ZFT, every flight doesn't have a Sky Marshall.
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From the New York Times,
A 2006 study by the F.A.A. of post-mortem toxicological evaluations of 4,143 pilots killed in accidents from 1993 to 2003 found that 223 were using mood-altering drugs like antidepressants, according to The Associated Press. Only 14 of the pilots who tested positive for the drugs reported a psychological condition on their medical forms, and only one reported using a mood-altering drug. None of the pilots determined to have used neurological medications had reported that on their medical forms, the AP reported.
I'll just note that the number of post-mortems seems high if all the accidents were plane crashes.