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Diphenhydramine; Uses, Side Effects, Interactions

Diphenhydramine is a histamine H1 antagonist used as an antiemetic, antitussive, for dermatoses and pruritus, for hypersensitivity reactions, as a hypnotic, an antiparkinson, and as an ingredient in common cold preparations. It has some undesired antimuscarinic and sedative effects.

Diphenhydramine is a first-generation antihistamine and ethanolamine with sedative and anti-allergic properties. Diphenhydramine competitively inhibits the histamine-1 (H1) receptor, thereby alleviating the symptoms caused by endogenous histamine on bronchial, capillary and gastrointestinal smooth muscles. This prevents histamine-induced bronchoconstriction, vasodilation, increased capillary permeability, and GI smooth muscle spasms.

Mechanism of Action of Diphenhydramine

Diphenhydramine competes with free histamine for binding at HA-receptor sites. This antagonizes the effects of histamine on HA-receptors, leading to a reduction of the negative symptoms brought on by histamine HA-receptor binding. Ethanolamine antihistamines have significant antimuscarinic activity and produce marked sedation in most patients. In addition to the usual allergic symptoms, the drug also treats irritant cough and nausea, vomiting, and vertigo associated with motion sickness. It also is used commonly to treat drug-induced extrapyramidal symptoms as well as to treat mild cases of Parkinson’s disease. Rather than preventing the release of histamine, as do cromolyn and nedocromil, diphenhydramine competes with free histamine for binding at HA-receptor sites. Diphenhydramine competitively antagonizes the effects of histamine on HA-receptors in the GI tract, uterus, large blood vessels, and bronchial muscle. Ethanolamine derivatives have greater anticholinergic activity than do other antihistamines, which probably accounts for the antidyskinetic action of diphenhydramine. This anticholinergic action appears to be due to a central antimuscarinic effect, which also may be responsible for its antiemetic effects,

Antihistamines used in the treatment of allergy act by competing with histamine for H1-receptor sites on effector cells. They thereby prevent, but do not reverse, responses mediated by histamine alone. Antihistamines antagonize, in varying degrees, most of the pharmacological effects of histamine, including urticaria and pruritus. Also, the anticholinergic actions of most antihistamines provide a drying effect on the nasal mucosa.

Indications of Diphenhydramine

Allergic Rhinitis
Insomnia
Allergic Reactions
Cold Symptoms
A cough
Extrapyramidal Reaction
Motion Sickness
Nausea/Vomiting
Pruritus
Urticaria
Anaphylaxis
Angioedema
Motion sickness
Oral mucositis
Parkinsonian syndromes
Pollen allergy
Skin irritation
Skin rash
Sleeplessness
Upper respiratory tract hypersensitivity reaction, site unspecified
Dermatographism
Anesthetics, Local; Anti-Allergic Agents; Antiemetics; Histamine H1 Antagonists; Hypnotics and Sedatives
Diphenhydramine is used as an antitussive for temporary relief of a cough caused by minor throat and bronchial irritation such as may occur with common colds or inhaled irritants.
Diphenhydramine is effective for the prevention and treatment of nausea, vomiting, and/or vertigo associated with motion sickness.

Contra-Indications of Diphenhydramine

Overactive thyroid gland
Increased Pressure in the Eye
Closed-angle glaucoma
High blood pressure
Stenosing Peptic Ulcer
Blockage of Urinary Bladder
Enlarged Prostate
Cannot Empty Bladder
Chronic idiopathic constipation

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Dosage of Diphenhydramine

Strengths: 12.5 mg; 25 mg; 50 mg; 10 mg/mL;12.5 mg/5 mL 6.25 mg/5 mL; 25 mg/5 mL; 50 mg/30 mL

Extrapyramidal Reaction

Oral: 25 to 50 mg orally 3 to 4 times a day
Parenteral: 10 to 50 mg deep IM or IV as needed, and may increase to 100 mg if required
Maximum dose: 400 mg/day

Insomnia

Diphenhydramine Citrate: 76 mg orally once a day at bedtime
Diphenhydramine Hydrochloride: 50 mg orally once a day at bedtime

