Side Effects of Chlorpheniramine
Chlorpheniramine commonly causes sedation, drowsiness, and cognitive impairment, with elderly patients being particularly susceptible to anticholinergic effects including confusion, urinary retention, and constipation.
Common Side Effects
Central Nervous System Effects
- Sedation and drowsiness - Most frequent side effect 1
- Dizziness and impaired coordination
- Reduced cognitive function and concentration 2
- Impaired performance in tasks requiring visual-motor coordination 2
- Difficulty with sustained attention and auditory stimulus detection 2
- Paradoxical excitation (especially in children) 3
Anticholinergic Effects
- Dry mouth
- Blurred vision
- Urinary retention (particularly in men with enlarged prostate) 4
- Constipation
- Increased intraocular pressure (caution in glaucoma) 4
Cardiovascular Effects
- Tachycardia
- Potential QT interval prolongation in overdose 3
- Possible hypertension or hypotension (in overdose situations) 3
Special Populations
Elderly Patients
- Higher risk of anticholinergic side effects
- Greater risk of cognitive decline and confusion 1
- Increased risk of falls due to sedation
- Potential for worsening cognitive function in those with pre-existing impairment 1
Children
- FDA recommends against use in children under 6 years 3
- Children 6-12 years should receive reduced dosing (2.5mg every 4-6 hours) 3
- Risk of paradoxical excitation, agitation, and confusion 3
- Potential for seizures in overdose situations 3
Patients with Comorbidities
- Respiratory conditions: May worsen breathing problems in emphysema or chronic bronchitis 4
- Glaucoma: Can increase intraocular pressure 4
- Prostatic hyperplasia: May cause urinary retention 4
- Liver disease: Avoid in severe liver disease as it may precipitate coma 1
- Renal impairment: May require dose adjustment 1
Long-Term Use Considerations
- Development of tolerance (subsensitivity) with chronic use 5
- Poor compliance with long-term therapy due to side effects 5
- Potential for medication-overuse symptoms with prolonged use
Rare but Serious Adverse Effects
- Rhabdomyolysis (in severe overdose cases) 6
- Acute kidney injury (in severe overdose cases) 6
- Seizures (particularly in overdose situations) 3
- Severe cardiac arrhythmias (in overdose) 3
Drug Interactions
- Increased sedation when combined with other CNS depressants (alcohol, benzodiazepines, opioids)
- Potential serotonergic effects due to SSRI-like properties 7
- May interact with MAOIs, potentially causing serotonin syndrome 1
- Pharmacokinetic interactions with drugs metabolized by cytochrome P450 system 1
Clinical Pearls
- First-generation antihistamines like chlorpheniramine have more pronounced CNS effects than newer non-sedating antihistamines
- Sedating effects may diminish with continued use but never completely disappear
- Consider timing of administration to minimize impact on daily activities (e.g., taking at bedtime)
- Monitor elderly patients closely for anticholinergic side effects and cognitive changes
- Be aware of the potential for impaired driving ability, even when patients don't subjectively feel impaired 2