First-Generation Antihistamines: Characteristics and Safety Considerations

Introduction to First-Generation Antihistamines

First-generation antihistamines, including diphenhydramine, hydroxyzine, chlorpheniramine, brompheniramine, clemastine, and cyproheptadine, are available for various formulations and dosing regimens 1, 2, 3, 4

Diphenhydramine is available in 25 mg and 50 mg tablets, dosed at 25-50 mg four times daily in adults, with a half-life of 2.6-4.3 hours and a 50% sedation rate 2, 5
Chlorpheniramine is available in 4 mg, 8 mg, and 12 mg formulations, dosed at 4 mg four times daily, with a half-life of 12-24 hours and approximately 45% sedation rate 2, 5
Hydroxyzine is dosed in adults with a half-life of approximately 20 hours and an 80% sedation rate 2, 5
Clemastine is available in 1.34 mg and 2.68 mg tablets, dosed twice to three times daily, with a half-life of 21.3 hours and a 14% sedation rate 2, 5
Cyproheptadine is available as 4 mg tablets and 2 mg/5 mL liquid, dosed at 4 mg three times daily, with a half-life of 16 hours 2, 3, 4

Drivers taking first-generation antihistamines are 1.5 times more likely to be responsible for fatal automobile accidents compared to non-users 1, 6
Performance impairment occurs even without subjective awareness of drowsiness, affecting work productivity and increasing occupational accidents 1, 5
Impaired learning and decreased school performance occur in children taking these medications 1, 6
Bedtime-only dosing does not eliminate daytime sedation due to prolonged plasma half-lives and persistent end-organ effects 1, 5
Dry mouth and eyes, constipation, urinary retention, and increased risk of narrow-angle glaucoma occur with all first-generation agents 1, 5
Older adults are at particularly high risk for cognitive impairment, falls, fractures, and subdural hematomas 1, 6
Cognitive decline is especially concerning in elderly populations with repeated use 3, 4
Concomitant use with alcohol, sedatives, hypnotics, or antidepressants significantly enhances performance impairment 1, 5

Cyproheptadine has dual H1-blocking and antiserotonergic activity, making it useful for diarrhea and nausea in mast cell activation syndrome 3, 4
The anticholinergic properties may provide better control of rhinorrhea compared to second-generation agents in select patients 1, 5
Ketotifen, though sedating, is used for dermatologic, gastrointestinal, and neuropsychiatric symptoms when compounded 3, 4

Before prescribing any first-generation antihistamine, physicians must ensure patients understand both the potential for adverse effects and the availability of safer second-generation alternatives 1