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Diphenhydramine HCl

Also known as: Diphenhydramine HCl, Benadryl, C17H22ClNO, Diphenhydramine Hydrochloride

Overview

Diphenhydramine hydrochloride is a first-generation antihistamine primarily used to treat allergies, insomnia, and symptoms of the common cold. It is a synthetic compound, not derived from natural sources, and functions as an inverse agonist of the histamine H1 receptor, possessing anticholinergic properties. While extensively studied for its antihistamine and sedative effects, research on its use in pain management is less comprehensive. Diphenhydramine is available in various forms, including oral, intravenous, intramuscular, and topical formulations. It is generally considered safe when used as directed, but it can cause significant side effects, especially in older adults. The quality of evidence supporting its analgesic effects varies, with some systematic reviews suggesting potential benefits as an adjunct in pain management.

Benefits

Diphenhydramine has demonstrated analgesic benefits as an adjunct in acute pain management. A meta-analysis indicated a 28% reduction in acute pain among patients, with a mean difference of -1.53 (95% CI: [-2.35 to -0.70]). It is also effective for its sedative properties and in treating insomnia. These benefits are particularly noted in emergency department settings. The effect size for pain reduction is moderate, suggesting clinical significance as an adjunct therapy. Maximal effects typically occur within two hours and can last up to seven hours. However, it is essential to note that diphenhydramine is primarily an antihistamine and sedative, and its analgesic use is secondary.

How it works

Diphenhydramine primarily works by blocking histamine H1 receptors, which reduces the effects of histamine and leads to antihistamine and sedative effects. It also exhibits anticholinergic effects by interacting with muscarinic receptors. The drug targets histamine H1 receptors and various muscarinic receptors (M1-M5). Oral bioavailability ranges from 40% to 60%, with peak plasma concentration occurring 2-3 hours after administration. By blocking histamine receptors, diphenhydramine reduces allergic symptoms and promotes drowsiness, while its anticholinergic action contributes to side effects like dry mouth and blurred vision.

Side effects

Diphenhydramine can cause several side effects, with drowsiness, poor coordination, and upset stomach being the most common. Uncommon side effects include dry mouth, blurred vision, and urinary retention. Rare but serious side effects, particularly in elderly patients, include confusion, delirium, and hallucinations. Diphenhydramine can interact with other sedatives, anticholinergics, and certain antidepressants, increasing the risk of adverse effects. It is generally contraindicated in patients with narrow-angle glaucoma, urinary retention, and certain gastrointestinal conditions. Caution is advised for older adults, and it should be avoided during breastfeeding. Due to its anticholinergic effects, it can exacerbate conditions like benign prostatic hyperplasia and cognitive impairment.

Dosage

For adults, the typical effective dose of diphenhydramine is 25-50 mg. For sedation and antihistamine effects, doses range from 25 to 100 mg. It is generally recommended not to exceed 100 mg per dose. For insomnia, it is best taken before bedtime, while for allergic reactions, it can be taken as needed. Diphenhydramine is available in oral, intravenous, intramuscular, and topical forms. Absorption can be affected by food intake and individual metabolism. It is crucial to follow the guidance of a healthcare provider to avoid misuse and potential adverse effects. Always start with the lowest effective dose to minimize side effects.

FAQs

Is diphenhydramine safe for older adults?

Diphenhydramine should be used cautiously in older adults due to the increased risk of side effects such as confusion, dizziness, and falls. Lower doses are often recommended.

Can I take diphenhydramine while breastfeeding?

No, diphenhydramine is not recommended for use during breastfeeding as it can pass into breast milk and may cause drowsiness or other adverse effects in the infant.

How long does it take for diphenhydramine to work?

Diphenhydramine typically provides relief from allergic symptoms or induces sleep within a few hours after administration. Its effects can last for several hours, depending on the dose and individual metabolism.

Is diphenhydramine a pain reliever?

Diphenhydramine is not a primary pain reliever but has been studied as an adjunct for acute pain management. It is primarily an antihistamine and sedative, not a dedicated analgesic.

Can I take diphenhydramine with other medications?

Diphenhydramine can interact with other sedatives, anticholinergics, and certain antidepressants. Consult a healthcare provider before combining it with other medications to avoid adverse effects.

Research Sources

https://www.jmlph.net/index.php/jmlph/article/view/55 – This systematic review and meta-analysis evaluated diphenhydramine's analgesic effects in acute pain management. The study included four trials with a total of 438 patients, finding that diphenhydramine reduced pain scores by 28% as an adjunct therapy. The authors concluded that diphenhydramine shows promise as an adjunct for acute pain, but noted limitations due to the small number of studies and potential bias.
https://pubchem.ncbi.nlm.nih.gov/compound/Diphenhydramine-Hydrochloride – This entry from PubChem provides detailed chemical and structural information about diphenhydramine hydrochloride. It includes identifiers, properties, and safety information, serving as a comprehensive resource for understanding the compound's basic characteristics. The resource is valuable for researchers and healthcare professionals seeking detailed chemical information.
https://archive.hshsl.umaryland.edu/bitstream/handle/10713/14921/Diphenhydramine_Final_2020_12.pdf?sequence=1 – This document provides a comprehensive overview of diphenhydramine, including its uses, side effects, and contraindications. It is designed as an educational resource for healthcare providers and patients. The document emphasizes the importance of using diphenhydramine cautiously, especially in older adults, and highlights potential drug interactions.
https://www.ncbi.nlm.nih.gov/books/NBK526010/ – This resource from NCBI Bookshelf offers a detailed review of diphenhydramine, covering its pharmacology, clinical uses, and adverse effects. It is a valuable reference for healthcare professionals seeking in-depth information on the drug's mechanisms and therapeutic applications. The review also addresses considerations for special populations, such as pregnant and breastfeeding women.
https://en.wikipedia.org/wiki/Diphenhydramine – The Wikipedia article on diphenhydramine provides a general overview of the drug, including its history, uses, and side effects. While not a primary research source, it offers a broad introduction to the topic and links to additional resources. The article is regularly updated and serves as a convenient starting point for learning about diphenhydramine.