Just found an FAA bulletin for AME's that says if any over the counter medications have side effects that could have an effect your job duties that you should wait double the time required for dosage. So if you would take nyquil every 4 to 6 hours, then you should wait 12 hours before flying. Makes sense, as long as you aren't talking about narcotics, or other more serious meds. Some meds like Hydrocodone (Vicodin) my AME says you need to wait 48 hours before flying.