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Diphenhydramine HCL Information

written by: Victoria Trix • edited by: Emma Lloyd • updated: 10/6/2009

Diphenhydramine HCL is also known as Sominex, Unisom, Benadryl and Genahist by pharmacists. This medication is an antihistamine used to relieve allergy symptoms as well as symptoms of the common cold and hay fever.

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    This drug is used to counteract symptoms of allergies and hay fever as well as the common cold such as rash, itching and watery eyes, cough, sneezing and runny nose. This is also used at times to prevent and treat nausea and dizziness that is associated with motion sickness as needed. This drug has also been used to treat tremors and muscle stiffness that is associated with Parkinson’s disease as well as side effects of psychiatric drugs.

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    Dosing Information

    This medication comes in a tablet or strip form, and should be allowed to completely dissolve on the tongue and then swallow. Water is not required for consumption of this medication. Chewable tablets are to be completely chewed and then swallowed, with or without water. Do not take more than your dose on a daily basis, as the dose is based on medical conditions, age and therapy response. When used to prevent motion sickness, take half an hour prior to travelling.

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    Those patients with breathing problems such as emphysema or asthma, persons with glaucoma, heart problems and high blood pressure should not take this medication. Those persons with liver disease, mental or mood changes, seizures, stomach problems like ulcers, overactive thyroid, or difficulty urinating should not take this medication. While taking this medicine, do not drive or complete any activity that requires you to be alert or have clear vision until you know when this can be done safely. Do not partake of alcohol or any other medicines that cause drowsiness while taking this drug.

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    Drug Interactions

    Medications such as belladonna alkaloids, atropine should be avoided. Any patient taking anticholinergics like benztropine or trihexyphenidyl as well as MAOIs, tricyclic antidepressants and scopolamine should alert their physician and pharmacist to these medications prior to filling this prescription, as it has severe side effects when mixed with any other drugs listed.

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    Side Effects

    Side effects of diphenhydramine HCL include dizziness, headache, constipation, stomach upset, drowsiness, blurred vision and irritability as well as decreased coordination and dry mouth, nose and throat. Relieving dry mouth can be done with ice chips or sugarless hard candy, or even by drinking water or chewing gum. Prevention of this side effect comes by drinking plenty of fluids while taking this medication.

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    If a patient is diabetic, it is best to avoid the liquid preparation of this medication, as it normally contains sugar and alcohol. Those with liver disease or alcohol dependence should avoid this medication due to the liquid mixture. Children are more sensitive to the effects of this medication, and can cause excitement. Elderly patients can have more side effects including dizziness, drowsiness and low blood pressure from this medication. This should not be used during pregnancy, because it does pass through breast milk.

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    Resources from the publication Diphenhydramine HCL