlarged prostate), or asthma. It should not be used concurrently with monoamine oxidase inhibitors (MAOIs). Patients must be warned against consuming alcohol or operating heavy machinery while taking this medication.

Pregnancy and Lactation
Chlorpheniramine is generally considered one of the safer antihistamines for use during pregnancy when necessary. However, it is excreted in breast milk and may cause drowsiness or irritability in nursing infants, and its anticholinergic effects may potentially inhibit lactation, so caution is advised during breastfeeding.