The FAA recently changed their time interval from the last dose os a DQ medication to flight duty. It is now 5 dosing intervals. Thus, if one took Zyrtec (dosing interval once a day) they would have to wait for 5 days until they flew. If the interval is every 8 hours, the time from the last dose to flight duty is 40 hours and so on. Hope this helps when making the choice about taking a medication.