There’s a vexing problem of drug abuse, and even alcohol abuse, in the workplace. One of the issues that employers face on a regular basis is whether than can send an employee out for a drug or alcohol test. The answer to that is, yes. You have control over the workplace and you have an obligation to keep the workplace reasonably safe under the circumstances. That means safe not only for you and the coworkers but also for the employee that you suspect may have a drug problem.
You can send somebody out for a drug test but you’ve got to make sure that you’ve got some reasonable basis that you could explain verbally to people why you’re doing so. Otherwise, then you’re acting arbitrarily and that’s never a good thing.