The number of people working or looking for work in the U.S. is near a four-decade low, and some experts believe that the opioid epidemic has played a key role in keeping a lid on participation.

According to Jeff Korzenik, chief investment adviser at Fifth Third Bank, the abuse of painkillers that has gripped the nation could be at the heart of a weaker reading of labor-force participation at 62.7%, near its 40-year lows. The rate has hovered around this level for the past five years even as the unemployment rate steadily declined over this period to below 4% from 7% in 2013.

For Korzenik, the reason for this shortage is clear. “The opioid epidemic is preventing a huge portion of the population that is sidelined from joining the labor force because labor intensive jobs are also the ones that require workers who can pass drug tests,” he said in an interview with MarketWatch.

Korzenik said this weakness in the workforce could have negative implications for a U.S. economy that some experts characterize as entering a waning period of expansion.

There are currently 6.7 million job openings according to the latest Job Openings and Labor Turnover Survey. Yet businesses have struggled to fill these positions, implying that there are aren’t enough skilled workers available.

Alan Krueger, a Princeton University economist and former chair of President Barack Obama’s Council of Economic Advisers, found that more than 6 million men between ages 25 and 54 weren’t in the labor force and nearly half of those were taking pain medication, according to a 2016 study.

Korzenik estimated that of those men, 1.4 million could be working but aren’t because they abuse opioids and/or have criminal convictions related to the abuse.

According to a 2016 report from the Department of Health and Human Services and the Substance Abuse and Mental Health Services Administration, more than 11 million people in the U.S. were addicted to opioids, which include prescription drugs.

The Fifth Third Bank investment adviser doesn’t think the labor shortage has reached the acute phase yet, but the signs that it is approaching it are already evident, appearing in the housing and oil markets.

“Home builders and developers aren’t able to meet the demand from growing population because there are not enough construction workers. In fact 80% of home builders report shortages in subcontractors,” Korzenick said.

Read:How good news on jobs could be bad news for industrial stocks

“At this stage of the economic cycle we would have expected a much faster pace of house building. Instead, the lack of supply is pushing home prices higher,” he added.

See also:We’re probably at peak housing. Here’s what that means.

Korzenik also suggested that the rally in oil prices US:CLU8 — 65% over the past 12 months — is partially driven by a shortage in truck drivers that is hampering production by U.S. producers.

“The oil patch is an industry that relies heavily on trucking, either to move equipment or goods and right now there is an acute shortage of truck drivers. Men, who traditionally fill these positions are not able to pass drug tests,” Korzenik said.

Read: U.S. economy can’t keep on truckin’ without more drivers to ease flatbed shortage

In a recent research note, Torsten Sløk, chief international economist at the Deutsche Bank Securities showed a significant link between levels of opioid prescriptions and labor-force participation.

In particular, states with a high level of opioid prescriptions have a lower labor-force participation rate, as seen in a chart below:

Tight labor markets, often associated with a peak of economic cycles, in the past have pushed wages higher and lured more people from the sidelines, thus pushing LFP rate higher. But not this time.

Wage increases have been nearly nonexistent. According to the BLS, from May 2017 to May 2018, real average hourly earnings were unchanged, on a seasonally adjusted basis.

“The labor shortage is creating bottlenecks in production. We are not that concerned about wage growth and higher labor costs, as productivity growth will offset that, but we are concerned about disruption to production,” Korzenik said.