"The COVID-19 pandemic intensifies the threat of mental health distress in several ways, including stress caused by financial, employment [and] child/family care instabilities, as well as fear [for] themselves or loved ones being exposed to or infected by COVID-19," reported the nonprofit member organization based in Itasca, Ill.

"Extended social isolation can lead to the development of substance use disorders. Those with previous substance use disorders are even more vulnerable due to decreased accessibility to treatment, recovery supports and harm reduction services, all a result of the pandemic."

Additionally, the NSC warned employers that they must prepare for an increase in substance misuse.

"This could be a serious threat to worker safety and cost tens of thousands in productivity losses, absenteeism and presenteeism, and workers' compensation claims if employers do not plan ahead," the NSC said in a statement.

It's up to managers, with support from their leadership teams, to stem the substance addiction tide as workers, emotionally fragile after months of lockdown, return to the workplace.

"The threat of post-COVID rise in mental health and substance abuse problems is definitely something that has to be monitored as people return to work," said Moe Gelbart, director of practice development for California-based Community Psychiatry, an outpatient mental health organization.

"It's important to have clear boundaries and expectations, primarily related to work performance. And it's essential for managers to follow through on behaviors [that] cross the line, with a zero-tolerance policy on abuse related to the workplace."

"Zero tolerance does not mean an employee is automatically fired, but getting appropriate assessment and help must be mandatory," Gelbart added.

Managers should take note because addiction and mental illness can poison a workplace culture.

"Substance abuse should be on companies' radars all the time and not just during a pandemic," said Mandie Conforti, director of health and benefits for Willis Towers Watson.

"According to the results of the Willis Towers Watson 2019 Emerging Trends Survey, only 22 percent of employers currently have a plan to implement programs to address opioid use, and 23 percent were considering initiatives to prevent opioid use."

While these data are specific to opioids, Conforti said employees suffering from any substance use disorder can impact workplace morale.

"Overall, substance use costs have risen for many employers, with several having high-cost claimants with over $200,000 in medical expenses over a three-month period," she said. "That's higher than many chronic medical conditions."

Tracking Potential Workplace Addiction Problems

Handling workplace addiction is a balancing act for managers.

The goal is to blend compassion for the employee with the need to keep the workplace running smoothly while ensuring that all company procedures and obligations are met.

Getting ahead of the issue is of paramount importance for employers. These action steps should streamline that process.

Check the big red flags, especially performance or production decline.

Managers should address any performance issues or problematic behavior, but not jump to conclusions about the underlying cause.

"Managers should discuss performance concerns and leave the door open for the employee to bring up any conditions or personal problems they are comfortable discussing," said Kristen Wilhelm, owner at Orchard HR LLC, a human resources outsourcing firm in Nashua, NH.

Note that help is available.

If the employee chooses to share information about a substance abuse issue, the manager should be ready to provide information about an available employee assistance program (EAP), leaves of absence or human resource contacts, Wilhelm advised.

"Managers must keep in mind that employees with current or past substance abuse conditions are covered to some extent by the Americans with Disabilities Act and may be eligible for accommodations to receive treatment," she said.

Don't let the situation escalate. If the behavior or warning signs are more egregious, a manager's response should be immediate with no need for the manager to talk to the employee.

"Once the signs of substance abuse are more obvious, such as an employee smelling of alcohol or exhibiting uncoordinated movements and slurred speech, this should be reported to human resources right away," Wilhelm said. "HR may then sit with the employee and his or her manager and express concern over the observed behaviors."

Don't make accusations.

It's never a good idea to accuse an employee directly of being intoxicated or impaired by drugs. "There may be other explanations for the behavior such as a medical condition or side effects from a legally prescribed medication, and the employee should be given the opportunity to provide this information to human resources," Wilhelm adds.

Begin a dialogue with staff as soon as possible.

It's important for an employer to foster an open environment where people can talk about substance use without stigma and with the full knowledge that people may still be dealing with COVID-19-related emotional issues.

"Many employers are launching awareness programs around substance use that include broad education, reducing stigma, and how to work with employees and family members who may have substance use struggles," Conforti said. "Also, many employers are looking at providing information, including resources and referrals to mental health professionals, through their EAP and making sure employees are aware of how to access care for themselves or family members."

Managers need training, too.

"Most employers are working on targeted manager training, including annual EAP training on mental health and substance use disorders," Conforti said. "Companies are also creating or updating their toolkits so their managers don't have to diagnose issues but can provide an environment where they know how to provide access to the right resources."

Addiction and Remote Workers

With COVID-19 displacing so many workers over the past several months and with more companies encouraging remote work, how does a manager recognize and handle a potential substance abuse situation with an at-home employee?

"Managers should set clear expectations around remote work," Wilhelm said. "This can include defining standard workdays and hours, reiterating the process for calling out sick or requesting time off, and clearly communicating deadlines and deliverables.

"Managers should stay on top of any problematic behavior—like missing calls or deadlines, or a decline in the quality of the work—that could indicate a condition that is impacting the worker's ability to meet the expectations of the job," Wilhelm added.

The process for addressing substance abuse and other addictions when they impact an employee's work are the same whether they're at the worksite or working remotely.

"The employee may qualify for a leave of absence or accommodation to seek treatment for the condition, and, fortunately, many providers now offer virtual counseling or treatment," Wilhelm said.

Clear Policies Need to Be in Place

One of the best ways to manage addiction in the workplace, especially given that COVID-19 may have thrown many managers off balance, is to have a clear set of rules to follow.

"Clear workplace policies, programs like an EAP and self-help, peer-based programs along with treatment resources and referrals are all great steps to help employers provide support for employees and their family members," Conforti said. "Addiction often impacts not only the addicted individual but the entire family system. Therefore, making sure that your insurance plan does not limit access to care and covers substance use is critical for making sure care is accessible and clinically appropriate."

Brian O'Connell is a freelance writer based in Bucks County, Pa. A former Wall Street trader, he is the author of the books CNBC Creating Wealth and The Career Survival Guide.