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Allerhist-D Side Effects

Generic name: clemastine / phenylpropanolamine

Note: This document contains side effect information about clemastine / phenylpropanolamine. Some dosage forms listed on this page may not apply to the brand name Allerhist-D.

Applies to clemastine/phenylpropanolamine: oral tablet extended release.

Nervous system

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

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.

Seizures may occur in rare cases of hypertensive crisis due to phenylpropanolamine and have been reported with normally recommended doses as well as in cases of overuse or overdose.

There have been anecdotal reports of cerebrovascular hemorrhage largely associated with an uneven pattern of cerebrovascular spasm referred to as vascular beading. Vascular beading has also been reported in the absence of hemorrhage. Intracranial hemorrhage has almost always been associated with hypertension.[Ref]

Central nervous system depression has commonly been associated with the use of clemastine. This may result in drowsiness, dizziness, and headache. Dyskinesias have rarely been reported following chronic use of antihistamines. Phenylpropanolamine can stimulate the nervous system, resulting in tremor, anxiety, insomnia, dizziness, and nervousness. Headache has also been associated with the use of phenylpropanolamine.[Ref]


Phenylpropanolamine causes vasoconstriction which usually does not result in blood pressure elevations in healthy adults given normally prescribed dosages. However, phenylpropanolamine administration may be problematic for patients with preexisting hypertension and those receiving higher dosages. In general, 75 mg of sustained-release phenylpropanolamine will not produce a significant increase in blood pressure in normotensive patients, but 150 mg of sustained-release phenylpropanolamine can.

The combination of caffeine and phenylpropanolamine is more apt to cause hypertension. Although caffeine is no longer added to phenylpropanolamine as an anorexiant, this combination is available as "look-alikes" for amphetamines. Hypertensive crisis has occurred occasionally subsequent to overusage, overdose, and ingestion of normally recommended doses. Hypertensive crisis may be accompanied by headache, blurred vision, confusion, intracranial hemorrhage, encephalopathy, or seizures.

Arrhythmias may be produced in predisposed patients. The majority of reports of arrhythmias involve overuse or overdose. Rarely, high doses of phenylpropanolamine may cause chest pain and evidence of myocardial injury.

One study reported that taking phenylpropanolamine increases the risk of hemorrhagic stroke in women. Men may also be at risk. Although the risk of hemorrhagic stoke is very low, the FDA recommends that all use of phenylpropanolamine be discontinued.[Ref]

Cardiovascular adverse effects may be associated with the use of phenylpropanolamine. Use of phenylpropanolamine can cause a significant rise in heart rate. Hypertension and arrhythmias may be problematic in susceptible patients. Cardiovascular effects of clemastine may include hypotension, tachycardia, and palpitations. Cardiovascular side effects have also included an increased risk of hemorrhagic stroke.[Ref]


Gastrointestinal reactions from clemastine include dry mouth and constipation due to its anticholinergic effects and may occur in up to one-third of patients. Gastrointestinal complaints most commonly associated with phenylpropanolamine include anorexia and gastric irritation. Nausea and vomiting have occurred in conjunction with hypertensive episodes.[Ref]


Ocular side effects of clemastine are primarily anticholinergic and may include blurred vision, diplopia, and dry eyes.[Ref]


Genitourinary side effects include dysuria, urinary hesitancy, and decreased urine flow. These side effects are thought to be due to the anticholinergic effects of clemastine. In rare cases, use of clemastine can precipitate acute urinary retention.[Ref]


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

Hematologic side effects have been associated with the use of antihistamines and include bone marrow suppression, thrombocytopenia, and aplastic anemia.[Ref]


Psychiatric reactions to phenylpropanolamine occur infrequently but include acute mania, anxiety, paranoia, confusion, agitation, and hallucinations. These reactions may be more common in women.[Ref]

Psychotic reactions to phenylpropanolamine have occurred in patients receiving normally recommended doses and in cases of abuse. In a few patients, phenylpropanolamine appears to have exacerbated an underlying bipolar disorder which was previously undiagnosed.

A psychotic episode consisting of abnormal behavior was reported in a young woman following a week of therapy with Naldecon (phenylephrine, phenylpropanolamine, chlorpheniramine, and phenyltoloxamine) and amantadine. The patient had no personal or family history of psychiatric illness and no history of recreational substance use. It is uncertain whether the episode was due to the amantadine, the phenylpropanolamine or another component in the Naldecon, or an interaction between the drugs.[Ref]


Hypersensitivity reactions to phenylpropanolamine have been reported.[Ref]


Renal side effects associated with the use of phenylpropanolamine include rare cases of acute interstitial nephritis.[Ref]

More about Allerhist-D (clemastine / phenylpropanolamine)

Related treatment guides


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Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Some side effects may not be reported. You may report them to the FDA.