Yes, Shash, it is safe to take both of these antihistamines at the same time. Zyrtec is a long-acting, second-generation antihistamine that is only taken once a day (experts recommend taking it at bedtime, so that it reaches peak effectiveness as you're waking up in the morning).
Benadryl is an older, short-acting (about 4 to 6 hours max) antihistamine that may provide some additional relief if the Zyrtec doesn't fully control your allergy symptoms. Unfortunately, Benadryl often has the side effect of drowsiness, so might not be a great choice if you have to work, drive or operate machinery.
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