"The 6th U.S. Court Circuit of Appeals has also held that “private employees are not protected from disciplinary action as a result of their use of medical marijuana, nor are private employers required to accommodate the use of medical marijuana in the workplace.”

"The 9th U.S. Circuit Court of Appeals has held that “the ADA does not protect medical marijuana users who claim to face discrimination on the basis of their marijuana use.” 

If Medical Marijuana has no Federal Court protection, recreational marijuana is unlikely to have it either.  

https://www.shrm.org/ResourcesAndTools/legal-and-compliance/state-and-local-updates/Pages/Marijuana-Use-Workplace-Drug-Policies.aspx

The stuff is still illegal at the Federal level.