I read this when you posted and I thought about the recent supervisor training I went through. Depending on where you work, your company may also have policies you need to follow as the supervisor, as well as diversity and ADA guidelines. Definitely seems like a tricky topic purely from a business standpoint.
Our company has an employee assistance program that is voluntary but can also be mandated if an addiction impacts work performance - a means to provide support for an employee to get back on track.