For most jobs, you probably cannot be denied a job due to being on methadone, because being addicted could be considered a disability, and the law prevents discrimination due to disability and requires employers to make reasonable accommodations for disabilities. Certain jobs, where being on methodone could impair performance, would be ones where the employer could refuse to hire you as could jobs where simply being an addict may pose some security risk or a risk to the employer's mission or purpose. These however, would be the exceptions, and are based on the effect of taking methadone or being an addict on the specific facts of the job.