Diphenhydramine

Introduction Overview

Generic Name

diphenhydramine

Brand Name

Allergy Relief
Allermax
Banophen
Benadryl
Compoz Nighttime Sleep Aid
Diphedryl
Diphenhist
Dytuss
Nytol QuickCaps
PediaCare Children's Allergy
Q-Dryl
QlearQuil Nightitme Allergy Relief
Quenalin
Scot-Tussin Allergy Relief Formula
Siladryl Allergy
Silphen Cough
Simply Sleep
Sleepinal
Sominex
Tranquil
Twilite
Unisom Sleepgels Maximum Strength
Valu-Dryl
Vanamine PD
Z-Sleep
ZzzQuil

Dosage Form

oral tablets, capsules, liquids; injectable formulation

Drug Class

Anticholinergic antiemetics
Anticholinergic antiparkinson agents
Antihistamines
Miscellaneous anxiolytics, sedatives and hypnotics

Pharmacokinetics

Absorption: Diphenhydramine is rapidly and almost completely absorbed from the gastrointestinal tract following oral administration. Peak plasma concentrations typically occur within 1 to 2 hours post-dose, with an onset of action of approximately 15 to 30 minutes. Both the hydrochloride and citrate salts are used, with slight variations in absorption rates but no clinically significant differences. Factors such as food intake may delay absorption slightly, but bioavailability is generally high. Distribution: Diphenhydramine exhibits a large volume of distribution, reflecting extensive tissue binding, including penetration across the blood-brain barrier, contributing to its central nervous system effects such as sedation and anticholinergic actions. It is about 90% bound to plasma proteins. The drug distributes widely throughout body tissues, including the central nervous system, explaining its sedative, antihistaminic, and anticholinergic effects. Special populations such as the elderly may display altered distribution volumes, influencing dosing considerations. Metabolism: Diphenhydramine undergoes extensive hepatic metabolism primarily via cytochrome P450 enzymes, especially CYP2D6 and CYP1A2 isoforms, resulting in multiple inactive metabolites. The drug undergoes N-demethylation and other phase I transformations, followed by phase II glucuronidation, facilitating elimination. Variability in metabolic rate due to genetic polymorphisms or drug interactions affecting CYP enzymes may alter therapeutic outcomes or toxicity risk. Elimination: Elimination of diphenhydramine and its metabolites occurs primarily via the kidneys, with a plasma elimination half-life of approximately 4 to 8 hours in healthy adults. Renal excretion accounts for the majority of inactive metabolites, while unchanged diphenhydramine is minimal in urine. In patients with renal or hepatic impairment, elimination may be prolonged, necessitating careful dosing. Clearance can be affected by age and disease states, and caution is advised to prevent accumulation and enhanced adverse effects. Clinical studies indicate that diphenhydramine's pharmacokinetics support typical dosing intervals of 4 to 6 hours for symptomatic relief while minimizing toxicity risk.

Dosage Forms

Available Dosage Forms: Diphenhydramine is available in multiple pharmaceutical formulations to accommodate various therapeutic needs, including:
- Oral capsules (25 mg)
- Oral tablets (25 mg, 50 mg)
- Oral liquids and syrups (e.g., 12.5 mg/5 mL, 25 mg/5 mL)
- Chewable tablets
- Injectable solutions for intramuscular or intravenous use (25 mg/mL concentration)
- Topical formulations (not covered in detail here)

Dosages for Adults and Adolescents: The typical adult oral dose for allergy or cold symptoms ranges from 25 mg to 50 mg every 4 to 6 hours, not exceeding 300 mg per day. Diphenhydramine is also dosed at 50 mg 30 minutes before exposure to motion sickness stimuli and continued every 4 to 6 hours as needed. As a sleep aid, a common dose is 25-50 mg administered 30 minutes before bedtime. Injectable forms are usually reserved for severe allergic reactions or when oral administration is not feasible, with doses of 10 to 50 mg administered intramuscularly or intravenously under medical supervision.

Specific Patient Populations: Dosage adjustments should be considered in elderly patients who may be more sensitive to anticholinergic and sedative effects; lower starting doses and careful titration are recommended. Children over 2 years old may receive weight-based dosing, with pediatric formulations such as liquids preferred. Diphenhydramine is contraindicated in children under 2 years due to risk of serious adverse effects. In patients with renal or hepatic impairment, dose reductions or extended dosing intervals may be necessary. Pregnant and breastfeeding women should use diphenhydramine only if the potential benefits outweigh risks, consulting with healthcare providers. Elderly patients should be monitored closely for increased side effects including confusion, dizziness, and falls.

Mechanism Of Action

Diphenhydramine acts primarily as a potent, first-generation inverse agonist of peripheral and central histamine H1 receptors. By competitively binding to these receptors, it competitively inhibits the action of histamine, a key mediator in allergic reactions and inflammation, thereby reducing vasodilation, increased capillary permeability, and sensory nerve stimulation responsible for symptoms like itching, swelling, and rhinorrhea.

