It began in 2013, with Colorado and Washington voting to legalize recreational marijuana. Alaska and Oregon will follow suit in 2014, according to The Kiplinger Letter. Next in line are Arizona, California, Massachusetts, Maine, Montana and Nevada, which are likely to legalize recreational marijuana in 2016, according to Kiplinger’s. And let’s not overlook the possibilities for New Hampshire, where the House of Representatives voted last month to legalize “recreational use.”

As the number of “recreational use” states grows, so will the challenges for employers in those states. Can they still refuse to hire an applicant who tests positive for marijuana? Or fire an employee who tests positive for marijuana? There have been a handful of legal challenges dealing with “medical marijuana” and, thus far, the outcomes have been favorable for employers. Most recently, on January 27, 2014, the Colorado Supreme Court agreed to review a decision which held that an employer did not violate the state’s legal activities law by terminating an employee who used “medical marijuana” off duty. Coats v. Dish Network, No. 13SC394 (CO. Jan. 27, 2014). To read the facts of that case and the appellate court’s decision, click here.

Meanwhile, marijuana remains classified as a Schedule I drug under the federal Controlled Substances Act.

Employers should follow these developments closely. Any diminution of an employer’s ability to enforce a “zero tolerance” policy with regard to substance abuse at work would have significant consequences, especially with regard to safety-sensitive positions.