Is safe if i take benadryl (diphenhydramine) six hours after take zyrtec?

Yes: Both can cause drowsiness.Saution when need to be at peak of concentration. Both can add to urinary steam reduction.

Should be ok: While taking Benadryl (diphenhydramine) after zyrtec should be ok, it would be even better to know the underlying trigger to allow less medication usage. An allergist could determine why symptoms are not controlled with less medication.