FDA: Allergy medications may make you too drowsy to drive

Dr. Holly Phillips reports on fall allergies and why this year may be worse than last.

The Food and Drug Administration (FDA) asked sleeping pill-makers in January to lower the dosage of their drugs, because medications like Ambien may leave some people feeling groggy in the morning, potentially leading to a traffic accident or other injuries.

Now, the FDA is reminding Americans that another common medication can lead to drowsiness behind the wheel: Allergy medicine.

With allergy season in full swing, the agency is saying people taking antihistamines should be especially cautious when getting behind the wheel.

"Any of these reactions can negatively interfere with driving or operating heavy machinery," Dr. Jane File, a medical officer at the FDA's Division of Nonprescription Regulation Development, said in the May 29 Consumer Update.

When your body comes in contact with something you're allergic to -- be it pollen, ragweed, molds, pet dander and dust mites, your body produces a chemical called histamines -- according to WebMD. These chemicals cause the nose tissue to swell, which makes you stuffy. It can also lead to other symptoms including runny nose, watery eyes, redness and itching. Collectively, these seasonal allergies are referred to as hay fever.

Taking an antihistamine like diphenhydramine -- sold as Benadryl -- can reduce or block histamines, thereby stopping the symptoms. But, the FDA says some antihistamines can make you feel drowsy, unfocused and slow to react.

Filie added that you may experience slower reaction time, haziness, or mild confusion -- even if you don't feel drowsy after taking antihistamines.

Dr. Tamara Kuittinen, director of medical education and an emergency medicine physician at Lenox Hill Hospital in New York City, told CBSNews.com that some hospitals give their patients diphenhydramine if they're having trouble sleeping, so the drowsiness effects are not benign.

She added that some people who take other popular allergy drugs like cetirizine (Zyrtec) and loratadine (Claritin) may also experience drowsiness.

Her advice? Read the label, know what you're taking, and act accordingly.

"If you have to be awake or doing something that requires operating machinery, it's better to take at nighttime," she said.

Some allergy medicines may look similar, but contain different ingredients, so she reiterated the importance of knowing exactly what you're taking.

"Really, checking the labels -- I think it's critical," she said.

The FDA agrees. The agency says different antihistamines may be dosed differently, so if you run out of one that works for you and pick up a different kind at the store, you might not be getting a drug that works the same.

"If one specific antihistamine worked for you before, take note of the dosage and make sure you get the same medication the next time," FDA pharmacist Ayana Rowley said in a statement.

Also, mixing antihistamines with alcohol, sleeping pills or tranquilizers can lead to more drowsiness, and should be avoided, the FDA said. That information can be found on the Drug Facts label, Fille pointed out.

Lastly, if you're having a bad allergy attack that still won't go away with the antihistamine, fight the temptation to take more, said Rowley.

"If the correct dosage isn't providing you the relief you expect, don't simply keep taking more and more of that product," she explained, "but instead, consult your health care professional".

The FDA has more information on driving on medication.

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