Are You Required to Notify All Current or Future Employers if You’ve Been in Drug Rehab?

If you’ve been in a drug rehabilitation program, and successfully completed it, do you have to notify any potential employers? Do you have to notify a current employer if you’re going into a rehab program?

I assume the laws are differen in every state, but specifically in Illinois or california? If anyone knows a website that would tell me all this i’d really appreciate it.