Testing for Marijuana: Is it Effective? 


October 2018

Employment and Labour Bulletin

Yes and no. Marijuana impairment can be detected through visible signs and confirmed by a biological test; however, the more useful metric of functional impairment cannot be reliably measured.

Visible signs of marijuana impairment include: fatigue, impaired memory and concentration, general disorientation, spontaneous laughter, dilated pupils, bloodshot eyes, nervousness, impaired motor skills, dry mouth, coughing, nausea, and increased appetite.[1],[2] But these are common symptoms with multiple possible causes and therefore cannot be relied upon as definitive proof of marijuana impairment.

Biological testing is the only way to confirm the presence of tetrahydrocannabinol, or THC – the primary psychoactive compound  in marijuana – in an individual’s body. Saliva and blood tests are the most accurate testing options and saliva is the best at indicating recent use.[3],[4] Unfortunately, THC remains in the body anywhere from four days to two months after last use,[5] while typical impairment lasts only 2 – 24 hours.[6],[7] As such, a positive THC test does not necessarily indicate current impairment.[8] 

Although reliable functional impairment testing is not yet possible, employers should develop a clear marijuana impairment policy based on fitness-to-work.[9] This is consistent with workers’ legal obligations to advise their employer if their ability to safely perform work is impaired.[10]

Employers should also consider impairment training for supervisory staff, and how to satisfy the need to have reasonable grounds if considering testing. It will be important for supervisors to not rely on stereotypes or personal belief if determining an employee is impaired.

by Natalie Cuthill

For more information on this topic, please contact:


Natalie Cuthill



a cautionary note

The foregoing provides only an overview and does not constitute legal advice. Readers are cautioned against making any decisions based on this material alone. Rather, specific legal advice should be obtained.

© McMillan LLP 2018


[1] Canadian Centre for Occupational Health and Safety, “Workplace Strategies: Risk of Impairment from Cannabis – 2nd edition,” January/February 2018, ISBN: 978-0-660-24755-7 [Workplace Strategies].

[2] Wayne K. Jeffery, BSc, MSc (Pharm), “Marijuana Interpretation: Biological Testing or Psychophysical Testing?” presentation slides [Marijuana Interpretation].

[3] Workplace Strategies, supra note 1.

[4] Marijuana Interpretation, supra note 2.

[5] Workplace Strategies, supra note 1.

[6] Workplace Strategies, supra note 1.

[7] Marijuana Interpretation, supra note 2.

[8] Canadian Human Rights Commission, “Impaired at Work: a guide to accommodating substance dependence - Frequently Asked Questions.” Minister of Public Works and Government Services 2017. Retrieved from: http://www.chrc-ccdp.gc.ca/eng/content/ impaired-work-guide-accommodating-substance-dependence

[9] WorkSafeBC, “Workplace impairment: A primer

on preparing for cannabis legalization,” May


[10] See, for example, sections 4.19 and 4.20 of the Occupational Health and Safety Regulation, BC Reg. 143/2017.