What is the recommended dosage of Diphenhydramine (Antihistamine)?

From the Guidelines

The recommended dosage of diphenhydramine is 1 to 2 mg/kg per dose, with a maximum dose of 50 mg IV or oral, as stated in the most recent and highest quality study 1. This dosage is applicable for adjunctive treatment in cases of anaphylaxis, and it is essential to note that diphenhydramine should not be used as a first-line treatment, but rather as a secondary measure to support the primary treatment with epinephrine. The medication comes in various forms, including tablets, capsules, liquid, and topical preparations, and it is crucial to start with the lowest effective dose, especially in elderly patients who are more sensitive to side effects. Some key points to consider when administering diphenhydramine include:

• The medication works by blocking histamine receptors, which helps relieve allergy symptoms, and also blocks acetylcholine, which can cause drowsiness and help with sleep.
• Common side effects include drowsiness, dry mouth, dizziness, and constipation.
• It is essential to avoid alcohol and other sedatives while taking diphenhydramine, as they can increase drowsiness and impair coordination.
• In cases of anaphylaxis, diphenhydramine should be used in conjunction with other treatments, such as epinephrine, bronchodilators, and supplemental oxygen therapy, as recommended in the guidelines 1.

From the FDA Drug Label

Active ingredient per 10 mL (1 Unit Dose) Diphenhydramine HCl 25 mg Each 10 mL of Diphenhydramine HCl Oral Solution contains Diphenhydramine HCl 25 mg The recommended dose of diphenhydramine is 25 mg per 10 mL.

• The dosage form is an oral solution.
• The strength is 25 mg/10 mL 2 2

From the Research

Diphenhydramine Dosage

• The dosage of diphenhydramine is not explicitly stated in the provided studies, but its effects are compared to other antihistamines.
• A study from 1999 3 compared the central nervous system effects of diphenhydramine to cetirizine and loratadine in elderly subjects, with diphenhydramine given at a dose of 50 mg.
• In this study, diphenhydramine was found to cause more central nervous system dysfunction than cetirizine and loratadine, as measured by latency of the P300 event-related potential and visual analogue scale for subjective somnolence.
• Another study from 2020 4 investigated the effects of diphenhydramine on tear volume, tear stability, and intraocular pressure, with diphenhydramine given at an unspecified dose.
• This study found that diphenhydramine use resulted in a significant decrease in tear volume, but had no effect on tear breakup time or intraocular pressure.

Comparison to Other Antihistamines

• Diphenhydramine is a first-generation antihistamine, which are known to have more sedating effects than second-generation antihistamines like loratadine and cetirizine 5.
• A study from 2000 5 found that loratadine, cetirizine, and fexofenadine are less likely to produce sedation and have excellent safety records, including in special populations like pediatric and elderly patients.
• In contrast, diphenhydramine has been shown to cause more central nervous system dysfunction and sedation, particularly in elderly subjects 3.