WorkerDrug use among American workers appears to be increasing, based on the results of drug tests.

Traces of drugs were found in 3.9 percent of urine tests conducted for employers last year, up from 3.7 percent in 2013, according to The Wall Street Journal.

Before 2013, positive drug tests decreased almost every year for 24 years, from 13.6 percent in 1988 to 3.5 percent in 2013.

Some positive drug test results are discounted if a worker can produce a doctor's prescription for a legal drug, but most of the positive tests reflect illicit use, according to Dr. Barry Sample of Quest Diagnostics, which released the findings this week. He said the rising positive drug test rate is driven by increases in marijuana, cocaine and methamphetamine use.

Marijuana, the drug most commonly found by testing, accounted for almost half of all positive tests. Other commonly found drugs included amphetamines, oxycodone, and benzodiazepines such as Xanax.

Sample said the increase in positive drug tests reflects a rise in substance use in the broader population. A report released last September found 9.4 percent of Americans ages 12 and older said they used illicit drugs in 2013, up from 9.2 percent in 2012 and 8.7 percent in 2011.

The 2014 report found almost 20 million people said they used marijuana, while 4.5 million Americans said they had taken prescription painkillers for nonmedical reasons in the past month. In addition, 1.5 million people said they used cocaine, 595,000 used methamphetamine and 289,000 used heroin.

Sample noted that positive drug tests increased sharply in 2013 in Colorado and Washington compared with the rest of the country.

Those two states have legalized recreational marijuana for adults 21 and older. Last year, the rate of increase in those two states was similar to the national average, which may indicate use in those states has "leveled off," he said.

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