Diphenhydramine is used to relieve red, irritated, itchy, watery eyes; sneezing; and runny nose caused by hay fever, allergies, or the common cold. Diphenhydramine is also used to relieve cough caused by minor throat or airway irritation. Chemically known as 2-(diphenylmethoxy)-N,N-dimethylethanamine, CAS 58-73-1, molecular formula C17H21NO and molecular weight 255.35 g/mol, diphenhydramine is commonly administered as the hydrochloride salt for improved water solubility; the free base is lipophilic and readily crosses the blood brain barrier. Typical physicochemical properties include a tertiary amine pKa near 9, logP in the low-to-mid 3 range, good solubility in organic solvents and increased aqueous solubility as the HCl salt. Pharmacokinetically, oral onset is usually within 15 to 60 minutes with a duration of 4 to 8 hours in most adults and an elimination half-life commonly reported in the 3 to 9 hour range depending on age and hepatic function; hepatic N-demethylation and other phase I and phase II pathways are principal metabolic routes. Pharmacodynamic features include H1 histamine receptor antagonism with substantial central nervous system penetration that produces antihistamine and sedative effects and measurable antimuscarinic activity at clinically relevant doses. Typical quality control and assay methods include HPLC-UV for potency and related substances, GC for residual solvents, and LC-MS for structure confirmation of impurities.
Parent: Diphenhydramine
Parent: Diphenhydramine
Parent: Diphenhydramine
Parent: Diphenhydramine
Parent: Diphenhydramine / Orphenadrine
Parent: Diphenhydramine
Parent: Diphenhydramine/ Ebastine/ Cyclizine/ Dimenhydrinate
Parent: Diphenhydramine/ Phenytoin/ Dimenhydrinate
Parent: Diphenhydramine Hydrochloride
Parent: Diphenhydramine
Parent: Diphenhydramine
Parent: Diphenhydramine
Parent: Azathioprine / Diphenhydramine / tamoxifen
Parent: Diphenhydramine/ Dimenhydrinate
Typical related compounds and degradants for diphenhydramine include N-desmethyl-diphenhydramine and other N-dealkylation products, diphenylmethanol (benzhydrol) and related benzhydryl fragments, benzophenone-type oxidation products, N-oxide species, and low-level alkylation or residual solvent impurities from manufacture; trace inorganic residues and catalysts can also be present depending on the synthetic route. Typical specification ranges used by manufacturers and pharmacopoeias are assay acceptance around 98.0 to 102.0 percent for the labelled substance and related substances limits expressed as individual impurity not greater than about 0.2 to 0.5 percent and total impurities not greater than about 1.0 to 2.0 percent, with lower reporting and identification thresholds in line with ICH Q3A guidance for specified and unspecified impurities; limits for genotoxic impurities follow established ppm-level criteria and residual solvents are controlled to ICH Q3C limits. Analytical control is normally performed by validated HPLC methods with system suitability criteria, complemented by orthogonal techniques such as LC-MS for identification and GC for volatile impurities to ensure material meets stability and safety requirements.
Diphenhydramine is used as an H1 antihistamine to relieve symptoms of allergic rhinitis and common cold such as red, irritated, itchy, watery eyes; sneezing; and runny nose, and is also indicated to relieve cough caused by minor throat or airway irritation; it is additionally used for short term symptomatic relief of allergies and as an over the counter sleep aid because of its sedating properties.
Cetirizine and diphenhydramine serve different clinical needs: cetirizine is a second generation H1 antihistamine with minimal sedation and longer duration of action, generally preferred for daytime allergy control, while diphenhydramine is a first generation H1 antihistamine with stronger central nervous system penetration and anticholinergic effects, useful when sedation is acceptable or desired; choice depends on symptom profile, need for sedation, age, comorbidities and drug interactions so clinical context should guide selection.
Diphenhydramine can produce sedation and is commonly used as a short term over the counter sleep aid, but safety considerations include next day drowsiness, impaired cognition and coordination, anticholinergic effects especially in older adults and those with glaucoma or urinary retention, and potential interactions with alcohol and other sedatives; it is advisable to consult a healthcare professional for recurrent insomnia or if taking other medications or if there are underlying medical conditions.
Diphenhydramine acts primarily as an H1 histamine receptor antagonist at peripheral and central H1 receptors, reducing histamine-mediated symptoms, and also exerts antimuscarinic activity at muscarinic acetylcholine receptors which contributes to drying of secretions and other anticholinergic effects; central nervous system penetration causes sedation and contributes to its use as a hypnotic at higher doses.
