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Table of Contents > Drug > Diphenhydramine (Systemic) Print

Diphenhydramine (Systemic)


Related terms
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    Related terms
    • U.S. Brand Names: Aler-Cap [OTC];Aler-Dryl [OTC];Aler-Tab [OTC];AllerMax® [OTC];Altaryl [OTC];Anti-Hist [OTC];BanophenT [OTC];Benadryl® Allergy Quick Dissolve [OTC];Benadryl® Allergy [OTC];Benadryl® Children's Allergy FastMelt® [OTC];Benadryl® Children's Allergy Perfect MeasureT [OTC];Benadryl® Children's Allergy [OTC];Benadryl® Children's Dye Free Allergy [OTC];Benadryl® Dye-Free Allergy [OTC];Compoz® [OTC];Diphen [OTC];Diphenhist® [OTC];Histaprin [OTC];Nytol® Quick Caps [OTC];Nytol® Quick Gels [OTC];PediaCare® Children's Allergy [OTC];PediaCare® Children's NightTime Cough [OTC];Siladryl Allergy [OTC];Silphen [OTC];Simply Sleep® [OTC];Sleep-ettes D [OTC];Sleep-Tabs [OTC];Sleepinal® [OTC];Sominex® Maximum Strength [OTC];Sominex® [OTC];Theraflu® Thin Strips® Multi Symptom [OTC];Triaminic Thin Strips® Children's Cough & Runny Nose [OTC];Twilite® [OTC];Unisom® SleepGels® Maximum Strength [OTC];Unisom® SleepMeltsT [OTC]
    • Canadian Brand Names: Allerdryl®;Allernix;Benadryl®;Nytol®;Nytol® Extra Strength;PMS-Diphenhydramine;Simply Sleep®;Sominex®
    • Pharmacologic Category: Ethanolamine Derivative;Histamine H1 Antagonist;Histamine H1 Antagonist, First Generation

    • It is used to stop coughing.
    • It is used to ease allergy signs.
    • It is used to help motion sickness.
    • It is used to treat hives.
    • It is used to treat itching.
    • It is used to treat Parkinson's disease.
    • It is used to treat swelling of the face, lips, tongue, or throat.
    • It is used to treat sleep problems.
    • Diphenhydramine lowers or stops the body's reaction to the allergen.
    • It helps treat a chemical imbalance.
    • It calms the brain.
    • It affects the throwing up (vomiting) center in the brain.


    How to take

    • Oral:
    • If you are taking this drug for an allergy, it is most useful if started before contact with the allergen. Take at least 1 to 3 hours before.
    • If you are taking this drug for motion sickness, take 30 minutes before travel.
    • If you are taking this drug for a sleep problems, take it 1 hour before going to bed.
    • Take with or without food. Take with food if it causes an upset stomach.
    • There is a liquid (elixir, syrup) if you cannot swallow pills.
    • Those who have feeding tubes can also use the liquid. Flush the feeding tube before and after this drug is given.
    • Oral-disintegrating tablet: Place on your tongue and let melt. Water is not needed. Do not swallow whole. Do not chew, break, or crush.
    • Chewable tablet: Chew or crush well. Mix crushed tablet with food. Do not swallow it whole.
    • Injection:
    • It is given as a shot into a muscle or vein.

    Missed Dose

    • Take a missed dose as soon as you think about it.
    • If it is close to the time for your next dose, skip the missed dose and go back to your normal time.
    • Do not take two doses or extra doses.
    • Many times this drug is taken on an as needed basis.


    • Oral:
    • Store at room temperature.
    • Protect capsules and tablets from water. Do not store in a bathroom or kitchen.
    • Injection:
    • This drug will be given to you in a doctor's office. You will not store it at home.



    • Talk with the doctor before giving this drug to a child. Check all of your child's drugs, including OTC, with doctor.


    • Do not give Benadryl® Children's Allergy Fastmelt® if your child is allergic to peanuts or soybeans.
    • If you have an allergy to diphenhydramine or any other part of this drug.
    • Tell your doctor if you are allergic to any drugs. Make sure to tell about the allergy and what signs you had. This includes telling about rash; hives; itching; shortness of breath; wheezing; cough; swelling of face, lips, tongue, or throat; or any other signs.
    • If you have asthma.
    • If you are breast-feeding.


    • Wear disease medical alert ID (identification).
    • If you are 65 or older, use this drug with care. You could have more side effects.
    • If you have a bowel block, talk with your doctor.
    • If you have an enlarged prostate, talk with your doctor.
    • If you have glaucoma, talk with your doctor.
    • If you have heart disease, talk with your doctor.
    • If you have high blood pressure, talk with your doctor.
    • If you have PKU, talk with your doctor. Some products have phenylalanine.
    • If you have thyroid disease, talk with your doctor.
    • If you have trouble passing urine, talk with your doctor.
    • Check all drugs you are taking with your doctor. This drug may not mix well with some other drugs.
    • Do not use with other products that have diphenhydramine.
    • You may not be alert. Avoid driving and doing other tasks or actions until you see how this drug affects you.
    • Avoid beer, wine, or liquor or other drugs and natural products that slow your actions and reactions.
    • Tell your doctor if you are pregnant or plan on getting pregnant.

    Side Effects

    • Feeling lightheaded, sleepy, having blurred eyesight, or a change in thinking clearly. Avoid driving and doing other tasks or actions that call for you to be alert or have clear eyesight until you see how this drug affects you.
    • Hard stools (constipation). Drinking more liquids, exercising, or adding fiber to your diet may help. Talk with your doctor about a stool softener or laxative.
    • Dry mouth. Good mouth care, sucking hard, sugar-free candy, or chewing sugar-free gum may help.


    • Change in the health problem being treated. Is it better, worse, or about the same?
    • Dry mouth may cause more cavities. Take good care of your teeth. See a dentist often.

    Contact a healthcare provider

    • If you think there was an overdose, call your local poison control center or ER right away.
    • Signs of a very bad reaction to the drug. These include wheezing; chest tightness; fever; itching; bad cough; blue skin color; seizures; or swelling of face, lips, tongue, or throat.
    • Not able to pass urine.
    • Feeling very tired or weak.
    • Any rash.
    • Health problem is not better or you are feeling worse.

    General Statements

    • If you have a very bad allergy, wear an allergy ID at all times.
    • Do not share your drugs with others and do not take anyone else's drugs.
    • Keep all drugs out of the reach of children and pets.
    • Most drugs can be thrown away in household trash after mixing with coffee grounds or kitty litter and sealing in a plastic bag.
    • In Canada, take any unused drugs to the pharmacy. Also, visit to learn about the right way to get rid of unused drugs.
    • Keep a list of all your drugs (prescription, natural products, vitamins, OTC) with you. Give this list to your doctor.
    • Call your doctor for help with any side effects. You may also call the FDA at 1-800-FDA-1088 or Health Canada's Vigilance Program at 1-866-234-2345.
    • Talk with the doctor before starting any new drug, including OTC, natural products, or vitamins.

    Author information
    • Copyright © 1978-2010 Lexi-Comp Inc. All rights reserved.

    Copyright © 2011 Natural Standard (

    The information in this monograph is intended for informational purposes only, and is meant to help users better understand health concerns. Information is based on review of scientific research data, historical practice patterns, and clinical experience. This information should not be interpreted as specific medical advice. Users should consult with a qualified healthcare provider for specific questions regarding therapies, diagnosis and/or health conditions, prior to making therapeutic decisions.

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