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,
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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
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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.
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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
- 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
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.
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