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Driving drowsy may be almost as risky as driving drunk. People who would never get behind the wheel after drinking alcohol frequently drive while impaired and don’t even realize it.

According to the Centers for Disease Control and Prevention, an extraordinary number of Americans drive drowsy, and many even fall asleep: “Among nearly 150,000 adults at least 18 years or older in 19 states and the District of Columbia, 4.2 percent reported that they had fallen asleep while driving at least once in the previous 30 days.”

Crashes that lead to injuries or death involve at least one drowsy driver about 2 percent of the time. That’s probably because sleepy people react slowly and have trouble paying attention.

The CDC points to people who skimp on sleep or who have disorders such as sleep apnea as being especially susceptible to this problem. But sometimes people take medications that can make driving dangerous, whether they realize it or not.

Even if people are trying to get the sleep they need, taking sleep aids can backfire. The Food and Drug Administration recently strengthened its warning about the popular sleeping pill zolpidem (Ambien). The agency has received 700 reports of impaired driving ability and road traffic accidents associated with this drug. Many other sleeping pills also can lead to diminished alertness or impaired driving the following morning.

One teacher who had been taking Ambien remarked: “I began noticing that I could not remember students’ names, although these were children I had taught for several years! The final straw was when I woke up and realized I had driven my car to a local store and purchased items. I was getting ready to go to the store to buy these items when I discovered that I had already bought them during an Ambien blackout.”

A father traumatized his daughter by repeatedly running into the curb as he drove her to school. He had taken Ambien the night before. Although he had gotten a full night’s sleep, he considered himself a dangerous driving “zombie” the next morning.

Sleeping pills are not the only problem. Allergy season is under way in many parts of the country, and this season is expected to be especially severe. The antihistamines many people rely on to ease nasal symptoms also can impair driving. Over-the-counter medicines such as diphenhydramine (Benadryl) can be as risky as alcohol (Annals of Internal Medicine, March 7, 2000).

Anti-anxiety drugs also can interfere with driving ability. Just as people who have had too many drinks are poor judges of their own ability to drive, people taking medications such as alprazolam (Xanax) may not realize they pose a hazard on the road (Psychopharmacology, February 2012). Warnings such as “Until you experience how this medication affects you, do not drive a car or operate potentially dangerous machinery” may be meaningless.

When you receive a prescription, ask your doctor and pharmacist whether it could affect your driving ability. Even drugs that don’t make people drowsy can impair reaction time and judgment.

In their column, Joe and Teresa Graedon answer letters from readers. Write to them via their website: www.peoplespharmacy.com. Their newest book is “Top Screwups Doctors Make and How to Avoid Them.”