Benadryl knocks me out. But Zyrtec does not.
In: Chemistry
Benadryl is a 1st generation antihistamine which crosses the blood-brain barrier and can therefore affect the brain, causing drowsiness.
Zyrtec is a 2nd generation antihistamine, which does not cross the blood-brain barrier as much as Benadryl, and so has less effects on your brain like drowsiness.
The ELI5 answer is that some medicines affect your brain as a side effect of treating the illness. Older drugs like Benadryl are more likely to have this effect than newer ones like Zyrtec. As researchers got better at targeting the desired symptoms, they got better at not having side effects. The medicine we take for allergies impacts the chemicals that the body uses to signal and communicate internally, leading to drowsiness sometimes.
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