Beyond its antihistaminic activity, diphenhydramine possesses significant anticholinergic properties by blocking muscarinic acetylcholine receptors, contributing to reduced secretions, sedation, and antiemetic effects. This anticholinergic blockade explains its utility in treating Parkinsonian symptoms and motion sickness as it modulates central cholinergic pathways involved in balance and movement control.

At the cellular level, diphenhydramine crosses the blood-brain barrier effectively, exerting pronounced central nervous system depressant effects. This characteristic enables its use as a short-term sedative and sleep aid, although it also accounts for common adverse effects such as drowsiness, dizziness, and cognitive impairment. Additionally, blockade of H1 receptors in the medullary vomiting center contributes to its antiemetic efficacy in preventing nausea and vomiting.

Comparatively, as a first-generation antihistamine, diphenhydramine differs from later generations by its higher lipophilicity and nonselective receptor blockade, leading to sedative and anticholinergic side effects. This pharmacodynamic profile has been extensively validated in clinical studies demonstrating efficacy in allergic conditions, motion sickness, and as a mild hypnotic agent, though with limitations due to its CNS effects. Ongoing research explores analogs to reduce side effects while maintaining therapeutic actions.

Pharmacological Actions

Potent H1 histamine receptor inverse agonist, blocking histamine-mediated allergic responses
Anticholinergic activity via muscarinic receptor antagonism, reducing secretions and exhibiting antiemetic effects
Central nervous system depressant causing sedation and mild hypnotic effects
Anti-motion sickness properties through CNS inhibition of vestibular stimulation
Reduction of pruritus and allergic rhinitis symptoms by preventing histamine-induced nerve activation
Inhibition of vasodilation and capillary permeability associated with allergic inflammation
Modulation of extrapyramidal symptoms via anticholinergic effects in Parkinson’s disease
Suppression of cough reflex through central mechanisms
Antiemetic effects by acting on the medullary vomiting center
Decreases restlessness and irritability secondary to allergic reactions
Relieves hypersensitivity reactions such as urticaria and angioedema
Exerts mild local anesthetic activity by stabilizing neuronal membranes
Crosses the blood-brain barrier leading to CNS effects uncommon in second-generation antihistamines
Demonstrates rapid onset of action with sustained effects lasting several hours
Displays dose-dependent pharmacodynamic effects requiring cautious titration

Therapeutic Or Clinical Uses

Treatment of allergic conditions including hay fever, urticaria, and allergic rhinitis
Relief of symptoms caused by the common cold such as sneezing, runny nose, and itchy watery eyes
Management of uncomplicated acute allergic reactions including hives and skin rash
Prevention and treatment of motion sickness via CNS vestibular suppression
Short-term management of insomnia and transient sleep disturbances
Adjunctive therapy for Parkinson’s disease symptoms due to anticholinergic effects
Control of cough and cold symptoms with sedative properties to facilitate rest
Use as an antiemetic in postoperative nausea and vomiting or other causes of nausea
Relief of pruritus from insect bites, minor skin irritations, or dermatitis
Emergency use in anaphylaxis as part of combination treatment (commonly injectable form)
Symptomatic relief during allergic asthma exacerbations (adjunctive treatment)
Management of extrapyramidal side effects induced by antipsychotic medications
Reduction of sneezing and rhinorrhea in allergic conjunctivitis
Treatment of minor allergic skin conditions with topical preparations
Occasional off-label use for anxiety due to sedative effects, though not first-line

Side Or Adverse Effects

Drowsiness and sedation, often dose-dependent and potentially impairing activities requiring alertness
Dizziness and loss of coordination, increasing fall risk especially in elderly patients
Dry mouth, nose, and throat due to anticholinergic effects
Constipation and gastrointestinal discomfort secondary to decreased gastrointestinal motility
Blurred vision and dry eyes resulting from muscarinic receptor blockade
Confusion, especially in older adults, with potential for delirium
Urinary retention or difficulty urinating, particularly in patients with prostatic hypertrophy
Palpitations or tachycardia in susceptible individuals
Hypotension and dizziness upon standing (orthostatic hypotension)
Paradoxical excitation including nervousness, irritability, or insomnia, more common in children
Allergic reactions to diphenhydramine itself, such as rash or anaphylaxis, though rare
Gastrointestinal disturbances including nausea and vomiting rarely reported
Headache and dizziness reported at higher doses
Potential for overdose leading to severe anticholinergic toxicity including seizures
Photosensitivity reactions, though uncommon, may occur

Precautions

Use caution when operating machinery or driving due to sedative effects
Avoid concomitant use with alcohol or other CNS depressants due to additive sedation
Not recommended in children under 2 years old due to serious adverse events
Use cautiously or avoid in patients with glaucoma due to risk of increased intraocular pressure
Monitor patients with urinary retention, prostate hypertrophy, or bladder obstruction
Exercise caution in patients with asthma, chronic obstructive pulmonary disease (COPD), or other respiratory disorders
Dose adjustments may be necessary in elderly patients due to increased sensitivity
Use carefully in patients with liver or kidney impairment as drug clearance may be reduced
Avoid in patients with known hypersensitivity to diphenhydramine or other antihistamines
Do not use to induce sleep in children; risk of paradoxical excitation exists
Pregnant and breastfeeding women should consult a healthcare provider prior to use
Avoid use with other anticholinergic drugs to prevent additive adverse effects
Be cautious in patients with cardiovascular disease due to potential for tachycardia or arrhythmia
May interfere with allergy skin testing; inform healthcare providers of use
Discontinue use and seek medical attention if urinary retention, palpitations, or confusion develop

