And unless there's a safety issue involved (a position that requires operating heavy machinery or driving, for example), many human relations legal experts suggest thinking carefully before placing a blanket ban on employee medical marijuana use.

Valerie Samuels, an attorney with Posternak Blankstein & Lund LLP in Boston and co-chair of the firm's employment law group, says employees can petition to have their medical marijuana use accommodated under the Americans with Disabilities Act—but that doesn't mean employers must comply.

“You cannot force an employer to accommodate an illegal activity, and marijuana remains illegal under federal law,” she points out. “As long as it's illegal under federal law, employers won't have to accommodate it. And under federal law, if an employee is fired for having marijuana in his or her system, the employee would have no recourse.”

Danielle Urban, a partner at Fisher & Phillips in Denver, has become an expert on pot-related employment questions. Colorado is one of two states, along with Washington, that legalized pot for recreational use back in the fall.

“Although marijuana use is not protected under the ADA,” she explains, “if someone comes to you and says they have a debilitating medical condition, you can't just say, 'Well, we're not going to accommodate marijuana use,' without looking at other possible accommodations. It would be unwise to end the conversation without exploring any other solutions to their condition. And in a state like Arizona, you may have no choice but to incorporate marijuana use unless it's in a safety position.”

One of the biggest problems for employers when it comes to marijuana is that, unlike alcohol, there's no way to gauge impairment through a scientific test; daily marijuana users—which many medical users are—will have a much higher tolerance for the active chemicals in marijuana than will those who only use it occasionally.

“In Colorado, a lot of employers don't want to say to their employees, 'You can't use it even on your own time,'” Urban notes. “They don't want to be involved in their employees' personal lives—and probably some of the employers voted for the Amendment. But there's no easily accessible way to test impairment, so employers often still have zero-tolerance policies. Because the law is still so gray, if someone hurts someone else at work and they test positive for marijuana, the burden is going to be to show that they weren't impaired—and that is difficult to do. Employers may want to have a philosophy of permitting employees to do what they want on their personal time, but the current testing technology makes that difficult.”

Samuels says employers should have a clear policy about incorporating disabilities in the workplace.

“They should include in their policy that they're not going to accommodate anything that's a health and safety violation or that's unlawful. But if an employer called me and said, 'I have an employee who uses medical marijuana, and he's a good employee, and I want to keep him,' then I would say if the employer wants to accommodate that and is comfortable with it, that's fine.”

All in all, communication between employers and employees is key, experts say.

“It's a good idea to tell your employees how you're going to handle the issue,” Urban says. “Employees in states like Colorado and Washington are shocked to learn that they could lose their jobs for testing positive. So it's a good idea to tell your employees, 'By the way, this is our policy. You'll be tested for these reasons on these occasions, and if you test positive, this is what will happen.'

“I think in states where medical marijuana is permitted, employees think that having the card means they can use it with impunity, but that's not the case. It's better to avoid that—and don't surprise your employee,” she says.