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Sedating h1

The term “antihistamine” generally is used to describe those medications that antagonize histamine activity at H1 receptors. Currently the only FDA-approved histamine antagonist medications block the effects of histamine at H1 or H2 receptor sites. 6 Antihistamine medications block or reduce histamine-mediated effects at one of four identified histamine receptors (see Table 1 below).These include the first-generation H1 antagonists, or sedating antihistamines (eg, diphenhydramine, chlorpheniramine), and second-generation H1 receptor blockers, or nonsedating antihistamines (eg, loratadine, cetirizine).First-generation antihistamines are widely available without a prescription and commonly used to treat allergic symptoms, including rhinitis, conjunctivitis, pruritus, eczema, urticaria, and anaphylactic reactions.

These medications decrease gastric acid secretion in the stomach by antagonizing the effects of histamine at H2 receptors found in gastrointestinal parietal cells.Ongoing research efforts aim to develop potential agents to target the H3 and H4 receptor sites.H3 receptor antagonists could provide new treatment options for sleep disorders, weight loss, neuropathic pain, obesity, movement disorders, schizophrenia, attention deficit disorders, and Alzheimer’s dementia, while the development of antagonists for H4 receptors may lead to new treatment options for autoimmune inflammatory diseases.1,2 The first H1 sedating antihistamines have been available for more than 60 years and were synthesized based on a chemical structure similar to that used to develop cholinergic muscarinic antagonists, tranquilizers, and antihypertensive agents.These antihistamines have low receptor specificity and interact with both peripheral and central histamine receptors and readily cross the blood-brain barrier.This leads to significant central nervous system side effects, including sedation, drowsiness, somnolence, fatigue, cognitive decline, psychomotor effects, and loss of coordination.These antihistamines also are potent muscarinic receptor antagonists that can lead to serious anticholinergic side effects, such as sinus tachycardia, dry skin, dry mucous membranes, dilated pupils, constipation, ileus, urinary retention, and agitated delirium.3 The mnemonic “blind as a bat, dry as a bone, red as a beet, mad as a hatter, and hot as a hare” often is used to help describe and identify patients suffering from anticholinergic syndrome (see Table 2 below).Urinary retention and difficulty urinating can be particularly troublesome in male patients with enlarged prostates, and this retention can increase the risk of urinary tract infections, especially in women.4 The second-generation nonsedating antihistamines generally are considered as safer alternatives for use in older adults who require treatment for allergic rhinitis and other allergy symptoms.These antihistamines are more selective on peripheral H1 receptors and have a lower affinity for cholinergic and alpha-adrenergic receptor sites, which reduces the risk of anticholinergic and central nervous system side effects.Older adults are especially sensitive to the central nervous system- and anticholinergic-related side effects of sedating antihistamines because of decreased cholinergic neurons or receptors in the brain, reduced hepatic and renal function, and increased blood-brain permeability.These patients also often have coexisting conditions and often take multiple medications that increase the risk of drug-drug interactions and the potential for sedative adverse effects.Even when first-generation antihistamines are used at the lowest doses recommended by the manufacturer, they can cause serious central nervous system side effects, including dizziness, hypotension, and next-day sedation.

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