Our company allows medical marijuana, can you suggest some language for our drug free workplace policy?

Yes, here you go:

An employee of the Company who has been authorized by a licensed physician to use marijuana for medicinal purposes is permitted to do so in accordance with applicable laws, rules and regulations of their state of residence, provided such usage does not impair or otherwise impede his or her ability to safely carry out assigned duties and responsibilties.  This does not apply to DOT regulated safety sensitive positions.

Employees enrolled in a medical marijuana program, and who occupy non-DOT safety-sensity positions, remain subject to random drug and alcohol screenings. In the event such an employee is randomly selected for testing, he or she must comply with the testing order. However, the employee may make known their participation in the medical marijuan program to the Company’s drug free workplace official or Designated Employer Representative (DER).