I have Graves Disease. One of the not-so-fun symptoms is Graves’ Ophthalmopathy.  Some of the symptoms of Graves’ Ophthalmopathy are (a) inflamation/swelling of the eyes, (b) dryness/redness of the eyes and (c) eyelids do not retract quickly.  So, the customs officers see big red irritated eyes and think the worst (drugs).  Based on my personal