antihistamines are commonly classified by functional class (e.g., first generation [sedating] and second generation [nonsedating]).
With similar efficacy, adverse effects depend on the ability of the H antihistamines (i.e., brompheniramine, chlorpheniramine, clemastine, dexbrompheniramine, diphenhydramine) cross the blood–brain barrier and are associated with central nervous system adverse effects (e.g., drowsiness, fatigue, dizziness, impaired thinking and memory, agitation, and hallucinations; children and the elderly may experience paradoxical excitation and agitation).
With the exception of cetirizine, sedation is not a noteworthy issue with usual doses of the second-generation nonprescription H antihistamines block alpha-adrenergic receptors, serotonin receptors, and cardiac ion currents, contributing to cardiovascular toxicities such as dysrhythmias, prolongation of the QT interval, and postural hypotension.
Physiologic changes associated with aging may increase the risk of medication-related adverse effects.
Comorbidities may increase the number of medications taken by the elderly, increasing the risk for drug–disease and drug–drug interactions. Census Bureau predicts that adults 65 years or older will represent 19.6% of the population by 2030, antihistamines with anticholinergic adverse effects are of great concern.
Mental status changes and fall risk are of special concern for adults 65 years or older. The American Geriatrics Society (AGS) updated Beers list of potentially inappropriate medications in older adults includes five nonprescription first-generation H antihistamines continue to be prescribed for and self-selected by adults 65 years or older.
First generation antihistamines include brompheniramine, diphenhydramine, carbinoxamine, chlorpheniramine, and clemastine.
These antihistamines all have anticholinergic properties, making sedation one of the major side effects.
Confusingly, these are sometimes also referred to as third generation antihistamines.
They are all effective antihistamines that are less sedating and have relatively few side effects.
Second generation antihistamines are less likely to cause impairment of cognitive facilities or performance skills.
They have the added benefit that they usually only need to be taken one or twice daily to be effective.
Cetirizine and loratadine are once daily antihistamines, while fexofenadine is taken once or twice daily, depending on its strength.