A number of employers have expressed “grave concerns” about potential spillover effects of legal marijuana in the workplace, a labour lawyer said at a panel for employers on cannabis use Wednesday.

“There are a lot of employers across the country who are saying this is going to be a groundswell of change in the way employees conduct themselves in the workplace,” Darryl Hiscocks of Torys LLP advised a room of employers curious about the impact of marijuana legalization expected a year from now.

He believes murkiness over marijuana use in the workplace will become a legal battleground.

Cannabis in the workplace is nothing new: employers have had to accommodate medical users since a court ruling in 2001, but experts expect workplace incidents to increase once it’s legal and as the number of medical users grows.

Recreational pot will be subject to the same restrictions as alcohol; just as an employee cannot show up drunk at work, they cannot show up high.

Though regulations have yet to be written for the Cannabis Act, introduced in April, Canada’s federal task force on marijuana legalization highlighted the importance of “workplace safety” in its report released in November. It noted that employers it consulted with called for more guidance from governments about appropriate policies.

But employers are worried that they are receiving little guidance from the federal government on how to enforce workplace safety under the new regime, Hiscocks said.

“The federal government’s response has been ‘you’ll be fine, nothing’s changed, good luck.”

There are a number of governance issues for workplaces to consider when adopting policies, including provincial human rights codes and collective agreements, under which employers might require the union’s consent to any policy changes.

Hiscocks believes that legalization will remove the stigma associated with pot use and “employees are going to be much more open and in your face about it,” he said. “But that doesn’t mean the employee holds the trump card.”

Employers still have the right to ask questions of employees and their doctors when an employee shows up with a prescription to determine whether they can be reasonably accommodated and whether doing so poses health and safety risks, he said.

In Colorado and Washington, the first two states to legalize marijuana, drug testing has shown an increase in positive results for cannabis. Marijuana remains illegal at the federal level in the U.S and employers can refuse to hire anyone who uses it, even if they have a prescription.

Workplace drug testing is much more constrained in Canada and employers are limited to testing after an incident or in workplaces that have an enhanced safety risk or high prevalence of drug use, he said.

The Supreme Court of Canada ruled that employers can only conduct drug tests in dangerous workplaces, if there is a history of abuse and there is “reasonable” suspicion they are impaired on the job.

Unlike a breathalyzer for alcohol, there is no reliable test to determine whether someone is high at the moment. Current tests can only test whether someone has consumed in the past few weeks, leaving many employers scrambling to determine how to proceed when they suspect an employee is under the influence.

Savvy employers should be using the next year to review their policies to determine if they adequately cover legal marijuana, said Karen Borden, lead employment counsel at Manulife Financial.

However, she said, it’s probably unnecessary to develop a new workplace cannabis policy and changes can likely be included in existing procedures in which alcohol is mentioned, such as those covering workplace events.

Given that the inability to determine whether someone is high, the best approach is likely having a frank and open, if uncomfortable, conversation with the employee. And that will require employers to educate employees about the implications of legalization, she told the panel.

“It’s not about a written policy, it’s about knowledge and training.”