In the US I found this comment that might be helpful here…
A: Your drug policy prohibiting all legal drug use may
be too broad and most likely violates the Americans with Disabilities Act
(ADA).
The ADA specifically limits what medical inquiries you can make of
current employees to those that are job-related and consistent with
business necessity. According to the Equal Employment Opportunity
Commission (EEOC) regulations and guidances, a medical inquiry or
examination of an employee meets this business necessity standard
when:
1. An employee is having difficulty performing the job effectively,
such as when the employee falls asleep on the job, is absent
excessively, or experiences a decline in the quantity or quality of work;
2. An employee is injured or becomes ill and the employee’s ability
to perform the essential functions of the job is in question;
3. An employee requests an accommodation for an alleged
disability;
4. The employer reasonably believes that an employee’s medical
condition will cause the employee to pose a direct threat;
5. The employer receives information from a credible third party
that an employee has, or the employer observes symptoms indicating an
employee may have, a medical condition that will impair his ability to
perform the essential job duties or will pose a direct threat; or
6. The inquiry or examination is required by law, such as medical
examinations required by OSHA or the Department of Transportation
regulations governing interstate bus and truck drivers, railroad engineers,
and airline pilots.