Impairment from medication, particularly over the counter (OTC) medication, has been cited in a number of accidents in general aviation. In a 2011 study from the FAA’s CAMI Toxicology Lab, drugs/medications were found in 570 pilots (42%) from 1,353 total fatal pilots tested. Most of the pilots with positive drug results, 511 (90%), were flying under 14 CFR Part 91.

Fact sheet: https://www.faa.gov/news/safety_briefing/2018/media/SE_Topic_18-10.pdf