Even when performance is not affected, you may wish to offer EAP assistance. I'm not sure the situation is that bad. You are not expected to be an expert. You need not decide the extent of the problem - that's EAP's responsibility. If there is a problem which affects your department's performance or which creates an unsafe environment for employees , your job is to document and suggest referral.
Performance is always the bottom line.
Of course, if you have direct observation of drug dealing or use, it should be reported to your manager. Either tell your staff nothing or that you are dealing with a performance problem. Individuals at all levels of an organization are vulnerable to substance abuse. Document your observations. List performance and behavioral concerns as you would list them for an employee who works for you, then consult your personnel office or your boss' supervisor.