Drug Lesson #4: Diphenhydramine
Mostly known for its brand name Benadryl, the FDA first approved this drug6. Even after 75 years, diphenhydramine is still widely used by healthcare professionals and is available over the counter for patients to use.
Mostly known for its brand name Benadryl, the FDA first approved this drug6. Even after 75 years, diphenhydramine is still widely used by healthcare professionals and is available over the counter for patients to use. In the next installment of my drug lessons, I’ll be discussing the basics of what diphenhydramine is and how it is used in clinical practice.
What is diphenhydramine?
Diphenhydramine belongs to a group of medications called H1 receptor antagonists.¹ The H stands for histamine, an inflammatory chemical in the body that’s released when there is physical damage, infection, or an allergic reaction.
When histamine is released and binds to its receptor, the muscles of the lungs and stomach contract. Additionally, the blood vessels dilate and become more permeable, meaning that white blood cells can easily pass back and forth from the blood vessel to the rest of the body.² histamine also increases gastric acid secretion in the stomach and accelerates the heart rate.
Diphenhydramine binds to the histamine receptor and prevents histamine from binding and exerting its action. All the effects I mentioned in the paragraph above won’t happen if diphenhydramine does this.
When do we use it?
The FDA approved diphenhydramine for use in allergic reactions, common cold symptoms, and motion sickness.¹ In most cases, diphenhydramine is given to those with common allergic symptoms like runny nose, watery eyes, itching, and mild swelling.
With a severe allergic reaction like anaphylaxis, EMTs can give diphenhydramine on the way to the hospital before the patient receives epinephrine. Even though diphenhydramine works for allergic reactions, epinephrine is the best option for severe cases.
What about the side effects?
The most prominent side effect of diphenhydramine is sedation.¹ When patients take diphenhydramine, they report sleepiness and drowsiness after an hour or so. This is because the drug gets into the brain and works on the histamine receptors there. Sedation with this drug can be helpful for those who have a hard time falling asleep at night.
Other side effects seen with diphenhydramine include the following¹:
- dry mouth
- blurred vision
- constipation
- urinary retention
- nausea, diarrhea, or vomiting
When should a patient NOT be taking diphenhydramine?
There are a few patient populations who should not under any circumstance take diphenhydramine and those are¹:
- allergic to diphenhydramine
- neonates
- breastfeeding people
Diphenhydramine passes into breast milk and potentially to the baby, so taking diphenhydramine while breastfeeding is not recommended. The safety data isn’t there, so it’s better to avoid it.
Healthcare professionals must be cautious when giving diphenhydramine to elderly patients because of the sedation and drying out of the mucous membranes. Diphenhydramine is not recommended as a sleep aid or antihistamine for an extended time in the elderly.
Can pregnant people take diphenhydramine?
Fetal toxicity has been reported when pregnant patients take diphenhydramine, so it is not recommended.¹ Pregnant people can take antihistamines, but only the non-drowsy ones. Examples of non-drowsy antihistamines include cetirizine (Zyrtec) and loratadine (Claritin).
Extra Tips!
- I recommend not engaging in any coordination activities while taking diphenhydramine for the first time.
- Stop taking diphenhydramine if you develop any anaphylactic symptoms like swelling, extreme itching, or shortness of breath.
References
- Diphenhydramine monograph, Lexicomp Medical
- “Histamine.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., https://www.britannica.com/science/histamine.