I wouldn't disclose to them that you have a problem with opiates since they have a no tolerance policy regarding drugs. This type of honesty may backfire on you and you could lose your job. However, you can be honest and tell your boss that you are having a personal problem or problems that are related to anxiety and depression. In order to deal with these problems, you may need to take some time off.
I am pretty open about my alcoholism, but I do show judgment and restraint when I tell others in work and professional related areas.