Diphenhydramine; Uses, Side Effects, Interactions

Last updated: February 7, 2026Reviewed date: February 7, 2026Reading time: 7 min read
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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...

For severe symptoms, danger signs, pregnancy, child illness, or sudden worsening, seek urgent medical care.

বাংলা রোগী নোট এখনো যোগ করা হয়নি। পোস্ট এডিটরে “RX Bangla Patient Mode” বক্স থেকে সহজ বাংলা সারাংশ যোগ করুন।

এই তথ্য শিক্ষা ও সচেতনতার জন্য। এটি ডাক্তারি পরীক্ষা, রোগ নির্ণয় বা প্রেসক্রিপশনের বিকল্প নয়।

Article Summary

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....

Key Takeaways

  • This article explains Mechanism of Action of Diphenhydramine in simple medical language.
  • This article explains Indications of Diphenhydramine in simple medical language.
  • This article explains Contra-Indications of Diphenhydramine in simple medical language.
  • This article explains Dosage of Diphenhydramine in simple medical language.
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Emergency safety firstUrgent warning signs are highlighted below.

Seek urgent medical care if you notice

These warning signs are general safety guidance. Local emergency numbers and clinical judgment should always come first.

  • Severe symptoms, breathing difficulty, fainting, confusion, or rapidly worsening illness.
  • New weakness, severe pain, high fever, or symptoms after a serious injury.
  • Any symptom that feels urgent, unusual, or unsafe for the patient.
1

Emergency now

Use emergency care for severe, sudden, rapidly worsening, or life-threatening symptoms.

2

See a doctor

Book a professional medical evaluation if symptoms persist, worsen, recur often, affect daily activities, or occur in a high-risk patient.

3

Learn safely

Use this article to understand possible causes, tests, treatment options, prevention, and questions to ask your clinician.

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,

or

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

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

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

More common

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

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.

References

 

Diphenhydramine; Uses, Side Effects, Interactions

Doctor visit helper

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A simple rural-patient checklist to help you explain symptoms clearly, ask better questions, and avoid unsafe self-treatment.

Safety note: This is not a prescription or diagnosis. For severe symptoms, pregnancy danger signs, children with serious illness, chest pain, breathing difficulty, stroke-like weakness, or major injury, seek urgent care.

Which doctor may help?

Start with a registered doctor or the nearest qualified health center.

What to tell the doctor

  • Write when the problem started and how it changed.
  • Bring old prescriptions, investigation reports, and current medicines.
  • Write allergies, pregnancy status, diabetes, kidney/liver disease, and major past illnesses.
  • Bring one family member if the patient is weak, elderly, confused, or a child.

Questions to ask

  • What is the most likely cause of my symptoms?
  • Which danger signs mean I should go to hospital quickly?
  • Which tests are necessary now, and which can wait?
  • How should I take medicines safely and what side effects should I watch for?
  • When should I come for follow-up?

Tests to discuss

  • Vital signs: temperature, pulse, blood pressure, oxygen saturation
  • Basic physical examination by a clinician
  • CBC, urine test, blood sugar, or imaging only when clinically needed

Avoid these mistakes

  • Do not use antibiotics, steroid tablets/injections, or strong painkillers without proper medical advice.
  • Do not hide pregnancy, kidney disease, ulcer, allergy, or blood thinner use.
  • Do not delay emergency care when danger signs are present.

Medicine safety and first-aid guide

This section is for patient education only. It does not replace a doctor, pharmacist, or emergency care.

Safe first steps

  • Avoid heavy lifting, sudden bending, and prolonged bed rest.
  • Use comfortable posture and gentle movement as tolerated.
  • Discuss physiotherapy, X-ray, or MRI only when clinically needed.

OTC medicine safety

  • For mild back pain, pain-relief medicine may be discussed with a doctor or pharmacist.
  • Avoid repeated painkiller use if you have kidney disease, stomach ulcer, uncontrolled blood pressure, or are taking blood thinners.

