According to the Colorado Supreme Court, you can absolutely be fired for consumption of cannabis, even if it is legal under state law, even if it was purchased legally under state law, even if you consumed it only during off-work hours, even if you use it for a valid medical purpose: their answer is a resounding “Yes.”
Colorado State law allows for the legal sale, use, and possession of cannabis, and prohibits employers from firing employees for engaging in any legal activity outside of work. However, “legal activity” isn’t specific to State or Federal law. Which defines “legal?”
Their highest court answered loud and clear: there is no such thing as legal weed in America, even in Colorado. Cannabis is illegal under Federal law, therefore, if an employee can be fired for illegal activity outside of work, then use of cannabis is an illegal action that gives an employer a valid reason to fire someone.