Sleep Disorders

Updated 11 August 2014

Antihistamines can impair driving skills

We hear plenty about the effects of alcohol on driving, but during hay fever season we also need to worry about how antihistamines impair our road skills.

We hear plenty about the effects of alcohol on driving, but during hay fever season we also need to worry about how antihistamines impair our road skills.

Experts say the problem stems from over-the-counter medications; they may stop your nose from running, but they bring your alertness level down a few notches as well.

"Antihistamines are really among the most notorious in causing drowsiness," says Dr Charles Ruchalski, an assistant professor at Temple University's School of Pharmacy in Philadelphia.

Antihistamines + alcohol = drowsiness
Products such as Benadryl, which contains diphenhydramine, induce a high level of sedation that not only can cause dizziness and make driving or operating machinery dangerous, but they can be exacerbated by even a small amount of alcohol, explains Ruchalski.

"You may normally be used to drinking maybe one or two glasses of wine, for instance, and feeling just fine, but if you are taking an antihistamine, it could make some people feel almost three times as drowsy," he says.

Size also plays a role
While everyone can be affected by the sedation of antihistamines, like alcohol, the ingredients can have varying effects on people according to their size, explains Dr Lloyd Van Winkle, an associate clinical professor at the University of Texas at San Antonio.

"The volume of distribution, meaning the size of the person, affects the dosage. So, smaller people will only need smaller doses, and if that dose isn't adjusted for weight, you'll see an even greater effect," he says.

Medication can be worse than alcohol
The American National Transportation Safety Board says medications are a seriously under-recognised threat because drivers are rarely tested for over-the-counter drugs after a crash.

Ruchalski suspects if drivers were tested for over-the-counter drugs, that number would be much greater.

"I think the numbers of accidents in which [over-the-counter] drugs played a role are definitely under-reported because authorities don't typically test for it," he says.

"If you've been in an accident, and your blood alcohol test shows that you're under the legal limit, they simply don't blame alcohol," says Ruchalski. "But if you had just also taken two Benadryl, it in fact may have made your sedation much worse."

Drugs act on central nervous system
According to Van Winkle, the ingredients in antihistamines may work to control nasal and allergy symptoms such as sneezing, runny nose, itching, watery eyes, hives, rashes and itching, but they must go through the brain to do so, causing the drowsy side effects.

"The antihistamine ingredients actually act through the central nervous system, so the mechanism of action is in the part of the brain affects alertness and sedation," he says.

Of course, there are "non-drowsy" cold and flu concoctions of all sorts, but Van Winkle says they probably don't contain antihistamines.

"The 'non-drowsy' formulas may contain things like decongestants to clear up stuffy noses, but they don't contain antihistamines," he says.

Non-drowsy medicines require prescriptions
Heavily promoted medications including Claritin and Allegra have gained popularity due to their ability to reduce symptoms such as runny nose and sneezing without causing drowsiness. However, they both require prescriptions.

Allergic symptoms are also treated with topical steroid medications, such as Nasonex and Beconase, which are sprayed into the nose, but the treatments are, again, currently offered only by prescription and can take up to a week to work.

Read more:
How drinking affects your driving


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Dr Alison Bentley is a general practitioner who has consulted in sleep medicine and sleep disorders, in both adults and children of all ages, for almost 30 years. She also researches and publishes on a number of sleep-related topics both in formal research journals and lay publications including as editor of Sleep Matters, an educational newsletter on sleep disorders for doctors.

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