Diphenhydramine

A classic first-generation antihistamine widely used for allergy, cold, cough, and sleep aid.

What is Diphenhydramine?

Diphenhydramine is a first-generation antihistamine used to relieve symptoms of allergies, common cold, and cough. It is also well known for its sedative (sleep-inducing) properties.

Because it crosses the blood–brain barrier easily, it causes drowsiness and is often used in night-time cold and cough formulations.

History & Background

Diphenhydramine was one of the earliest antihistamines developed and has been in use since the 1940s. Over time, it became popular not only for allergies but also as a cough suppressant and sleep aid.

Chemical Structure & Properties

• IUPAC name: 2-(diphenylmethoxy)-N,N-dimethylethanamine
• Molecular formula: C₁₇H₂₁NO
• Molar mass: 255.36 g/mol
• Functional groups: Ether, tertiary amine, aromatic rings

Its lipophilic structure allows easy penetration into the central nervous system, which explains both its effectiveness and sedative effects.

Mechanism of Action

Diphenhydramine blocks histamine H₁ receptors centrally and peripherally. It also has anticholinergic properties.

• Reduces sneezing and runny nose
• Suppresses cough reflex
• Produces sedation and sleepiness

Pharmacokinetics

• Absorption: Rapid oral absorption
• Peak plasma time: 2–3 hours
• Protein binding: ~78%
• Metabolism: Liver
• Half-life: 4–8 hours
• Excretion: Urine

Medical Uses

• Allergic rhinitis
• Common cold
• Dry cough
• Motion sickness
• Short-term insomnia

Dosage Guidelines

Adults: 25–50 mg every 6–8 hours

Children: Only under medical advice

⚠️ Causes drowsiness — do not drive or operate machinery.

Side Effects

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

Warnings & Precautions

• Not recommended for elderly patients
• Avoid alcohol
• Use caution in glaucoma and asthma

Drug Interactions

• Alcohol
• Sedatives
• Antidepressants

Diphenhydramine vs Chlorpheniramine

Diphenhydramine causes stronger sedation, while chlorpheniramine is slightly less sedating.

Frequently Asked Questions

Is diphenhydramine an antibiotic? – No.

Can it help with sleep? – Yes.

Is it safe daily? – Only short-term.

Why is it used at night? – Due to strong sedative effect.

⚠️ Educational content only. Not a substitute for professional medical advice.