I used to use Cetirizin and it didn’t make me drowsy until the next day when it hit me like a truck and had to take a nap before driving home. I was certain I’d fall asleep when driving…but to answer your question based on my doctor’s response years ago:
Antiallergics can make you feel tired if they pass the blood-brain barrier and affect the nerve cells in the brain. For these cells, histamine produced by the body in very small amounts is a kind of “wake-up call” that promotes alertness. If the histamine access is blocked by a medication, the result is fatigue. Newer allergy medications cannot overcome the blood-brain barrier well.
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