Chlorpheniramine
Chlorpheniramine
Generic Name
Chlorpheniramine
Mechanism
- H₁ receptor antagonism: Competitively blocks peripheral and central H₁ receptors, reducing vasodilation, capillary permeability, and smooth‑muscle contraction.
- Anticholinergic effect: Inhibits muscarinic (M₁–M₄) receptors, contributing to sedation, xerostomia, and anticholinergic toxicity in high doses.
- Limited blood‑brain barrier penetration leads to significant CNS side effects relative to newer antihistamines.
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Pharmacokinetics
| Parameter | Typical values |
| Absorption | Rapid; peak plasma concentration 0.5–1 h after oral dosing |
| Bioavailability | ~70 % (oral) |
| Distribution | Highly protein‑bound (~99 %); crosses BBB |
| Metabolism | Hepatic via CYP2D6; oxidative deamination and glucuronidation |
| Elimination | Renal (≈70 % unmetabolised), hepatic excretion of metabolites |
| Half‑life | 5–10 h (may extend to 12‑15 h in hepatic/renal impairment) |
| Onset of action | 15–20 min (rapid relief of allergic symptoms) |
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Indications
- Acute allergic rhinitis (hay fever)
- Urticaria (hives)
- Seasonal allergic conjunctivitis
- Non‑infectious rhinitis/eczema-associated itching
- First‑line adjunct in post‑operative sedation or nausea (when anticholinergic benefit desired)
*(Note: In many countries, over‑the‑counter formulations are limited to single‑dose use for acute symptoms.)*
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Contraindications
- Absolute contraindications: Severe cardiovascular disease (e.g., uncontrolled arrhythmias, ischemic heart disease), narrow‑angle glaucoma, prostatic hypertrophy with urinary retention, pregnancy (Category C).
- Precautions:
- Elderly: increased sensitivity to sedation & anticholinergic effects.
- Children <6 yrs: contraindicated; use caution in 6‑12 yrs for CNS effects.
- Patients with liver or kidney impairment → dose adjustment.
- Drug interactions:
- MAO inhibitors, tricyclic antidepressants, sedatives, alcohol → additive CNS depression.
- CYP2D6 inhibitors (e.g., fluoxetine) → elevated plasma levels.
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Dosing
| Age/Weight | Adult | Pediatric (6‑12 yrs) | Infant (≤6 yrs) |
| Oral | 4 mg every 4‑6 h (max 24 mg/day) | 0.25 mg/kg/dose every 8 h (max 12 mg/day) | Contraindicated |
| Intramuscular / Subcutaneous | 4–10 mg IM/SC (single dose) | Not recommended | |
| Nebulised | 2 mg/mL for respiratory allergies | 0.5 mg/kg/dose (max 3 mg) | – |
*Adjust dose downward in hepatic/renal dysfunction.*
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Adverse Effects
Common
• Drowsiness & sedation
• Dry mouth, blurred vision
• Headache, dizziness
• Light‑headedness, orthostatic hypotension
Serious
• Anticholinergic toxicity (confusion, urinary retention, tachycardia)
• Severe hypersensitivity (rash, angioedema)
• Cardiac arrhythmias (rare, especially in patients with pre‑existing arrhythmias)
• CNS depression (when combined with other CNS depressants)
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Monitoring
- Vital signs: BP, pulse, ECG in patients with cardiac disease
- CNS status: level of alertness, sedation score (e.g., Ramsay)
- Respiratory: especially if used IV/IM in patients with airway obstruction
- Urinary function: in patients with prostatic hypertrophy
- Laboratory: liver & renal panels if chronic use or dose adjustment needed
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Clinical Pearls
1. Anticholinergic profile matters – Use caution in elderly or patients with glaucoma; consider switching to a second‑generation antihistamine (cetirizine, loratadine) when anticholinergic side‑effects are problematic.
2. Sedation is a therapeutic advantage – In postoperative patients or those with severe allergic reactions causing sleep‑deprived symptoms, chlorpheniramine’s sedating properties can aid in rest and comfort.
3. Drug‑drug synergy – Combine with decongestants (pseudoephedrine) only after ruling out cardiac risk; the anticholinergic effect of chlorpheniramine can mask tachycardia induced by sympathomimetic decongestants.
4. Pediatric dosing formula – 0.25 mg/kg/dose; never exceed total of 12 mg/day; double‑check weight calculations to avoid overdose.
5. Over‑the‑counter caution – OTC products may contain 4‑mg tablets; a single dose is safe for adults, but repeat dosing >24 h without physician review can increase risk of overdose.
6. Renal impairment → dose reduction – Reduce to 2 mg every 6 h or halve the total daily dose; monitor serum creatinine.
7. Use a symptom‑based approach – If itching or sneezing is the only symptom, a second‑generation antihistamine may be preferable to avoid sedation.
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• > Key Takeaway: *Chlorpheniramine* remains a useful, readily available first‑generation antihistamine for acute allergic symptoms, especially when sedative effect is desired. Its anticholinergic activity and CNS penetration, however, necessitate mindful dosing and monitoring, particularly in vulnerable populations.