Warnings

May cause significant drowsiness and impair mental or physical abilities required for activities such as driving or operating machinery
Concomitant alcohol use may potentiate CNS depression and increase risk of accidents
Should not be used in children younger than 2 years as fatal overdoses have occurred
Risk of serious allergic reactions including anaphylaxis exists, requiring immediate medical attention
Use with caution in patients with heart disease, low blood pressure, or other cardiovascular conditions
Pregnancy Category B; safety in pregnancy not definitively established—only use if clearly needed
May cause paradoxical excitation in children and elderly, manifesting as agitation or confusion
Overdose can result in serious, potentially fatal anticholinergic toxicity including seizures and coma
Risk of respiratory depression when combined with other CNS depressants, opioids, or sedative-hypnotics
May reduce breast milk production; avoid in nursing mothers unless benefits outweigh risks
Use caution in patients with thyroid disorders due to potential exacerbation of symptoms
Avoid use in patients with a history of drug hypersensitivity reactions to diphenhydramine
Excessive dosing may lead to cardiac arrhythmias or conduction disturbances
Discontinue prior to allergy skin testing to avoid false-negative results
Older adults are at increased risk for side effects such as confusion, dizziness, and falls

Contraindication

Known hypersensitivity or allergy to diphenhydramine or other first-generation antihistamines
Children under 2 years of age due to risk of fatal respiratory depression and misuse
Use in patients with narrow-angle glaucoma unless under careful medical supervision
Patients with prostatic hypertrophy complicated by urinary retention
Individuals with known severe liver or kidney impairment without appropriate monitoring
Concurrent use with monoamine oxidase inhibitors (MAOIs) increases risk of adverse effects
Patients with acute asthma attacks or severe chronic obstructive pulmonary disease
In patients with known blockage in the gastrointestinal tract or other obstruction
Individuals with bladder obstruction or urinary retention issues
Patients with colostomy or ileostomy may require careful use or avoidance
Concurrent use with other strong anticholinergic drugs without clinical indication
Known hypersensitivity to any of the excipients in diphenhydramine formulations
Use in patients prone to seizures without adequate medical supervision
Patients on medications that could cause additive CNS depression without dose adjustment
Individuals with severe cardiovascular disease unless benefits clearly outweigh risks

Drug Interactions

Concomitant use with alcohol or other CNS depressants (e.g., benzodiazepines, opioids) may result in additive sedation and respiratory depression
Co-administration with monoamine oxidase inhibitors (MAOIs) may prolong and intensify anticholinergic effects
Enhanced sedative effects when combined with muscle relaxants or antianxiety medications
Potential interaction with other anticholinergic drugs leading to increased risk of anticholinergic toxicity
Concurrent use with thyroid medications may exacerbate cardiac side effects
Diphenhydramine may interfere with drugs metabolized by CYP2D6 due to competitive enzymatic inhibition
Use with other antihistamines can increase sedation and other side effects
May reduce the efficacy of cholinesterase inhibitors used in Alzheimer's disease
Combination with other QT-prolonging drugs may increase risk of cardiac arrhythmias
Interaction with potassium supplements requiring monitoring due to renal clearance considerations
Use caution with medications that impair cognitive function or cause dizziness
May alter the results of allergy skin testing due to H1 receptor blockade
Synergistic CNS depressant effects when combined with sedative-hypnotics or antipsychotics
Concurrent administration with anesthetics may increase sedative complications
Potential to alter the metabolism of other drugs through hepatic enzyme modulation

Preparations Of The Drug

Oral tablets available in 25 mg and 50 mg strengths
Capsules formulated as 25 mg doses for oral administration
Oral liquid formulations commonly dosed at 12.5 mg/5 mL and 25 mg/5 mL concentrations with dosing devices for pediatric use
Injectable diphenhydramine hydrochloride solution for intramuscular and intravenous administration, typically at 25 mg/mL
Chewable tablets designed for ease of administration especially in pediatric allergic symptom relief
Combination formulations with other cold, cough, and allergy medications for multi-symptom management
Topical formulations (creams, gels) for localized allergic or itching conditions
Sublingual and rapidly dissolving strips available for quick onset of action
Sleep aid preparations available over-the-counter in liquid and capsule forms branded as Nytol, Sominex, or ZzzQuil
Formulations with varied excipients to cater for patients with specific allergies or intolerances
Pediatric-specific oral suspensions with standardized dosing for safety and efficacy in children
Extended-release preparations are generally not available due to the drug’s rapid pharmacokinetics
Pharmaceutical preparations conforming to FDA and other regulatory authority standards for quality and potency
Medical-grade diphenhydramine for hospital use in injectable and oral forms
Generic and brand name manufacturers provide a wide global availability of diphenhydramine products