Pharmacology Flashcards: Antihistamines, Decongestants, Allergy, Anaphylaxis Drugs

First-generation antihistamines like diphenhydramine and chlorpheniramine cross the blood-brain barrier and cause significant sedation, dry mouth, urinary retention, and blurred vision due to their anticholinergic properties. Second-generation antihistamines like cetirizine, loratadine, and fexofenadine are less lipophilic, minimally cross the blood-brain barrier, and cause much less sedation. Both generations block H1 histamine receptors to reduce allergy symptoms including sneezing, itching, runny nose, and hives.

Epinephrine is an adrenergic agonist that is the only first-line treatment for anaphylaxis because it rapidly counteracts all the life-threatening features of a severe allergic reaction. It constricts blood vessels to reverse hypotension and shock through alpha-1 stimulation, relaxes bronchial smooth muscle to open airways through beta-2 stimulation, increases heart rate and contractility through beta-1 stimulation, and suppresses further mediator release from mast cells. Intramuscular injection in the anterolateral thigh is the preferred route.

Pseudoephedrine is an oral decongestant that works by stimulating alpha-adrenergic receptors in nasal blood vessels, causing vasoconstriction and reducing nasal congestion. Common side effects include insomnia, restlessness, nervousness, elevated blood pressure, tachycardia, and headache. It is contraindicated in patients with uncontrolled hypertension, severe coronary artery disease, and those taking monoamine oxidase inhibitors. Pseudoephedrine is kept behind pharmacy counters in the United States because it can be used to manufacture methamphetamine.

Patients with a history of anaphylaxis should carry at least two epinephrine auto-injectors at all times, such as EpiPen or generic equivalents, and know how to use them. They should administer epinephrine immediately at the first sign of a severe allergic reaction and call emergency services even if symptoms improve, because biphasic reactions can occur hours later. Antihistamines like diphenhydramine are secondary treatments that help with itching and hives but cannot reverse airway obstruction or cardiovascular collapse.

First-generation antihistamines like diphenhydramine are included on the Beers Criteria list of potentially inappropriate medications for older adults because their strong anticholinergic effects increase the risk of confusion, delirium, cognitive impairment, falls, urinary retention, and constipation in elderly patients. Older adults are more susceptible to these effects due to decreased hepatic metabolism, increased blood-brain barrier permeability, and pre-existing cholinergic deficits. Second-generation antihistamines like cetirizine and loratadine are preferred alternatives. ---