Managers are not trained to manage. You are made a manager because you are good at what you do e.g. accounting, computers or nursing. This may lead to dangerous situations and liability on the part of a company or individual. It is much better for the manager to focus on performance, on a subject they know about. A little knowledge can be a dangerous thing.
Denial is very powerful on the part of the addict.
Facts that are demonstrable and measurable are the best way to break through and let an employee know there is a problem. Obvious syptoms such as alcohol on an employees breath can be documented. Slurred speach or staggering. However other medical conditions can cause these problems too. Managers and coworkers should be able to recognize drug addiction when they encounter it.