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Diphenhydramine is an antihistamine used to relieve symptoms of allergy, hay fever, and the common cold, as well as for its sedative properties to induce sleep.

Last Modified: 20 May 2024

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What is Diphenhydramine? 

Diphenhydramine is an antihistamine used to relieve symptoms of allergy, hay fever, and the common cold, as well as for its sedative properties to induce sleep. 

Global Name

Generic name: Diphenhydramine
Brand name: Benadryl, Sominex, Nytol


What is Diphenhydramine used for? 

Diphenhydramine is used to alleviate allergic reactions, treat motion sickness, nausea, suppress cough (antitussive) and as a sleep aid. 

How do you use Diphenhydramine? 

This medication is available in tablet, liquid, and topical forms. 

Dosage information: 

  • Adults: 25-50 mg every 4-6 hours for allergies; up to 50 mg at bedtime for sleep aid. 
  • Children: Dosage varies by age and weight; consult with a pediatrician. 


  • Children: Yes, with pediatrician's approval. 
  • Adults: Yes, as recommended. 
  • Elderlies: Lower doses may be required. 
  • Pregnancy: Not recommended unless prescribed. 
  • Lactation: Not recommended without medical advice. 


  • Allergies: To diphenhydramine or excipients in the formulation. 
  • Medical Conditions: Asthma, glaucoma, heart disease, high blood pressure, thyroid disease, or enlarged prostate. 

Diphenhydramine can cause marked drowsiness. Avoid alcohol, and use caution when driving or performing tasks requiring alertness. 

Concurrent use with other sedatives or alcohol can increase drowsiness. Monoamine oxidase (MAO) inhibitors, an anti-depressant medication, can prolong and intensify the anticholinergic effects of antihistamines. 

Side Effects

Side effects may include drowsiness, dry mouth, dizziness, blurred vision, increased heartbeat and difficulty urinating.

Pharmacist Tips

  • Storage: Keep in a cool, dry place away from children. 
  • Alcohol: Avoid alcohol as it increases sedative effects. 
  • Topical Use: Do not apply creams to large areas of the skin or open wounds. 
  • Elderly Caution: Be particularly cautious in elderly patients due to increased risk of confusion and falls.


Where is Diphenhydramine available in Hong Kong?

 Diphenhydramine is an over-the-counter (OTC) medication that can be sold without a prescription. It is found in all chain drug stores, including Mannings and Watsons.


Is Diphenhydramine available over-the-counter?

Yes, it is available OTC for both allergy relief and sleep aid.

Can Diphenhydramine be taken daily for allergies?

It can be taken daily, but long-term use should be under medical advice.

How quickly does Diphenhydramine take effect?

It typically begins to work within 30 minutes of ingestion.

How long does Diphenhydramine last?  

Diphenhydramine typically lasts between 4 to 6 hours, with the peak effects occurring within the first 1 to 3 hours after ingestion. 


This website provides content related to drug use and medicine for informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Do not use the information on this website to diagnose or treat a health problem or disease without consulting a qualified healthcare provider. The website's owners and contributors are not liable for any repercussions, adverse effects, or consequences resulting from the use of any medication or information provided herein.

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