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While more commercial motor vehicle drivers tested positive for marijuana in 2015 than any other drug screened in mandated pre-employment and random tests given by motor carriers, the rate of positive tests for amphetamines continues to climb at a rapid rate. That's according to new numbers reported by the Department of Transportation.

In all, 47,782 of the 6.3 million individuals��— an estimated 97% of whom are drivers��— tested by U.S. Department of Health and Human Services-certified labs in 2015 flunked their drug tests for marijuana use last year, compared��with 47,524 in 2014, according to DOT’s Office of Drug and Alcohol Policy and Compliance.��That's a growth of just 0.5%.

But positive drug test results for amphetamines were the second-highest in number last year, with 37,619 drug test takers registering positive for amphetamine use, compared��with 34,830 in 2014. That's a growth of 8% in one year and 165% since test results were first reported in 2009, when 14,192 positives were found.

The new numbers were made public at a May 20 meeting of the Substance Abuse and Mental Health Administration’s drug testing advisory board.

Positive results for cocaine were indicated in 15,513 tests in 2015, compared��with 14,909 the year prior.

A total of 14,925 individuals tested positive for opiate use, and 1,265 for phencyclidine (PCP) in 2015. The 2014 numbers for the two drugs, respectively, were 13,446 and 1,295.

Overall, the rate of positive test results was 1.85% in 2015, compared��with 1.77% in 2014, officials said.

The rate of suspected tampering or cheating on tests “remained low, but is a continuing concern,” according to a DOT drug program update document.

Abigail Potter, a research analyst for American Trucking Associations, said drug use among drivers is fairly rare.

“However, when people are using when they’re on duty, it can be extremely costly to life and financially,” Potter said. “These are people we don’t want on the highways driving a very large truck.”