Tests on pilots killed in plane crashes in the United States since 1990 have found a four-fold increase in the use of legal and illegal drugs, according to a study released on Tuesday by the National Transportation Safety Board.
Of the more than 6 600 pilots who died in the crashes, 96% were private, rather than commercial pilots.
The proportion of pilots whose bodies tested positive for at least one legal or illegal drug increased to 40% in 2011 from less than 10% in 1990, the study found.
None of the pilots who died in large airline accidents had recently used illicit drugs, though some had been using potentially impairing medications, the agency reported.
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Despite the rising drug use, the board said there has not been a corresponding increase in the proportion of accidents in which drug use contributed to the accident.
The most common drug was antihistamines, which generally are available over the counter and can cause drowsiness.
They were found in 9.9% of pilots on average from 2008-2012, nearly double the 5.6% who tested positive from 1990 to 1997.
The percentage of pilots testing positive for at least one illegal drug increased to 3.8% in the 2008-2012 period from 2.3% in the 1990-1997 period.
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Marijuana was the most commonly identified illegal drug, and the percentage of pilots in the study testing positive for marijuana increased to 3% in 2008-2012 from 1.6% in 1990-1997.
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