Diphenhydramine (Benadryl)

Diphenhydramine (Benadryl)


July 7, 2023

Dear colleagues:

Welcome back to UH EMS Institute’s Pharmacy Phriday.  With the arrival of summer, it seems we start hearing about those incidents of bee, wasp, and hornet stings again!  And it serves as a good time to review Benadryl’s use in treating the allergic reactions to those stings as well as the other indications included in the UH Protocols.

While our focus in this article is the review of Benadryl, an overwhelming point regarding an allergic reaction is recognizing the difference of a mild allergic reaction to that of anaphylaxis and the proper treatment for both.  What comes to mind regarding your assessment and treatment of these patients?

Let us start with a quick physiology review.  When an allergen, be it an insect bite, a sting, a medication, animal dander, molds, etc., is introduced into the body through ingestion, inhalation, absorption, or injection (envenomation) the body’s immune system responds to defend itself and eliminate the foreign substance.  During this protective response, one of the chemical mediators released is histamines. 

Histamines produce a wide variety of actions within various body systems, including increased vascular permeability, promotion of vasodilation and flushing, decreased AV node conduction time, stimulation of nerves that produce coughing, smooth muscle contraction in the bronchioles and GI tract, and stimulation of the allergic immune response. In an allergic reaction, this defense mechanism is exaggerated.  In anaphylaxis, the protection ceases to be a help and becomes an immediate life threat.

Benadryl is an antihistamine that is helpful in the treatment of symptoms during an allergic reaction.  The antihistamine prevents the histamine responses of the body that result in signs and symptoms such as rhinitis, watery eyes, itching, rashes, swelling, wheezing, etc.

Benadryl also has an anticholinergic (drying) effect on the body, which helps during an allergic reaction.  The anticholinergic effect is a reason Benadryl is helpful in dystonia or extrapyramidal symptoms (EPS) and is included in the treatment when giving Haldol. Benadryl will affect the dopaminergic-cholinergic balance in the brain). 

Additionally, Benadryl readily crosses the blood-brain barrier affecting the CNS receptors and causes a sedative effect on the body.  This is why it may be used in our protocols for the pediatric patient during behavioral emergencies.

Benadryl is supplied in 50mg/1ml vials.  The dosing of Benadryl under the UH Protocols is 25-50 mg IV/IO/IM for the adult patient.  The pediatric dose is 1mg/kg.  The presence of narcotics, alcohol, and sedatives in the patient should be considered when dosing Benadryl, as their effects can be increased. Like most medications we administer, Benadryl should be given as a slow IV push.  The onset of Benadryl is usually about 10-15 minutes if given IV.  The IV route is preferred in cases of dystonia and EPS.  The AEMT may administer Benadryl, but only in cases of allergic reactions and anaphylaxis.

Some of the most common adverse effects from the use of Benadryl include drowsiness, sedation, palpitations, and anxiety. Due to drying secretions in the body, Benadryl can sometimes make an asthma attack worse.  Tachycardia and QT prolongation are also possible, and some of the reasons caution is advised when treating a patient with a known cardiac history.  Patients should be monitored carefully for these and other side effects following the administration of Benadryl.

Once again, remember to watch the patient carefully for the progression of an allergic reaction to one of anaphylaxis.  The typical allergic reaction, which may seem a “local” reaction, can quickly progress to a life-threatening “systemic” reaction. 

While Benadryl is the first-line treatment for a mild allergic reaction, in the case of anaphylaxis, it is not the “primary” treatment.  Benadryl should only be given following the use of Epinephrine. Anaphylaxis and Epinephrine will be the focus of our next installment of Pharmacy Phriday.

Until then, stay safe!


The UH EMS-I Team

University Hospitals