As of April 2017, there are 29 states that allow marijuana use for medical purposes. There is considerable variation in medical cannabis laws from state to state, including how it is produced and distributed, how it can be consumed and what medical conditions it can be used for.
At the federal level, cannabis is still a prohibited substance. However in 2014, the Rohrabacher–Farr amendment
was signed into law, prohibiting the Justice Department from spending funds to interfere with the implementation of state medical cannabis laws.
The following excerpt from a Society for Human Resource Management
article gives a general overview of how medical marijuana impacts employers and the issues that need to be considered if you are in one of the 29 states: