Research has found that cold and allergy medicine can impair driving more than being legally drunk can.
Researchers at the University of Iowa studied the driving performance of 40 people, ages 25 to 44, who had hay fever and were given two types of antihistamines; alcohol; and a placebo, in consecutive trials. The result: Their driving after taking over-the-counter products like Benadryl Allergy/Cold tablets or Tylenol Allergy Sinus Night Time was more impaired than when they were legally drunk. The active ingredient of the two allergy remedies is Diphenhydramine (DPH).
Be aware that Diphenhydramine is also found in many cough medicines like AllerMax and in a variety of pain relievers including Alka-Seltzer PM, Excedrin PM, and Tylenol PM.
The good news is that a new generation of allergy medicines like Allegra, which contains fexofenadine, doesn't impair a user's ability to drive. In addition, steriod nasal sprays such as Beconase, Flonase, Nasacort, Nasalide, Nasonex, or Rhinocort prevent allergy symptoms without causing drowsiness. However, keep in mind that these are only available by prescription.