Diphenhydramine (First-Generation Antihistamine)

Diphenhydramine is a first-generation antihistamine. It is mainly used to treat symptoms of allergies which commonly include itchiness, runny nose, watery eyes, and sneezing. Another effect of diphenhydramine includes sedation which is useful for treating motion sickness and insomnia.

Uses

What is diphenhydramine used for?

• Allergic conditions
• Motion sickness
• Insomnia
• Parkinson’s disease
• Pruritic skin disorders

How should I take diphenhydramine?

Read the directions on the packaging for complete information. Check the label and expiration date.

For oral dosage forms, swallow it whole without chewing, crushing, or dissolving it in liquid. For syrups, use a medical-grade measuring cup not a household spoon.

Apply topical preparations to the affected area. Wash your hands thoroughly afterwards.

How do I store diphenhydramine?

This product is best stored at room temperature away from direct light and moisture. To prevent drug damage, you should not store it in the bathroom or the freezer. 

There may be different brands of this drug that may have different storage needs. So, it is important to always check the product package for instructions on storage, or ask your pharmacist. For safety, you should keep all medicines away from children and pets.

You should not flush this product down the toilet or pour them into a drain unless instructed to do so. Additionally, it is important to properly discard this product when it is expired or no longer needed. Consult your pharmacist for more details about how to safely discard your product.

Precautions & warnings

What should I know before using diphenhydramine?

Before using this drug, tell your doctor if you are/have:

• Pregnant or breastfeeding.
• Taking any other medicines. This includes any prescription, OTC, and herbal remedies.
• An allergy to any of the ingredients of this product.
• Any other illnesses, disorders, or medical conditions.

Is it safe during pregnancy or breastfeeding?

Unfortunately, there isn’t enough information about the safety of using this drug during pregnancy and breastfeeding. However, it is likely safe to take. Please always consult with your doctor to weigh the potential benefits and risks before taking any medication.

This medication is pregnancy risk category B according to the US Food and Drug Administration (FDA).

FDA pregnancy risk category reference below:

• A= No risk
• B= No risk in some studies
• C= There may be some risk
• D= Positive evidence of risk
• X= Contraindicated
• N= Unknown

Side effects

What side effects can occur from diphenhydramine?

Like all drugs, this product may have side effects. If they occur, side effects are generally mild and resolve once treatment is finished or the dose is lowered. Some reported side effects include:

• Chest tightness
• Palpitations
• Tachycardia (fast heart rate)
• Sweating
• Skin rash
• Decreased appetite
• GI disturbances
• Dry mouth
• Changes in urination
• Dizziness
• Drowsiness
• Excitation
• Tremor
• Blurred vision
• Tinnitus
• Insomnia
• Memory impairment
• Cognitive dysfunction

However, not everyone experiences these side effects. In addition, some people may experience other side effects. So, if you have any concerns about a side effect, please consult your doctor or pharmacist.

Interactions

What drugs may interact with diphenhydramine?

This medication may interact with other drugs that you are currently taking, which can change how your drug works or increase your risk for serious side effects. 

To avoid any potential drug interactions, you should keep a list of all the drugs you are using (including prescription drugs, nonprescription drugs, and herbal products) and share it with your doctor and pharmacist. 

Drugs with known interactions:

• Other antihistamines
• CNS depressants
• MAOIs
• Anticholinergic agents

If you experience an adverse drug interaction, inform your doctor immediately to reevaluate your treatment plan. Approaches include dose adjustment, drug substitution, or ending therapy.

Does food or alcohol interact with diphenhydramine?

This drug may interact with food or alcohol by altering the way the drug works or increase the risk for serious side effects. Do not consume alcohol while taking this drug. Please discuss with your doctor or pharmacist any potential food or alcohol interactions before using this drug.

What health conditions may interact with diphenhydramine?

This drug may interact with underlying conditions. This interaction may worsen your health condition or alter the way the drug works. Therefore, it is important to always let your doctor and pharmacist know all the health conditions you currently have.

Dosage

The information provided is not a substitute for any medical advice. Therefore, you should always consult with your doctor or pharmacist before using any medication.

What is the dose for an adult?

Allergies & motion sickness

Oral

Take 25-50 mg 3-4 times per day. The maximum dose is 300 mg per day.

If used to prevent motion sickness, administer the drug 30 minutes before traveling or other activities.

IV/IM

Give 10-50 mg (up to 100 mg, if needed) via IV injection at a rate not exceeding 25 mg/min or via deep IM injection. The maximum daily dose is 400 mg.

If used to prevent motion sickness, administer the drug 30 minutes before traveling or other activities.

Topical

Apply a thin layer of cream to the affected areas 1-2 times per day. Do not use for more than 3 days.

Short-term treatment of insomnia

Oral

Take 50 mg, 30 minutes before bedtime.

Parkinson’s disease

IV/IM

Give 10-50 mg (up to 100 mg, if needed) via IV injection at a rate not exceeding 25 mg/min or via deep IM injection. The maximum daily dose is 400 mg.

*Note: only use the parenteral route when oral therapy is impossible or contraindicated.

What is the dose for a child?

Allergies & motion sickness

Oral

• Age 2-6 years: give 6.25 mg every 4-6 hours.
• 6-12 years: give 12.5-25 mg every 4-6 hours.
• >12 years: same as the adult dose.

IV/IM

Give 5 mg/kg via IV injection at a rate of 25 mg/min or deep IM injection in 4 divided doses. The maximum daily dose is 300 mg.

If used to prevent motion sickness, administer the drug 30 minutes before traveling or other activities.

Topical

For children 2 years and above: same as adult dose.

How is diphenhydramine available?

Diphenhydramine is available in the following brands, dosage forms, and strengths:

• Capsule: 25 mg, 50 mg
• Tablet: 50 mg
• Syrup: 12.5 mg/5 mL
• Solution for injection: 50 mg/mL

What should I do in case of an emergency or overdose?

In case of an emergency or an overdose, call your local emergency services or go to the nearest emergency room.

What should I do if I miss a dose?

If you miss a dose, take it as soon as possible. However, if it is almost time for your next dose, skip the missed dose and take your regular dose as scheduled. Do not take a double dose.