Avoid these mistakes

  • Do not start antibiotics without a proper medical decision.
  • Do not use steroid tablets or injections casually for quick relief.
  • Do not delay emergency care because of home remedies.

Get urgent help if

  • Back pain with leg weakness, numbness around private area, loss of urine/stool control, fever, cancer history, or major injury needs urgent care.
Medicine names, dose, and timing must be decided by a qualified clinician or pharmacist after checking age, pregnancy, allergy, other diseases, and current medicines.

For rural patients and family caregivers

Patient health record and symptom diary

Write your symptoms, medicines already taken, test results, and questions before visiting a doctor. This note stays on your device unless you print or copy it.

Doctor to discuss: Orthopedic / spine specialist, physical medicine doctor, or qualified clinician
Tests to discuss with doctor
  • Neurological examination for leg power, sensation, reflexes, and straight leg raise
  • X-ray only if injury, deformity, long-lasting pain, or doctor suspects bone problem
  • MRI discussion if severe nerve symptoms, weakness, bladder/bowel problem, or persistent symptoms
Questions to ask
  • What is the most likely cause of my symptoms?
  • Which warning signs mean I should go to emergency care?
  • Which tests are really needed now?
  • Which medicines are safe for my age, pregnancy status, allergy, kidney/liver/stomach condition, and current medicines?
  • Is physiotherapy, posture correction, or activity modification needed?

Emergency warning signs such as chest pain, severe breathing difficulty, sudden weakness, confusion, severe dehydration, major injury, or loss of bladder/bowel control need urgent medical care. Do not wait for online information.

Safe pathway to proper treatment

Care roadmap for: Diphenhydramine; Uses, Side Effects, Interactions

Use this simple roadmap to understand the next safe steps. It is educational and does not replace examination by a doctor.

Go to emergency care if you notice:
  • Severe or rapidly worsening symptoms
  • Breathing difficulty, chest pain, fainting, confusion, severe weakness, major injury, or severe dehydration
Doctor / service to discuss: Qualified healthcare provider; specialist depends on symptoms and examination.
  1. Step 1

    Check danger signs first

    If danger signs are present, seek emergency care and do not wait for online information.

  2. Step 2

    Record the symptom story

    Write when symptoms started, severity, medicines already taken, allergies, pregnancy status, and test results.

  3. Step 3

    Visit a qualified clinician

    A doctor, nurse, or qualified healthcare provider can examine you and decide which tests or treatment are needed.

  4. Step 4

    Do only useful tests

    Do tests after clinical assessment. Avoid unnecessary tests, random antibiotics, or repeated medicines without diagnosis.

  5. Step 5

    Follow up and return early if worse

    If symptoms worsen, new warning signs appear, or treatment is not helping, return for review quickly.

Rural patient practical tips
  • Take a written symptom diary and all previous prescriptions/test reports.
  • Do not hide medicines already taken, even herbal or over-the-counter medicines.
  • Ask which warning signs mean urgent referral to hospital.

This roadmap is for education. A real diagnosis and treatment plan requires history, examination, and clinical judgment.

RX Patient Help

Ask a health question safely

Write your symptom story. A health professional or site editor can review it before any answer is prepared. This box is not for emergency care.

Emergency first: Severe chest pain, breathing trouble, unconsciousness, stroke signs, severe injury, heavy bleeding, or rapidly worsening symptoms need urgent local medical care now.

Frequently Asked Questions

Is this article a replacement for a doctor?

No. It is educational content only. Patients should consult a qualified clinician for diagnosis and treatment.

When should I seek urgent care?

Seek urgent care for severe symptoms, rapidly worsening condition, breathing difficulty, severe pain, neurological changes, or any emergency warning sign.

References

Add references, clinical guidelines, textbooks, journal articles, or trusted medical sources here. You can edit this area from the RX Article Professional Blocks panel.

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