Military accidents (at least ones that result in major injury, death, or
over a million dollars damage require tox screens on all flight crew
members). Many OTC sleep aids contain benadryl, the antihistamine.

The issue regarding reporting is more philosophical: if the drug wasn't on
an approved list, and there were an accident or even a fatality and the drug
was found in a pilot's blood, an insurance company might not pay, with the
presumption that the individual was not flying with a valid medical. In
other words, taking an unapproved medication (or having a procedure that was
grounding, but was not reported) might void the medical, and therefore the
insurance company would not need to pay on any claim.

At the very least, some careful homework would be in order prior to making
the decision.