In treating allergies or a cold, you may experience drowsiness, a common side effect in antihistamines that help you control your runny nose, sneezing and itchy nose and eyes.
How does this happen? Histamine is a chemical produced by the immune system to fight off allergens and germs. When there is a threat of allergies or infection (such as the common cold), histamine is even more active. In its search-and-destroy process, it can cause allergy or cold symptoms. Antihistamine medications relieve these symptoms by blocking the unwanted effects of histamine.
– See more at the Asthma & Allergy Network website: here