Leader Allerhist Side Effects

Generic Name: clemastine

Note: This page contains information about the side effects of clemastine. Some of the dosage forms included on this document may not apply to the brand name Leader Allerhist.

Not all side effects for Leader Allerhist may be reported. You should always consult a doctor or healthcare professional for medical advice. Side effects can be reported to the FDA here.

For the Consumer

Applies to clemastine: oral oral solution, oral tablets

Side effects include:

Sedation, sleepiness, dizziness, disturbed coordination, epigastric distress, thickening of bronchial secretions.

For Healthcare Professionals

Applies to clemastine: compounding powder, oral syrup, oral tablet

Nervous system

Nervous system side effects have included central nervous system depression which has commonly resulted in drowsiness, dizziness, and headache. Dyskinesias have rarely been reported following chronic use of antihistamines.

Nearly all patients experience drowsiness. This drowsiness may subside in some patients with extended use. Patients should be warned against driving, as well as concomitant ingestion with alcohol and other sedative-hypnotic drugs.

Few cases of dyskinesias and tremors, often of the face, have been reported in patients whose chronic use of antihistamines extended over a period of 3 to 10 years. Some of these cases were only partially relieved by discontinuation of the drug. Haloperidol was successful in relieving symptoms.


Gastrointestinal side effects have included dry mouth and constipation due to anticholinergic effects. This has occurred in up to one-third of patients.


Cardiovascular side effects have included hypotension, tachycardia, and palpitations.


Ocular effects are due to the anticholinergic effects and may include blurred vision, diplopia, and dry eyes.


Genitourinary side effects have included dysuria, urinary hesitancy, decrease in urine flow, and in rare cases, acute urinary retention.


Hematologic side effects that have been reported with antihistamines include bone marrow suppression, thrombocytopenia, and aplastic anemia.

A fatal case of agranulocytosis has been reported in a patient taking chlorpheniramine, pseudoephedrine, acetaminophen, dextromethorphan, phenylpropanolamine, and aspirin. Chlorpheniramine was felt to be the cause.


Respiratory side effects have included thickening of bronchial secretions, wheezing, and tightness in the chest.


At least one case of toxic pustuloderma has been reported during clemastine (the active ingredient contained in Leader Allerhist) therapy.

Dermatologic side effects have included toxic pustuloderma.

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