Will getting a medical marijuana card cause disability payments to be denied?

Richard you ask an interesting question.  From the way you phrase the question, it sounds as though your brother-in-law is already receiving disability payments, that is, the Social Security Administration (SSA), has found him disabled.  If that is so, he will be reviewed by SSA sometime in the future to determine if he is still disabled. Not being from your area, I will presume a physician prescription is required to obtain a "medical marijuana card."  If this is the case, and it is used within the terms of the prescription, it should not be a problem.  If there is "addiction" regarding the use of marijuana that changes things.     According to the regulations of SSA, "If we find that you are disabled and have medical evidence of your drug addiction or alcoholism, we must determine whether your drug addiction or alcoholism is a contributing factor material to the determination of disability, unless we find that you are eligible for benefits because of your age or blindness."  Note that the first determination is whether the person is disabled, then another decision must be made whether drug addiction is a "material factor" of the disability.  You can read the complete regulation if you click here.  If the limitations that would remain if the use of the drug (or alcohol) stopped would not be disabling, SSA will find the person not disabled.  If the limitations that would remain if the use of the drug (or alcohol) stopped would be disabling, SSA will find the person disabled.  This law regarding DA&A was created through an Act of Congress in 1996.  An Emergency Teletype with Questions and Answers about the changes appear at the following link, and are still useful today. I would recommend contacting a local attorney that is well versed in Social Security Disability law.  Click the house in the upper left "Find A Lawyer" if you need help finding a local attorney.   hope this answers your question, and good luck.