Employers report they are having difficulty finding workers who can pass a pre-employment drug test, The New York Times reports.

Drug testing is becoming increasingly common at companies of all sizes. In some industries, such as trucking, drug testing is mandated by federal law for safety reasons.

The trend reflects an increase in the use of marijuana, as well as heroin and other opioid drugs, the article notes.

In June 2015, Quest Diagnostics found the percentage of American workers testing positive for illicit drugs such as marijuana, cocaine and methamphetamine increased for the second consecutive year in the general U.S. workforce. The positivity rate for approximately 6.6 million urine drug tests increased to 4.7 percent in 2014, compared with 4.3 percent the previous year.

Dr. Barry Sample, Quest’s Director of Science and Technology, said the problem used to be worse. “If we go back to 1988, the combined U.S. work force positivity was 13.6 percent when drug testing was new,” he said. Sample added he considers two consecutive years of increases to be worrisome.

According to the National Survey on Drug Use and Health, in 2014, one in 10 Americans ages 12 and older said they had used illicit drugs in the past month—the highest percentage since 2001.

According to John Sambdman, who employs about 100 people in Atlanta at Samson Trailways, many potential employees “just don’t bother to show up at the drug-testing place.”

Last August, Georgia Governor Nathan Deal said he would develop a program to help because so many business owners complain “the No. 1 reason they can’t hire enough workers is they can’t find enough people to pass a drug test,” he said. The program is still being discussed.