What it's for (Indications)
- Diphenhydramine, a first-generation antihistamine, is indicated for a range of conditions primarily related to its antihistaminic and anticholinergic properties.
- These include the temporary relief of symptoms associated with upper respiratory allergies or the common cold, such as sneezing, runny nose, itching of the nose or throat, and watery eyes.
- It is also utilized for the temporary relief of nocturnal insomnia, leveraging its sedative effects.
- Furthermore, diphenhydramine can be employed in the prevention and treatment of motion sickness, as well as for the management of mild symptoms of Parkinsonism and drug-induced extrapyramidal symptoms.
- Its antitussive properties also make it useful for the temporary relief of cough due to minor throat and bronchial irritation.
- The clinical application of diphenhydramine should always consider its pronounced sedative and anticholinergic side effects, which can be both therapeutic and dose-limiting, necessitating careful patient selection and education regarding its appropriate use.
Dosage Information
| Type | Guideline |
|---|---|
| Standard | Dosage of diphenhydramine varies significantly based on age, indication, and formulation, and should always be determined by a healthcare professional. For oral administration in adults and children 12 years and older, the typical dose is 25 mg to 50 mg every 4 to 6 hours, not to exceed 300 mg in 24 hours. For children 6 to 11 years, common dosing is 12.5 mg to 25 mg every 4 to 6 hours, with a maximum of 150 mg per 24 hours, often based on weight (e.g., 5 mg/kg/day, not to exceed 300 mg/day). For temporary relief of insomnia, a single dose of 50 mg at bedtime is commonly recommended for adults. Parenteral formulations (intravenous or intramuscular) are available for severe allergic reactions or other situations where oral administration is not feasible, with doses typically ranging from 10 mg to 50 mg, administered slowly, with a maximum daily dose not exceeding 400 mg in severe cases. Dosage adjustments may be necessary in patients with hepatic impairment or in the elderly, who may be more susceptible to adverse effects. |
Safety & Warnings
Common Side Effects
- Diphenhydramine is associated with a wide array of potential side effects, primarily due to its potent antihistaminic and anticholinergic actions.
- The most common adverse effects include significant central nervous system (CNS) depression, manifesting as drowsiness, sedation, dizziness, and impaired coordination, which can significantly affect activities requiring mental alertness like driving or operating machinery.
- Anticholinergic effects are also prominent and include dry mouth, blurred vision, urinary retention, and constipation.
- Less frequently, but still notable, are gastrointestinal disturbances such as nausea, vomiting, and epigastric distress.
- Paradoxical CNS excitation, characterized by restlessness, nervousness, tremors, and insomnia, can occur, particularly in children and the elderly.
- More serious, albeit rare, adverse effects encompass palpitations, tachycardia, hypotension, headache, tinnitus, convulsions, and hypersensitivity reactions.
- Prolonged use or overdose can exacerbate these effects, leading to anticholinergic toxicity syndrome, which may include delirium, hallucinations, and severe cardiovascular compromise.
Serious Warnings
- Black Box Warning: While diphenhydramine does not carry a formal FDA Black Box Warning, several critical safety considerations warrant attention comparable to a serious warnings section due to the potential for significant harm. Extreme caution is mandated in its use across vulnerable populations. Specifically, diphenhydramine is contraindicated in neonates and premature infants due to a heightened risk of central nervous system depression, including respiratory arrest. The elderly are particularly susceptible to severe anticholinergic adverse effects, including confusion, delirium, hallucinations, urinary retention, and increased risk of falls, necessitating careful dosing and monitoring. The profound central nervous system depressant effects of diphenhydramine are significantly additive with alcohol and other CNS depressants, leading to potentially life-threatening sedation and respiratory depression. Patients must be rigorously cautioned against combining these substances and should be advised against operating machinery or driving while under the influence of diphenhydramine. There is also a recognized potential for misuse or abuse, especially among adolescents and young adults, due to its sedative and hallucinogenic properties at supratherapeutic doses, which can lead to severe cardiovascular events (e.g., arrhythmias, QTc prolongation), seizures, coma, and death. Healthcare providers should emphasize the importance of strict adherence to prescribed dosages and the critical need to store this medication securely, out of reach of children and individuals prone to substance misuse, to prevent accidental or intentional overdose.
- Patients should be advised that diphenhydramine can cause significant drowsiness and impair their ability to perform tasks requiring mental alertness, such as driving a car or operating machinery.
- Concurrent use of alcohol or other central nervous system depressants, including sedatives and tranquilizers, should be strictly avoided as it can potentiate the sedative effects of diphenhydramine and lead to severe respiratory depression or profound sedation.
- Caution is particularly warranted in the elderly, who are more susceptible to adverse anticholinergic effects like confusion, delirium, urinary retention, and constipation, as well as increased risk of falls.
- Patients with certain pre-existing medical conditions, such as narrow-angle glaucoma, prostatic hypertrophy (due to risk of urinary retention), asthma, chronic obstructive pulmonary disease (COPD), hyperthyroidism, and cardiovascular disease (due to potential for tachycardia), should use diphenhydramine with extreme caution and only under medical supervision.
- The drug should not be used in premature or newborn infants due to increased susceptibility to adverse effects.
How it Works (Mechanism of Action)
Diphenhydramine primarily exerts its pharmacological effects through competitive antagonism of histamine H1 receptors. By blocking histamine's action at these receptors, it effectively suppresses the symptoms of allergic reactions such as sneezing, itching, rhinorrhea, and urticaria. This antagonism occurs both peripherally and centrally, as diphenhydramine readily crosses the blood-brain barrier. Its central H1 receptor blockade contributes significantly to its pronounced sedative properties. Beyond its antihistaminic activity, diphenhydramine possesses potent anticholinergic (antimuscarinic) effects, which contribute to side effects like dry mouth, blurred vision, and urinary retention, but also contribute to its utility in motion sickness and drug-induced extrapyramidal symptoms. Additionally, it exhibits antiemetic properties and, to a lesser extent, local anesthetic action by blocking sodium channels, though this is not its primary therapeutic indication for systemic use. Its broad pharmacological profile underlies its diverse clinical applications.