From the Guidelines
Diphenhydramine (Benadryl) should be used with caution in patients at risk of hypotension, with recommended starting doses of 12.5-25 mg (half the standard adult dose) in these vulnerable populations. Patients with cardiovascular disease, the elderly, those on antihypertensive medications, and individuals with autonomic dysfunction are particularly susceptible to hypotensive effects. When administering diphenhydramine to these patients, blood pressure should be monitored before and after administration, especially with parenteral (IV or IM) forms which pose a greater hypotension risk than oral formulations. The medication should be administered slowly when given intravenously (over 2-5 minutes) to minimize blood pressure drops. Patients should be advised to change positions slowly to avoid orthostatic hypotension and to remain hydrated. The hypotensive effect occurs because diphenhydramine blocks histamine H1 receptors and has anticholinergic properties that can cause peripheral vasodilation, reducing vascular resistance and potentially lowering blood pressure. If hypotension develops, patients should be placed in a supine position with legs elevated, and in severe cases, IV fluids or vasopressors may be required, as suggested by the management guidelines for infusion reactions to systemic anticancer therapy 1.
Some key points to consider when using diphenhydramine in patients at risk of hypotension include:
- Monitoring blood pressure before and after administration
- Administering the medication slowly when given intravenously
- Advising patients to change positions slowly and remain hydrated
- Being prepared to manage hypotension with IV fluids or vasopressors if necessary
- Considering alternative dosing regimens, such as using less-sedating second-generation antihistamines, as suggested by guidelines for the diagnosis and management of food allergy in the United States 1.
It's also important to note that the combined use of H1 and H2 antagonists, such as diphenhydramine and ranitidine, is superior to the use of either agent alone in the management of anaphylaxis, as recommended by the guidelines for the management of infusion reactions to systemic anticancer therapy 1. However, the most recent and highest quality study on this topic, published in 2017, suggests that the use of diphenhydramine should be cautious and tailored to the individual patient's needs, taking into account their risk factors for hypotension and other potential adverse effects 1.
From the FDA Drug Label
The FDA drug label does not answer the question.
From the Research
Precautions for Diphenhydramine Use in Patients at Risk of Hypotension
- Diphenhydramine can cause sedation and has a risk of toxicity in overdose, including cardiac toxicity 2
- There is no direct evidence in the provided studies that links diphenhydramine use with hypotension
- However, diphenhydramine can cause alterations in heart rate and blood pressure as part of infusion reactions 3
Dosage Considerations
- A dose-response relationship was demonstrated for most adverse outcomes in older hospitalized patients 4
- The studies do not provide specific dosage considerations for patients at risk of hypotension
- It is recommended to carefully review the use of diphenhydramine in vulnerable populations, such as older adults 4, 5
Alternative Antihistamines
- Second-generation antihistamines are available and have fewer adverse effects compared to diphenhydramine 2, 5
- Cetirizine appears to be a viable substitute for diphenhydramine for the prevention of infusion reactions 3
- Further studies are needed to determine the efficacy and safety of alternative antihistamines compared to diphenhydramine 6, 3