Motion Sickness

Oral

Prescription formulations: 25 to 50 mg orally 3 to 4 times a day, with the first dose given 30 minutes before exposure to motion and repeated before meals and upon retiring throughout the duration of the journey.
Parenteral: 10 to 50 mg deep IM or IV as needed, and may increase to 100 mg if required
Maximum dose: 400 mg/day

Allergic Rhinitis

Oral

Diphenhydramine Citrate: 38 to 76 mg orally every 4 to 6 hours as needed
Maximum dose: 456 mg/day
Diphenhydramine Hydrochloride: 25 to 50 mg orally every 4 to 6 hours as needed
Maximum dose: 300 mg/day
Parenteral: 10 to 50 mg deep IM or IV as needed, and may increase to 100 mg if required
Maximum dose: 400 mg/day

Cold Symptoms

Oral

Diphenhydramine Citrate: 38 to 76 mg orally every 4 to 6 hours as needed
Maximum dose: 456 mg/day
Diphenhydramine Hydrochloride: 25 to 50 mg orally every 4 to 6 hours as needed
Maximum dose: 300 mg/day
Parenteral: 10 to 50 mg deep IM or IV as needed, and may increase to 100 mg if required
Maximum dose: 400 mg/day

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Pruritus

Oral

Diphenhydramine Citrate:

38 to 76 mg orally every 4 to 6 hours as needed
Maximum dose: 456 mg/day

Diphenhydramine Hydrochloride

25 to 50 mg orally every 4 to 6 hours as needed
Maximum dose: 300 mg/day
Parenteral: 10 to 50 mg deep IM or IV as needed, and may increase to 100 mg if required
Maximum dose: 400 mg/day

Side Effects of Diphenhydramine

The most common

Dysphoria
Hallucinations (auditory, visual, etc.)
Heart palpitations
heart palpitations
increased appetite
weight loss
dizziness
drowsiness
dry mouth
headache
joint pain
nausea and vomiting
Severe stomach ache
diarrhoea,
anorexia,
flatulence,
headache,
fainting, fast or pounding heartbeats.

More common

weight loss
Syndrome of inappropriate secretion of antidiuretic hormone
Fast or irregular heartbeat
fever
Back pain
dizziness
headache
increased cough
Acid or sour stomach
decreased appetite
Agitation
chest congestion
chest pain
cold sweats
confusion
decreased sexual ability or desire
diarrhea or loose stools
heartburn
sleepiness or unusual drowsiness
stomach or abdominal cramps, gas, or pain
trouble sleeping

Less common

Abnormal dreams
change in sense of taste
congestion
discouragement, feeling sad, or empty
Acting on dangerous impulses
Aggressive or violent behavior
Thoughts about suicide or dying
New or worse depression
New or worse anxiety or panic attacks
Agitation, restlessness, anger, or irritability
Trouble sleeping
An increase in activity or talking more than normal

Drug Interactions of Diphenhydramine

Diphenhydramine may interact with following drugs, supplements & may decrease the efficacy of drugs

alcohol
anticholinergics (e.g., benztropine, oxybutynin)
antihistamines (e.g., brompheniramine, chlorpheniramine)
barbiturates (e.g., phenobarbital, butalbital)
benzodiazepines (e.g., diazepam, lorazepam, oxazepam)
muscle relaxants (e.g., cyclobenzaprine)
opioid medications (e.g., codeine, morphine)
other medications that cause drowsiness (e.g., chlorpromazine, gabapentin, loxapine, mirtazapine, quetiapine, zopiclone, droperidol)
warfarin

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Pregnancy & Lactation of Diphenhydramine

FDA Pregnancy Category B

Pregnancy

This medication should not be used during pregnancy unless the benefits outweigh the risks. If you become pregnant while taking this medication, contact your doctor immediately.

Lactation

The safety of using this medication while breastfeeding has not been established. If you are a breastfeeding mother and are taking this medication, it may affect your baby. Talk to your doctor about whether you should continue breastfeeding. The safety and effectiveness of using this medication have not been established for children less than 2 years old.