How are some allergy medicines non drowsy but others put you to sleep?

Question

Benadryl knocks me out. But Zyrtec does not.

In: Chemistry

Answer 1

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.