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Chlorpheniramine (chlorphenamine) is a first-generation alkylamine H1 antihistamine ubiquitous in cold and allergy preparations, providing symptom relief through histamine H1 receptor blockade while causing sedation and, via muscarinic acetylcholine antagonism at all five subtypes, anticholinergic effects that classify it among the deliriants. A distinctive and less appreciated property is that it is a comparatively potent serotonin reuptake inhibitor; rodent studies show it raises prefrontal extracellular serotonin and produces anxiolytic-like effects independent of H1 blockade, and its in vitro profile resembles that of antidepressants. This serotonergic action carries clinical risk, since chlorpheniramine has contributed to serotonin toxicity in overdose, particularly when combined with dextromethorphan. High doses otherwise produce the anticholinergic delirium typical of the class.
- Relieves allergic rhinitis, hay fever and hives
- Eases sneezing, runny nose and itchy eyes in cold and allergy products
- Sedating properties can help with allergy-related sleeplessness
- Antihistamine that also inhibits serotonin reuptake
- Drowsiness and impaired coordination or driving
- Dry mouth, blurred vision and constipation
- Urinary retention and confusion, especially in older adults
- Delirium and toxidrome at high doses
Mechanism
Chlorpheniramine is an inverse / at H1 receptors; blocking H1 in the periphery calms allergic symptoms (itching, runny nose, watery eyes) and blocking H1 in the brain causes drowsiness because histamine promotes wakefulness. Like other first-generation antihistamines it also crosses the and antagonizes receptors, which is the source of its anticholinergic side effects and its deliriant potential at high doses. Chlorpheniramine additionally has modest and reuptake inhibition, which is part of why some of its relatives were explored as antidepressants. At high doses the muscarinic blockade dominates and produces the anticholinergic toxidrome.
receptor fingerprint
H1 receptorAntagonist (inverse agonist)
receptorsAntagonist
and transportersReuptake inhibitor
Safetyrisks and cautions, not medical advice
At high doses chlorpheniramine causes anticholinergic delirium with realistic hallucinations mistaken for reality, confusion and memory loss, plus the full toxidrome: blurred vision, dangerous overheating, flushed dry skin, dry mouth, racing heart, urinary retention, seizures and, in serious overdose, death. Because it is in so many combination cold products, it is easy to double up unknowingly or to stack anticholinergic burden with other sedating antihistamines and deliriants. High anticholinergic burden is linked to greater dementia risk in older people and with long-term use. Not medical advice.
Interactionsdocumented pairs only, not exhaustive
Chlorpheniramine is a sedating first-generation antihistamine, and nearly all of its interactions are additive rather than metabolic. Alcohol, benzodiazepines, opioids, barbiturates, sedating antidepressants and antipsychotics all compound its central depression, and the combined effect on alertness and driving is larger than either agent produces alone. Its anticholinergic activity stacks the same way with tricyclics, oxybutynin, scopolamine and low-potency antipsychotics, producing dry mouth, blurred vision, urinary retention, constipation and confusion, with older adults at the highest risk of delirium.
Monoamine oxidase inhibitors prolong and intensify the anticholinergic effects of antihistamines, so that combination is generally avoided.
Chlorpheniramine also interacts with CYP2D6 and can slow the clearance of drugs handled by it. That matters most in the combination cold products where it usually appears: dextromethorphan in the same tablet is a CYP2D6 substrate with serotonergic activity, and phenylephrine adds a pressor effect that turns dangerous alongside an MAO inhibitor.
Checking a whole stack? Run it through interactions + stacks.
Subjective profileweighing the evidence above
It works on allergies, but the sedation and anticholinergic load make it a poor daily choice and a genuinely bad one for older adults, where confusion and urinary retention are the price. A non-sedating antihistamine is the better pick for almost everyone.
Resources
This entry is here for reference.
Research
- 1990first citedH1-receptor antagonist treatment of seasonal allergic rhinitis
- 2002controlled trialDifferential cognitive effects of ebastine and (+)-chlorpheniramine in healthy subjects: correl…
- 2022most recentClinically Relevant Drug Interactions with Monoamine Oxidase Inhibitors
- 1.H1-receptor antagonist treatment of seasonal allergic rhinitis
- 2.Clinically Relevant Drug Interactions with Monoamine Oxidase Inhibitors
- 3.Differential cognitive effects of ebastine and (+)-chlorpheniramine in healthy subjects: correlation between cognitive impairment and plasma drug concentration
- 4.Affinities of brompheniramine, chlorpheniramine, and terfenadine at the five human muscarinic cholinergic receptor subtypes.
- 5.Chlorpheniramine exerts anxiolytic-like effects and activates prefrontal 5-HT systems in mice.
- 6.Dextromethorphan, chlorphenamine and serotonin toxicity: case report and systematic literature review.
- 7.Human lymphoblastoid cell line panels: novel tools for assessing shared drug pathways.
- 8.First-generation H1 antihistamines found in pilot fatalities of civil aviation accidents, 1990-2005.
8 listed here; entry last updated August 2026
Reviews
My notesprivate to this device
FAQ
Is chlorpheniramine an anticholinergic even though it is an antihistamine?
Yes. First-generation antihistamines like this one block muscarinic acetylcholine receptors in addition to histamine H1 receptors, which is why the anticholinergic side effects and deliriant risk exist.
Why is it in so many cold medicines?
It dries secretions and eases allergy-type symptoms. The downside is that its wide availability makes it easy to accidentally exceed safe amounts or stack anticholinergic burden.
Can high doses cause hallucinations?
Yes. At high doses it produces an anticholinergic delirium with realistic hallucinations mistaken for reality, which is dangerous and unpleasant rather than recreational.
Adverse effects
- Drowsiness and impaired coordination or driving
- Dry mouth, blurred vision and constipation
- Urinary retention and confusion, especially in older adults
- Delirium and toxidrome at high doses