Diphenhydramine, a first-generation antihistamine, was developed in the 1940s and became widely used for medical purposes in the 1950s. Its history is marked by its effectiveness in treating allergic reactions, motion sickness, and insomnia, as well as its sedative properties. Diphenhydramine acts as a competitive antagonist of H1 histamine receptors, reducing symptoms such as itching, swelling, and runny nose. It is available as both a standalone medication and in combination products for allergies, cold relief, and sleep aids. Over the years, diphenhydramine has been widely used in over-the-counter formulations, and its safety profile has been extensively studied, though caution is advised due to sedation, anticholinergic effects, and potential interactions with other central nervous system depressants.
BRAND NAMES
Benadryl – widely used for allergies, hay fever, and urticaria.
Sominex – marketed as a short-term sleep aid.
Nytol– over-the-counter sleep aid formulation.
MECHANISM OF ACTION
Diphenhydramine is a first-generation H1 receptor antagonist that works by competitively binding to histamine H1 receptors, preventing histamine from exerting its effects on target tissues. By blocking H1 receptors, diphenhydramine reduces vasodilation, increased vascular permeability, and sensory nerve stimulation, which alleviates symptoms such as itching, swelling, hives, and nasal congestion.
PHARMACOKINETICS
Absorption
Diphenhydramine is rapidly and well absorbed after oral administration, with peak plasma concentrations typically reached within 1 to 2 hours. Its oral bioavailability can vary due to first-pass metabolism in the liver, but sufficient drug levels are achieved to exert both peripheral and central effects.
Distribution
diphenhydramine exhibits a large volume of distribution, estimated at approximately 3–5 L/kg, reflecting its extensive distribution into body tissues, including the central nervous system.
Metabolism
The Diphenhydramine is extensively metabolized in the liver following oral administration, undergoing significant first-pass metabolism. Its biotransformation is primarily mediated by the cytochrome P450 enzyme system, especially CYP2D6, which catalyzes oxidative reactions such as N-demethylation and deamination.
Elimination
The elimination of Diphenhydramine occurs primarily through the kidneys after it has been extensively metabolized in the liver. Most of the drug is excreted in the urine in the form of inactive metabolites, such as diphenylmethoxyacetic acid, with only a very small fraction eliminated as unchanged diphenhydramine.
PHARMACODYNAMICS
The pharmacodynamics of Diphenhydramine are primarily related to its action as a first-generation H1 antihistamine. It works by competitively blocking Histamine H1 receptor sites, thereby preventing the effects of histamine, a chemical released during allergic reactions. This results in reduced symptoms such as itching, sneezing, rhinorrhea, and urticaria.
ADMINISTRATION
The administration of Diphenhydramine can be carried out through multiple routes depending on the clinical situation. It is most commonly given orally in the form of tablets, capsules, or liquid preparations for the relief of allergic symptoms, motion sickness, or as a sleep aid. For faster onset of action or in acute conditions, it may be administered via intravenous (IV) or intramuscular (IM) injection, particularly in hospital settings for allergic reactions.
DOSAGE AND STRENGTH
The dosage and strength of Diphenhydramine depend on the patient’s age, clinical condition, and route of administration. In adults and children aged 12 years and above, the usual oral dose is 25–50 mg every 4–6 hours as needed, while children between 6 and 11 years typically receive 12.5–25 mg at similar intervals. Younger children may require lower doses under medical supervision.
DRUG INTERACTIONS
The drug interactions of Diphenhydramine are mainly related to its sedative and anticholinergic properties, as well as its metabolism in the liver. When taken with other central nervous system (CNS) depressants such as opioids, benzodiazepines, alcohol, or sedative-hypnotics, diphenhydramine can produce enhanced drowsiness, respiratory depression, and impaired coordination.
FOOD INTERACTIONS
The food interactions of Diphenhydramine are generally minimal, but certain substances can influence its effects. The most important interaction is with alcohol, which can significantly enhance the drug’s central nervous system depressant effects, leading to increased drowsiness, dizziness, and impaired coordination.
CONTRAINDICATIONS
The contraindications of Diphenhydramine include conditions where its use may be unsafe due to its antihistaminic and anticholinergic effects. It is contraindicated in individuals with known hypersensitivity to diphenhydramine or other similar antihistamines.
SIDE EFFECTS
Drowsiness and sedation
Dizziness
Dry mouth
Blurred vision
Constipation
Urinary retention
Palpitations
OVER DOSAGE
Overdosage of Diphenhydramine can be serious and potentially life-threatening, primarily due to its anticholinergic and central nervous system depressant effects. Symptoms may range from mild to moderate, including extreme drowsiness, dizziness, dry mouth, blurred vision, nausea, vomiting, and flushed skin.
TOXICITY
Toxicity of Diphenhydramine arises primarily from excessive anticholinergic and central nervous system (CNS) effects. It can occur after acute overdose or chronic ingestion of higher-than-recommended doses. Clinical manifestations include severe sedation, confusion, agitation, hallucinations, tremors, seizures, and coma. Cardiovascular effects such as tachycardia, hypotension, or arrhythmias may also occur, while respiratory depression can be life-threatening in severe cases.
