The following information is not legal advice and should not be relied on as such.
Employers must take necessary precautions to ensure the safety, health and welfare of workers, and to provide and maintain a safe workplace, as set out by The Workplace Safety and Health Act and Workplace Safety and Health Regulation. Specifically, under Part 2.19 (1) & (2) of the Workplace Safety and Health Regulation for Alcohol and Drug Consumption, an employer must take all reasonable steps to ensure that a worker does not work while under the influence of alcohol or a drug that impairs or could impair the worker's ability to perform work safely. A worker must not work while under the influence of alcohol or a drug that impairs or could impair the worker's abiilty to perform work safely.
Employers can demonstrate due diligence in preventing impairment in their workplace by having a drug and alcohol policy in place. A policy will show that the employer has identified hazards (i.e., the risk of drug or alcohol impairment in the workplace), made changes to correct these conditions as necessary and trained supervisors and workers on the drug and alcohol policy. Workplace Strategies: Risk of Impairment from Cannabis from the Canadian Centre for Occupational Health and Safety is one good resource with information on policies (this document addresses impairment using cannabis as an example).
Please note that employers should seek legal counsel when developing their own drug and alcohol policies, as each policy should be tailored to the unique needs of their workplaces. DO NOT COPY A POLICY.