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).
Because delirium and hallucinations can result from the use of anticholinergics including sedating antihistamines, patients receiving these medications are at risk of being prescribed antipsychotic medications.
Patients with Alzheimer’s disease are at particular risk as caregivers may purchase these medications without understanding the associated risk of exacerbating the disease.
Prescribers and nurses should discuss the potential risks of OTC medications during routine patients’ physician visits, and pharmacists should be proactive in seeking additional information from patients, including asking questions about OTC product use.
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.
Nasal and ocular topical antihistamines may be used in the treatment of allergic rhinitis (hay fever).