Active Ingredient

Diphenhydramine

Diphenhydramine is an antihistamine commonly used to relieve symptoms of allergy, hay fever, and the common cold. It works by blocking the action of histamine, a substance in the body that causes allergic symptoms.

It is also used to relieve symptoms of motion sickness and to induce sleep in people with insomnia.

Written By Douglas C. Goldberg, MD

Reviewed By Philip Schein, MD

Last Updated 28 Nov 2025

Available Diphenhydramine Brands

Unisom

Strengths 25mg

Diphenhydramine Uses

Diphenhydramine is used to treat a variety of conditions related to allergies and sleep issues.

Allergic reactions such as sneezing, itching, and runny nose
Hay fever
Common cold symptoms
Motion sickness
Short-term treatment for insomnia

How Diphenhydramine Works

Diphenhydramine works by blocking the effects of histamine, a chemical released during an allergic reaction. This helps to reduce symptoms like sneezing, itching, and runny nose.

It also has sedative properties, which can help induce sleep in people experiencing insomnia.

By affecting certain natural substances in the brain, it can also help prevent nausea and dizziness associated with motion sickness.

Common side effects of Diphenhydramine

Not everyone experiences side effects from diphenhydramine, and most are mild and temporary.

Drowsiness
Dizziness
Dry mouth, nose, or throat
Constipation
Blurred vision
Upset stomach
Loss of appetite

If you experience any severe or persistent symptoms, contact your doctor.

Avoid driving or operating heavy machinery until you know how diphenhydramine affects you.
Limit alcohol consumption, as it can increase drowsiness and dizziness.
Inform your doctor about any other medications you are taking to avoid potential interactions.
Do not use diphenhydramine if you have certain health conditions like severe breathing problems, unless advised by a doctor.
Stay hydrated and maintain a balanced diet to help manage side effects like dry mouth and constipation.

Always consult your healthcare provider for advice tailored to your personal health situation.

Frequently Asked Questions

How quickly does diphenhydramine work?

Diphenhydramine typically starts to work within 30 minutes to an hour after taking it.

How long do the effects of diphenhydramine last?

The effects of diphenhydramine usually last for 4 to 6 hours.

Is it safe to use diphenhydramine regularly for sleep?

Diphenhydramine is not recommended for long-term use as a sleep aid. Consult your doctor for advice on managing insomnia.

Can I take diphenhydramine with other medications?

You should inform your doctor about all medications you are taking, as diphenhydramine can interact with other drugs.

What should I do if I miss a dose of diphenhydramine?

If you miss a dose, take it as soon as you remember. If it’s almost time for your next dose, skip the missed dose. Do not double up doses.