The laws surrounding marijuana are changing rapidly, with most states either legalizing it in some form, or considering legislation on the issue. A running list of states and their stance on marijuana can be found on Wikipedia. Be careful to note the differences in state laws. For example, some states have only decriminalized marijuana instead of fully or partially legalizing it. Other state have legalized marijuana, but only for medical usage. Furthermore, there can be many significant but small differences between state laws, and it is best to seek advice from an attorney in your area if you have questions or concerns.
You may be protected by the Americans with Disabilities Act (ADA). Under the ADA, an employer cannot discriminate based on disability. However, if over time the disability no longer exists, if the medication is interfering with your ability to perform essential job functions with reasonable accommodations, or if you are taking the medication illegally, then you can be fired.
Yes. In most cases, an employee seeking first-time employment can be tested as a condition of employment, even if there is no cause or reason to believe that the prospective employee has been taking drugs. The employer, however, must test all incoming employees for drugs and may not single you out for special treatment.
Some states have imposed limitations on pre-employment drug testing. For example, California allows a drug test only after the applicant has received an offer of employment conditioned on passing the test. In other states, employers that drug test are required to provide written notice or indicate in their job postings that testing is required.
"There is no mind over matter. Mind is matter. You don’t get over an illness by not talking about it. It isn’t weak to ask for help. It isn’t weak to want to survive. It isn’t a character flaw to experience bad things."
–Mattz Haig
from National Center for Health Statistics (NCHS)