Can my employer mandate impatient rehab even though I am in treatment?

Maybe. The facts leading up to your inpatient treatment are important. If you voluntarily checked yourself in before you did something which would have justified your termination is much different than your employer giving you a chance to check yourself in after it discovered that you abused drugs or alcohol which abuse was affecting your job performance. Two very different scenarios. The law is not that persons who are addicted to drugs or alcohol are entitled to one "keep your job even if you are still addicted to drugs or alcohol". To be protected by laws you have to no longer abuse drugs or alcohol which means no longer being addicted. The recovery process is lifelong and if you do not remain recovering there is no protection.  So how you would up in a treatment program is critical. If your employer forced you into the program which no employer can do then no employer would have been obligated to keep you on its payroll either. The ADAAA protects employees who are recovering, meaning that they no longer abuse drugs or alcohol or entered programs of their own choice before anything adverse or potentially adverse happened at work. If your employer was the one which "forced" you into treatment then it did not have to keep your job open either but apparently decided to do so for other reasons. As long as you know that your employment is "at will" unless you have a union or private contract, then you probably know that no reason is a good reason to let you go. Most employees need no reason to lose their jobs. Perfectly able and competent employees lose jobs every day for no reason at all. And that is not illegal. So you have to be the one to decide whether to comply with any requests of your employer if you are like most, "at will" employed. It does not sound like you are in denial. You acknowledge your issue. Recovery is lifelong for most. Maybe the employer knows best since it has observed you, sees good qualities, but might not give you any more chances after this? Maybe it really wants to help you? Maybe you really want to keep this job? It's a free country. Employers fire employees every day with no notice and no reason. And employees quit their jobs every day with no notice and no reason. You are the only one who knows whether now is the time to move on.