Diphenhydramine Hydrochloride

What is Diphenhydramine HCL

It was first synthesized by the George Rieveschl and developed in early 1940’s. It is the prototype drug of ethanolamine class of first generation antihistamines which also includes orphenadrine, doxylamine and other halogenated diphendramine derivatives.

It is first generation antihistamine possessing anticholinergic, antitussive, antiemetic and sedative properties that mainly used to treat allergies. This is also used in the management of drug induced Parkinsonism and other extra pyramidal symptoms which is its secondary action. Diphenhydramine is also available in other conjugate forms like diphenhydramine citrate which has strong hypnotic effect and used as sleep aid.

Why Antihistaminic (Diphenhydramine) is prescribed

This kind of drugs are very useful for quick reduction of the naturally occurring chemical histamine in the body. Histamine produced in the human body above the tolerable limit causes symptoms like sneezing, runny nose, watery eyes, and itching.

Diphenhydramine is used to treat this kind of allergic symptoms by blocking the production of histamine and controls the symptoms like watery nose, runny nose, hives, skin rash, itching, and other cold or allergy symptoms.

Diphenhydramine is also used to treat motion sickness, to induce sleep, and to treat certain symptoms of Parkinson’s disease.


Majorly the people probably the adults are prone to so many types of allergic trigger’s such that they suffer from symptoms like sneezing, itching eyes, running nose, etc. where the underlying cause is the production of excess of histamines in the body which provokes this kind of symptoms .

Diphenhydramine is an H1 receptor agonist. It blocks the effect of histamine and in turn blocks its release. It has vasoconstriction effects, anticholinergic, antiemetic and also sedative side effects.

Due to the sedative side effect, most cough syrups have sleep inducing side effect. It is also antimuscarinic. It is effective for Parkinsons disease as it blocks the muscarinic acetylcholine receptors of brain. Diphenhydramine also blocks sodium channel and may also be used as local anaesthetic.


Diphenhydramine is absorbed from the GI tract and is also capable or passing through the BBB (blood brain barrier). It is 61% available orally and 78% is bound to the plasms.

It is metabolised into three components: nordiphenhydramine, dinorphenhydramine and diphenylmethoxyacetic acid. Nordiphenhydramine is the active metabolite amongst the three metabolites. Urinary excretion of unchanged drug is 1.9%. Plasma concentrations are reached in 2-3 hours.

Chemical Structure:

diphenhydramine hydrochloride structure

Chemical Formula: C17H21NO.HCl

IUPAC: Ethanamine,2(-diphenylmethoxy)-N,N-dimethyl-,hydrochloride
Chemical Properties:
Molecular weight: 291.81568 g/mol
Molecular Formula: C17H22ClNO
Rotatable Bond Count: 6
Exact Mass: 291.138992 gm/mol
Heavy atom count: 20
Formal Charge: 0
Isotope atom count: 0
Covalently bonded unit count: 2

Physical Properties: Diphenhydramine is a white crystalline powder. It is odourless and bitter in taste. The pH is 4-6. Melting point of the powder is around 331 – 338o F.

How does Diphenhydramine Hydrochloride work

Mechanism of action: It primarily by blocking the histaminic receptors located both peripherally and centrally and controls the excitability .There are different types of histaminic receptors are found in the human body ,it precisely act on the H1 type of histaminic receptors .

By reversing the effect of histamine on the capillaries. It can reduce the intensity of allergic reactions and symptoms significantly. It can cross the BBB (Blood Brain Barrier) easily and antagonizes the H1 receptors centrally. Its effect on central H1 receptors causes drowsiness.

As known every drug has its secondary binding site where it shows additive effects, as such diphenhydramine is also a potent antimuscarinic (a competitive antagonist of muscarinic acetylcholine receptors) and at high doses can cause anticholinergic syndrome.

The utility of diphenhydramine as an antiparkinson agent is the result of its blocking properties on the muscarinic acetylcholine receptors in the brain.

It also acts intracellularly as an sodium channel blocker, which is responsible for its actions as a local anaesthetic. Diphenhydramine has also been shown to inhibit the reuptake of serotonin. It has been shown to be a potentiator of analgesic induced by morphine, but not by endogenous opioids, in rats.


Uses: It is a first generation antihistamine and used in number of conditions like allergic symptoms which may include itchiness, insomnia, motion sickness and extrapyramidal symptoms. It has local anaesthetic properties.

  • Allergies: It is more effective than the second generation antihistaminics. If injected in combination with epinephrine can be used to treat anaphylaxis.
  • Movement disorders: Drug is mainly used to treat Parkinson’s disease.
  • Sleep: Diphenhydramine has sleeping inducing properties and is widely used as a non-prescription sleep aid for insomnia. The drug might cause minor psychological dependence. It can also be used as an anxiolytic.
  • Vomiting: The drug has antiemetic property and can be used for treating nausea and motion sickness.

Adverse Effects:

  • Dysphoria
  • Pupil dilation
  • Hallucinations
  • Heart palpitations
  • Extreme drowsiness
  • Severe dizziness
  • Abnormal speech
  • Flushed skin
  • Severe mouth and throat dryness
  • Tremors
  • Seizures
  • Urinary retention
  • Constipation
  • Photophobia
  • Temporary erectile dysfunction
  • Vomiting
  • Motor disturbances
  • Anxiety
  • Disorientation
  • Abdominal pain
  • Delirium
  • Coma
  • Death

Acute poisoning can be fatal, cardiovascular collapse and death can occur in 2-18 hours.
Indications: Use the drug as directed in the label or as prescribed by the doctor. Store at room temperature away from moisture and heat.

Diphenhydramine hydrochloride dosage

Dosage and Administration: It is administered by orally or by IV or deep IM injection. The salt of diphenhydramine citrate-containing preparations orally.

IV Administration: For solution and drug compatibility information, see compatibility under stability. IV injection preferred over deep IM injection. It can administered in a home-care setting should be employed under careful supervision.

Rate of Administration: <25mg/minute

Dosage: Available as diphenhydramine hydrochloride and diphenhydramine citrate; dosage is expressed in terms of diphenhydramine hydrochloride or diphenhydramine citrate.

Citrate salt is available only in fixed-combination preparations.

12.5mg diphenhydramine hydrochloride equivalent to 19mg diphenhydramine citrate.

: Available in the following strength: 50 mg/ml, 12.5 mg, 25 mg, 12.5 mg/ml, 50 mg, 10 mg/ml/ 19 mg, 6.25 mg/5ml, 25 mg/5ml, 50mg/30ml

Usual adult dose used for: Extrapyramidal reaction (10-50 mg i.v., 25-50mg oral), Insomnia (25-50 mg orally), Motion Sickness (10-50 mg i.v., 25-50mg oral), Cough 25 mg every 4 hours), Cold symptoms, Pruritus, Urticaria.

Usual paediatric dose used for: Allergic Rhinitis, Cold, Motion sickness, Insomnia, Cough, Extrapyramidal Reaction, Allergic reaction.

Contraindication: Contraindicated in infant neonates and lactating mothers. Also contradicted with the intake of alcohol.