Diphenhydramine Impairment

Diphenhydramine is an over-the-counter as well as prescription first generation antihistamine and H 1 receptor antagonist.1 It is available alone or in combination with other compounds and includes trade names such as Benadryl, Sominex, Diphenhist, Banophen, Hydramine, Dicopanol, Allermax and more. It is used to treat allergy and common cold symptoms such as sneezing; itching; watery eyes; hives; rashes; and cough. In addition to these symptoms diphenhydramine is also used in the treatment of motion sickness as well as to induce sleep. Drowsiness is often an unwanted side effect if the drug is used during normal day time activities such as operating a motor vehicle, machinery, or other hazardous activity. It is sold in tablet, capsule, chewable tablet, syrups/elixir, injectable, and topical forms.1

Alcohol can increase drowsiness, sedation and decrease motor skills when taken in conjunction with diphenhydramine. Monoamine Oxidase Inhibitors (MAOIs), diazepam, hypnotics, sedatives, tranquilizers, and other central nervous system depressants also increase the effects of diphenhydramine. “Laboratory studies have shown diphenhydramine to decrease alertness, decrease reaction time, induce somnolence, impair concentration, impair time estimation, impair tracking, decrease learning ability, and impair attention/memory within the first 2-3 hours post dose. Significant adverse effects on vigilance, divided attention, working memory, and psychomotor performance have been demonstrated. It is important to note that impairment has been shown to occur even in the absence of self-reported sleepiness or sedation.”1

In addition to the hazard posed to those operating a motor vehicle, we also find aviators taking sedating compounds such as diphenhydramine and operating aircraft.2 Aviation Medical Examiner’s, doctors and pharmacist are all great resources for patient education regarding the use of diphenhydramine. As it is readily available over-the-counter individuals may by-pass any possible patient education and/or underestimate the effect diphenhydramine may have on safety-sensitive activities.

References: 

1. Drugs and Human Performance FACT SHEETS-Diphenhydramine. National Highway Traffic Safety Administration. Accessed on April 28, 2013. <http://www.nhtsa.gov/People/injury/research/job185drugs/diphenhydramine.htm>

2. Antiemetic and Sedative Levels Found Together in 26 Civil Aviation Pilot Fatalities, 2000-2006. Aviation, Space, and Environmental Medicine. 2008 Jun;79(6):607-10.

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