Diphenhydramine: Drug Interactions, Side Effects & Overdose

Instructor: Amy Couture

Amy has a master's degree in nursing and has taught nursing students at the college level.

Diphenhydramine is an antihistamine that is used to treat allergies, cold symptoms, and insomnia. This lesson discusses the drug interactions, side effects, and how overdoses occur when using this medication.

What is Diphenhydramine?

Jake is headed on an outdoor camping trip next month with friends and is very excited. But then he remembers it will be spring and he begins to dread it. Spring means allergies from pollen. Will he have to spend his weekend with itchy, watery eyes, a runny nose and sneezing? Well, there's good news for Jake, he can try medications that contains diphenhydramine.

Diphenhydramine is an antihistamine that is used to treat symptoms such as itchy, watery eyes, sneezing, and runny nose caused by allergies or the common cold. It is also used to treat and prevent motion sickness and insomnia.

Diphenhydramine works by blocking a natural substance our bodies make during an allergic reaction called histamines. By blocking histamines, the side effects from the allergic reaction are relieved.

Diphenhydramine can also be used to help control abnormal movements in people who are in the early stages of Parkinson's disease. This disease is a progressive disorder of the nervous system which causes tremors, body stiffness, and slowing of movement.


You can find this medication in tablet, capsule, liquid, and injectable form and is sold over the counter. You may not have heard the name diphenhydramine, but you would probably recognize some of the common brand name medications it is in.

For example, Benadryl, Diphen, Hydramine, Sominex, and Unisom all have diphenhydramine as its main ingredient. Diphenhydramine also comes in combination with fever reducers, pain relievers, and decongestants.

Diphenhydramine can be used to treat common allergy symptoms

Drug Interactions

Jake knows to check nonprescription cough and cold products labels carefully before using products at the same time. These products may contain the same active medication and taking them together could cause an overdose. The usual adult dose of diphenhydramine for treatment of allergy or cold symptoms is 25-50 mg taken orally every 4 to 6 hours.

Jake also knows that he needs to stay away from drinking alcohol or other central nervous system depressants while taking this medication to avoid becoming overly drowsy. He also is aware of other medications he cannot take while he is on this medication such as:

  • anti-anxiety medications (for example Valium or Xanax)
  • narcotics (for example Percocet, Vicodin, or Dilaudid)
  • certain antidepressants (for example Elavil, Tofranil, or Norpramin)
  • antihypertensives (for example Catapres and Inderal).

He also cancels his plans for driving, operating heavy machinery, or doing any activity that requires alertness or clear vision until the medication wears off. Guess his friends will have to drive!

Side Effects

There are side effects which may occur when taking diphenhydramine. The common ones include:

  • dry mouth
  • drowsiness
  • dizziness
  • nausea
  • vomiting
  • constipation
  • nervousness
  • drying and thickening of oral and respiratory secretions.

Jake is not worried though as most people taking this medication do not experience serious side effects. He knows he should contact his doctor immediately if he experiences mental or mood changes such as restlessness or confusion, vision problems, or difficult or painful urination while taking this medication.

Very serious side effects are rare, but 911 should be called for anyone experiencing seizures while on this medication.

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