Feeling alert behind the wheel does not mean an allergy medicine has worn off. The U.S. Food and Drug Administration says medicines containing antihistamines can slow your reaction time, make it hard to focus or think clearly, and cause mild confusion even if you do not feel drowsy.

Antihistamines help relieve many types of allergies, including hay fever, but the FDA says some work for longer periods than others. For some medicines the effect on driving passes quickly, while for others it can last for several hours and even into the next day. The agency lists cold remedies, allergy medicines, nighttime sleep aids and cough medicines that contain an antihistamine among the products that could make driving dangerous.

Mixing matters too. The FDA says to avoid drinking alcohol while using some antihistamines, and to check with your health care professional before taking them alongside sleep medicines, because those combinations can increase drowsiness.

Most medicines will not keep you from driving safely, the agency says. Its advice is to follow the directions and warnings on the Drug Facts label, take a nonprescription medicine for the first time when you will not need to drive, and talk with your health care professional about side effects. They may be able to adjust when you take it or switch you to a medicine with fewer side effects.