HISTOP SYRUP 100ML

Indications

Approved Uses

Symptomatic relief of allergic conditions such as allergic rhinitis (hay fever), urticaria, pruritus and other uncomplicated allergic skin manifestations; may also relieve allergic conjunctivitis symptoms.

Off-Label Uses

Symptomatic relief of common cold symptoms (runny nose/sneezing) is commonly used; other off-label uses exist but are less routine.

Dosage & Administration

Dosing by Condition

Adults/≥12 years: 2 mg (5 mL of 2 mg/5 mL formulation) every 4-6 hours, max 12 teaspoonfuls (60 mL) in 24 hours; Children 6-12 years: 1 mg (2.5 mL) every 4-6 hours, max 6 teaspoonfuls (30 mL) in 24 hours; Children under 6 years: dosing not specified without doctor direction

Initial Dose

Adults: 2 teaspoonfuls (10 mL) every 4-6 hours for the 2 mg/5 mL formulation

Maintenance Dose

Adults: 2 teaspoonfuls (10 mL) every 4-6 hours for the 2 mg/5 mL formulation, not to exceed 12 teaspoonfuls in 24 hours

Maximum Dose

Adults and children 12 years and over: 40mL (16mg) per day; Children 6-12 years: 30mL (12mg) per day; Children 2-6 years: consult physician

Children's Dosage

Children 6 to under 12 years: 1 teaspoonful (5 mL) every 4-6 hours, not to exceed 6 teaspoonfuls in 24 hours. Children under 6 years: dosing not specified without doctor direction

Dose Adjustment Notes

Elderly: consider lower initial doses and use caution due to increased sensitivity to sedation/anticholinergic effects; renal or hepatic impairment: use with caution and consider dose reduction/extended dosing interval if clinically needed.

How to Take

Shake well before use; measure the dose with a calibrated oral syringe or measuring spoon/cup; take by mouth; may be taken with or without food.

Side Effects

Common Side Effects

Drowsiness/sedation, dizziness, dry mouth/nose/throat, blurred vision, constipation, urinary retention; paradoxical excitation/restlessness can occur (especially in children).

Side Effect Frequency

Most common: sedation/drowsiness and anticholinergic effects (e.g., dry mouth, blurred vision, constipation); less common/rare: urinary retention, tachycardia/palpitations/arrhythmias, paradoxical CNS excitation (especially in children), and very rare hematologic reactions (e.g., agranulocytosis, thrombocytopenia).

Safety & Warnings

Contraindications

Contraindications: hypersensitivity to chlorpheniramine/ingredients; use in neonates/premature infants; concomitant or recent (within 14 days) MAOI therapy; narrow-angle glaucoma; urinary retention/bladder neck obstruction (including symptomatic prostatic hypertrophy); stenosing peptic ulcer; pyloroduodenal obstruction.

Warnings & Precautions

Warnings/precautions: causes drowsiness-avoid driving/operating machinery; avoid alcohol and other CNS depressants; use caution in asthma/COPD or lower respiratory disease, glaucoma/increased intraocular pressure, prostatic hypertrophy/urinary retention, cardiovascular disease/hypertension, hyperthyroidism, hepatic/renal impairment, and in the elderly; paradoxical excitation may occur in children; avoid in neonates/premature infants.

Age Restriction

Not recommended in children <2 years unless directed/supervised by a physician.

Driving Warning

May Cause Drowsiness

Drug Interactions

Drug Interactions

Key interactions: MAO inhibitors (avoid); CNS depressants including alcohol, opioids, benzodiazepines/sedatives (additive sedation/respiratory depression); other anticholinergics and tricyclic antidepressants (additive anticholinergic/CNS effects).

Interaction Severity

MAJOR: Concomitant MAO inhibitors (or within 14 days) - may potentiate/prolong anticholinergic/CNS effects and is generally avoided/contraindicated; MODERATE: alcohol and other CNS depressants (opioids, benzodiazepines, sedatives) - additive sedation/respiratory depression risk; MODERATE: other anticholinergics (including TCAs) - additive anticholinergic effects.

Food Interaction

No specific food restrictions; may be taken with or without food.

Alcohol Interaction

Avoid

Special Populations

Pregnancy

Caution

Breastfeeding

Caution

Children

Children 6 to under 12 years: 1 teaspoonful (5 mL) every 4-6 hours, not to exceed 6 teaspoonfuls in 24 hours. Children under 6 years: dosing not specified without doctor direction

Storage & Patient Advice

Stopping the Medicine

Safe to stop anytime; no tapering required

Overdose

Overdose may cause marked sedation or paradoxical excitation (especially in children), anticholinergic toxicity (mydriasis, dry mouth, flushing, hyperthermia, tachycardia, urinary retention), hallucinations and seizures; management is urgent medical evaluation with supportive care (airway/ventilation), activated charcoal if appropriate, and benzodiazepines for seizures.

Patient Counseling

May cause drowsiness-avoid driving/operating machinery until effects are known; avoid alcohol and other sedatives; measure doses with a proper device; do not exceed recommended dose; seek advice before use in glaucoma, urinary retention/BPH, severe asthma/COPD, or if taking MAOIs; for young children (especially <2 years) use only with medical advice; see a clinician if symptoms persist/worsen.

Monitoring Requirements

No routine monitoring required for short-term OTC use; monitor for paradoxical excitation in children

Pharmacology

Mechanism of Action

Reversible competitive antagonist (inverse agonist) at peripheral and central H1 receptors, reducing histamine-mediated symptoms (sneezing, rhinorrhea, pruritus, wheal/flare).

Onset of Action

Approximately 30 minutes (often within 15-60 minutes).

Duration of Effect

4-6 hours.

Half-Life

Mean ~20 hours; reported range roughly 12-43 hours (can be longer in some patients).

Bioavailability

25-50%.

Metabolism

Extensively metabolized hepatically with significant first-pass metabolism; CYP2D6 is a major pathway (with minor contribution from other CYPs such as CYP3A4).

Excretion

Primarily renal excretion in urine as metabolites and some unchanged drug.

Product Information

Available Dosage Forms

Syrup (this product: 2 mg/5 mL); chlorpheniramine is also available in immediate-release tablets/capsules in some markets.

Composition per Dose

Each 5ml: 2mg chlorpheniramine maleate (0.4mg/ml)

Generic Availability

Yes

OTC Alternatives

Second-generation antihistamines (e.g., loratadine, cetirizine, fexofenadine) for less sedation; other sedating antihistamines (e.g., diphenhydramine) are alternatives when sedation is acceptable.

Symptom Target

Allergy, Runny Nose

Sedating

Yes

Reviewed by Al Mujtama Pharmacy Medical Review Team. Last reviewed 07-06-2026