When one looks at the pharmacologic properties of these drugs and their metabolism, it make sense to wait 24 hours. The
Guide for Aviation Medical Examiners states Applicants with seasonal allergies requiring antihistamines may be certified by the Examiner with the stipulation that they not exercise privileges of airman certification within 24 hours of experiencing symptoms requiring treatment or within 24 hours after taking an antihistamines. See the link
http://www.faa.gov/about/office_org/...ch/item26/amd/
Oh you doubting Thomas
