Can you take zyrtec and benadryl 12 hours apart?

If you are not sleepy from the Zyrtec you can take a Benadryl later, and the opposite is true as well. Be sure to avoid other possible causes of respiratory tract symptoms/irritation, in addition to the suspected allergen: no smoke of any kind (including candles), aerosols, and fragrances, which are bad for us all generally, see: http://www.idph.state.il.us/envhealth/factsheets/airquality.htm