Until 1996, substance abuse was considered to be a free standing disability under Social Security disability law, applicants could draw benefits based on addiction alone. That changed with the publication of a Ruling that has been in effect since then, not quite having the effect of a formal regulation, and applied unevenly across the country from hearing office to office.
A new Regulation has been proposed giving the previous policy the force of law. As before, if no disability except substance abuse exists, then the person is not considered disabled. The thinking goes that if the abuse stopped, the person would be able to work.
If the person is addicted but has another medical impairment, one that would not be affected if the abuse stopped, then there can be eligibility for benefits. For example, if someone has AIDS or cancer, plus an addiction, halting substance abuse will not change the fact that the underlying disability exists from the other illness. State or local medical marijuana laws have no bearing on federal law.
Cases involving diagnosed mental illness are the trickiest, since there is an underlying diagnosis, but it is arguable that stopping drug or alcohol abuse could affect the illness. This process is known as determining whether the substance abuse is “material” to the disability.