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Last updated on Apr 08, 2025.
Diphenhydramine is an antihistamine that helps relieve allergy symptoms.
Provides further relief from severe allergic reactions when used with Epinephrine.
Available as a 25 mg or 50 mg tablet.
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Diphenhydramine is primarily used as an antihistamine that relieves allergy symptoms like a runny nose, itching or watery eyes.Â
But it can also be given in addition to an epinephrine (adrenaline) pen to help treat a severe allergic reaction (known as anaphylaxis).
Diphenhydramine can’t treat anaphylaxis alone. Anaphylaxis needs to be treated as soon as possible with epinephrine (EpiPen), as delaying treatment can be dangerous.Â
Diphenhydramine may be taken after using an adrenaline pen to help further relieve symptoms.Â
Diphenhydramine is an antihistamine. This means that it blocks histamine receptors in the body. Histamine is one of the primary chemicals that causes allergy symptoms. By blocking its release, Diphenhydramine helps alleviate symptoms.[1]
When someone has a severe allergic (or anaphylactic) reaction, this histamine release can cause a dangerous drop in blood pressure, as well as significant swelling of the airways, leading to difficulty breathing. By counteracting histamine, diphenhydramine can help to stabilise these symptoms (when used in addition to an epinephrine injector).
Diphenhydramine comes as a tablet in two doses: 25 mg and 50 mg.Â
The usual dose for mild allergic reactions is one tablet taken every four to six hours. Your doctor will provide more information on the dosage based on the severity of your condition.
This page was written by The Treated Content Team.
How we source info.
When we present you with stats, data, opinion or a consensus, we’ll tell you where this came from. And we’ll only present data as clinically reliable if it’s come from a reputable source, such as a state or government-funded health body, a peer-reviewed medical journal, or a recognised analytics or data body. Read more in our editorial policy.
You’ll usually take Diphenhydramine alongside an epinephrine injection for anaphylaxis. Swallow the tablet whole with some water, with or without food. You’ll take a tablet every four to six hours as needed to manage your symptoms. If you’re taking multiple tablets daily, try to space the tablets out evenly throughout the day.[1]
Diphenhydramine isn’t used to treat anaphylaxis by itself – it only relieves mild allergy symptoms. To prevent anaphylaxis, make sure to use an epinephrine – then you can use Diphenhydramine to provide further relief.
Diphenhydramine takes 20 to 30 minutes to start working and can last up to 4-6 hours.
Diphenhydramine can’t be used to treat anaphylaxis alone as it works too slowly. If you’re experiencing symptoms of anaphylaxis, take epinephrine immediately.
If you forget to take Diphenhydramine, it shouldn’t cause any issues. Just take it as soon as you remember.
If you accidentally took more than the recommended dose, you might feel drowsy or sleepy. If you do, don’t drive or operate heavy machinery.
In some cases, you might experience overdose symptoms like a fast heartbeat or even lose consciousness; in these cases, contact 911 immediately for medical advice.
This page was written by The Treated Content Team.
How we source info.
When we present you with stats, data, opinion or a consensus, we’ll tell you where this came from. And we’ll only present data as clinically reliable if it’s come from a reputable source, such as a state or government-funded health body, a peer-reviewed medical journal, or a recognised analytics or data body. Read more in our editorial policy.
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Disclaimer: The information provided on this page is not a substitute for professional medical advice, diagnosis, or treatment. If you have any questions or concerns about your health, please talk to a doctor.
