The stereotype of impairment at work generally suggests substance abuse, but impairment can arise from a variety of sources and not all of them are illegal. The employer's job isn't to monitor what a worker may ingest, but to ensure they can be safe at work and do not pose a risk to others.
Impairment can arise from mental or physical fatigue. It can be related to illness or prescribed medication. It can also be the result of distraction due to personal or family crises, stressful or traumatic events, harassment or bullying.
When obviously impaired, an employee should be safely sent home, or to an appropriate healthcare practitioner if they are not. In some cases, a fit-to-work certificate can be requested before their return.
But sometimes it is not as clear whether there is impairment or if the employee is just having a bad day. In this case, the employer may wish to determine if the employee's cognitive ability or judgement has been impacted. Each individual's situation should be assessed, without making assumptions or blanket observations.
To learn more about responding to potential employee impairment, including some questions to ask, you can visit Impairment and substance use.