Know your rights.

It is important to know that there are certain laws that have been established to protect you. Substance abuse disorder can be classified as a disability. Because of this, you may be protected by the ADA (American Disability Act).
The WIA (Workforce Investment Act) prohibits employers from discriminating against those who are in recovery or treatment as long as you are able and qualified for the position. Also, current employees are entitled to FMLA if they need to use the time for treatment. The FHA is similar. It protects discrimination of those who are in recovery or treatment.
There are loop holes to consider though. If you are currently using illegal substances, they would not be protected, except for health care. For example, employers and landlords can prohibit the use of illegal substances to qualify for employment or housing.
Other areas that must not discriminate include government assistance, health care and private education. Discrimination happens often and will continue until more people understand the specifics. Many times, people of authority do not even realize that they are discriminating. Knowledge is power and it is up to you to know and understand your rights in order to protect yourself.