Benadryl: What You Should Know

Taking Benadryl After Surgery

Nausea and Vomiting
Benadryl Can Decrease Nausea and Vomiting.

Benadryl (Diphenhydramine) Explained

Diphenhydramine hydrochloride is most commonly known by the brand name Benadryl. To be clear, some Benadryl brand products contain medications other than diphenhydramine, but, generally speaking, when people say Benadryl they usually mean diphenhydramine.  

This medication is used for multiple reasons, but most commonly to decrease the body’s response to allergens, ranging from medications that cause itching, to life-threatening anaphylactic reactions to bee stings and food allergies.

 This medication is frequently used during before and after surgery.

What Does Benadryl Do?

Benadryl is a first generation H1 receptor antagonist.  That is a very technical way of saying that diphenhydramine was one of the first drugs created to reduce the effects of histamine, which the body releases when fighting off an allergen or pathogen. In other words, diphenhydramine reduces the symptoms we experience when we have allergies.

Histamine has its purposes, it does more than cause misery during allergy season.  It helps our bodies decide when to be awake and when it is time to sleep, and it helps to regulate our appetite and body temperature. Most people know about histamine for its role in fighting off foreign pathogens (like the ones we wash our hands to get rid of after going to the bathroom), but we most often notice the effects of histamine when our allergies are causing annoying symptoms, like itching, watery eyes, trouble breathing, a runny nose and sneezing.


For people with allergies, diphenhydramine is one of many “histamine blocker” medications that can improve symptoms, but it comes with side effects.  For some people, the side effects of the medication are the true desired effects.

For individuals with an itchy rash, such as poison ivy, a spray is available that can be applied to the skin.

How Benadryl (Diphenhydramine) Is Taken

Benadryl can be taken orally, as a liquid, a pill, or a fast melting tablet.  It can also be given through an IV or as an injection into the muscle. It can also be purchased as a topical spray.

Diphenhydramine can be purchased over the counter to be taken by mouth or to spray on the skin, but injectable forms of the drug require a prescription.

Why is Benadryl Used Before and After Surgery

It may seem strange to take a medication for allergies during recovery after surgery, but there are many uses for this drug, some of which are very useful in the hours and days following surgery.  

Diphenhydramine is frequently given before and after surgery to prevent a very common complication of surgery: nausea and vomiting.  Studies have shown that when used with other medications that prevent this unpleasant side effect, diphenhydramine is very effective for prevention and treatment.

For other individuals, diphenhydramine is given to reduce the itching that people may experience when taking prescription pain medications.

  It is also frequently used as a mild sleep aid when insomnia is present.

Other Uses For Benadryl

This medication is also used to treat and prevent motion sickness, to reduce the symptoms of Parkinson’s disease, and to reduce cold and flu symptoms.

Side Effects of Benadryl

Diphenhydramine has several notable side effects.   The most common of these include a drying effect mouth and eyes, sedation and sleepiness.  In higher doses--usually over 25mg every 4-6 hours, these side effects can be more notable.

The effects of diphenhydramine last between 2.5 hours to 9 hours in most individuals, but can last longer in those with liver disease.

Newer antihistamines are available that decrease these side effects, which is beneficial to those who experience allergy symptoms but want to avoid feeling sleepy.

Risks of Taking Benadryl

  • Benadryl is a category B medication, which means it is considered safe to use during pregnancy, but it should be avoided when breast feeding as it can pass through milk to the infant.  
  • It should be avoided by those who have glaucoma, prostate problems that make urination difficult, and those with serious breathing issues unless approved by their healthcare provider.
  • Not everyone experiences drowsiness with this medication, some individuals can have the opposite reaction and may feel jittery or overly excited.  This effect is more common in children.  Children under the age of 6 should not be given this medication without first discussing in with their healthcare provider. 
  • Do not operate heavy machinery or drive when taking this medication.
  • Diphenhydramine should not be combined with other sedatives, including alcohol. 
  • Diphenhydramine should be avoided in individuals taking MAO inhibitors.
  • This medication was used in the past to decrease sensation in the skin prior to procedures, such as having an IV placed.  It is no longer recommended for that purpose. 


Diphenhydramine Hydrochloride Monograph.  Accessed February, 2016.

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