The ADA applies to all government employers and all private employers with at least 15 employees. Those employers are covered employers. Under the ADA the covered employer may not discriminate against you because of any disability that you have (or that the employer perceives you to have). Assuming, as is likely, that your ADHD would qualify as a disability, that means that the covered employer may discriminate against because you have ADHD. Nor may the covered employer discriminate against you because of any legal drugs you take to treat your disability. However, your employer always may fire you or take other action against you if your job performance does not measure up to what the employer expects, even if your performance problems are caused by the disability you have or the drugs you take to treat it. A covered employer must, however, provide you assistance to help you overcome the obstacles caused by your disability if you ask for it and so long as doing so does not impose an undue burden on the employer. This is known as providing reasonable accommodation.
As it applies to drug tests, the ADA prohibits an employer from singling you out for a drug test because of your disability. But if the employer tests all employees or has a truly random drug testing program, the employer may require you to take those tests unless some state or local law prohibits it. Whether you may be required to disclose the medications you are on and the disability that requires you to take them will depend greatly on whether the employer is a government agency or private business and what federal, state, and local laws apply to that particular employer. I would say, however, that if the drugs are likely to show up on the drug screen and might make it appear you take illegal drugs then it would in most circumstances be in your interest to tell the employer of the drugs you take, and do that before the test rather than after, even if you are not required to do that.

