Medicine Name

Diphenhydramine (pronounced as Diphen – hydramine)

Drug Class

First-generation antihistamine
(Central and peripheral H1 receptor blocker – sedating)

H1 receptors are present in:

Brain → Sedation (minimal effect )
Skin → Itching, urticaria
Lungs → Allergic responses
Nose → Sneezing, rhinorrhea
Blood vessels → Vasodilation, edema

Peripheral H1 receptors are present in

Lungs → Allergy symptoms
Skin
Nose
Blood vessels

Central H1 receptors are present in

Brain
⚠️ Diphenhydramine significantly crosses the blood–brain barrier, causing sedation.

Brand Names

India:

Benadryl
Restamine
Caladryl (combo)

USA

Benadryl

UK / Europe:

Nytol, Benadryl

Approved By

• FDA (United States)
• CDSCO (India)
• EMA (Europe)

Available Strengths & Dosage Forms

• Tablet: 25 mg, 50 mg
• Capsule: 25 mg, 50 mg
• Syrup: 12.5 mg / 5 ml
• Injection: 10 mg / ml

Purpose / Indications

Diphenhydramine is used to treat allergic conditions, motion sickness, and insomnia. It blocks both central and peripheral H1 receptors and produces sedative effects.

Symptoms It May Cure

• Sneezing
• Runny nose
• Itching (skin, nose, throat)
• Watery eyes
• Urticaria (hives)
• Allergic skin rash
• Motion sickness symptoms
• Short-term insomnia

Dosage

Adults

Allergy: 25–50 mg every 6–8 hours
Insomnia: 50 mg at bedtime
Motion sickness: 25–50 mg

Children

As prescribed based on age and weight

⚠️ Use caution in elderly patients due to sedation risk.

Mechanism of Action of Diphenhydramine

Diphenhydramine blocks both peripheral and central H1 histamine receptors.

• Prevents histamine from binding to H1 receptors
• Reduces vasodilation
• Decreases capillary permeability
• Controls itching and mucus secretion

Unlike second-generation antihistamines:

• Causes significant sedation
• Crosses the blood–brain barrier
• Has strong anticholinergic effects

Detailed Explanation

During an allergic reaction:

• Mast cells release histamine
• Histamine binds to H1 receptors
• Causes itching, redness, swelling, sneezing

Diphenhydramine blocks these receptors and prevents allergic symptoms.

Because it enters the brain, it also causes:
Sedation
Sleep induction
Anti-motion sickness effect

Bioavailability

Definition:

Bioavailability refers to the percentage of a drug that reaches systemic circulation in an active form after administration.

Bioavailability:

Approximately 50–60%

Well absorbed after oral administration.

Half-Life

Definition:

Half-life is the time required for the plasma concentration of a drug to reduce to half of its original value.

Half-Life:

Approximately 4–8 hours

This is why multiple doses per day may be required.

Summary Statement

Diphenhydramine is a first-generation H1 antihistamine that effectively treats allergic conditions but causes sedation due to penetration into the brain.

Additional Pharmacological Points

• Crosses blood–brain barrier significantly
• Causes sedation
• Has strong anticholinergic effects
• Also acts as antiemetic
• Used as sleep aid

Side Effects

Common Side Effects

• Drowsiness
• Dry mouth
• Dizziness
• Blurred vision
• Constipation

Less Common / Rare

• Urinary retention
• Confusion (especially in elderly)
• Tachycardia

Contraindications

• Hypersensitivity to diphenhydramine
• Newborn or premature infants
• Narrow-angle glaucoma
• Severe asthma attack
• Prostatic hypertrophy

Dosage Frequency

Every 6–8 hours (for allergy)
Once daily at bedtime (for insomnia)

Not ideal for daytime use due to sedation.

Use in Special Populations

Pregnancy

• Previously Category B (old classification)
• Generally considered safe
• Use only if clinically needed

Lactation

• Passes into breast milk
• May cause sedation in infant
• Use with caution

Elderly

• Higher risk of sedation
• Risk of confusion and falls
• Use cautiously

Drug Interactions

• Alcohol (increases sedation)
• CNS depressants
• Anticholinergic drugs
• Sedatives

Storage Instructions

• Store at room temperature
• Protect from moisture
• Keep out of reach of children

Disclaimer

This information is for educational purposes only and should not replace professional medical advice. Always consult a doctor before using any medication.

Extras

It is preferred for:
✔️ Night-time allergy relief
✔️ Motion sickness
✔️ Short-term insomnia

Not ideal for:
Daytime use
Elderly patients
Long-term use

Key points / Revision

Point	Details
Drug Class	First-generation antihistamine
Receptor Blocked	Central and Peripheral H1
Blood–Brain Barrier	✔️ Crosses significantly
Sedation	High
Generation	First
Main Uses	Allergy, insomnia, motion sickness
Bioavailability	~50–60%
Half-life	4–8 hours
Pregnancy	Generally safe
Lactation	Use with caution
Best For	Night-time allergy
Not Ideal For	Daytime use