Unfortunately, the genreal rule is that employment in the state of AZ is "at will" absent an express agreement to the contrary. At will employment can be terminated for any reason not amounting to discrimination on the basis of a legally protected trait (race, religion, gender, etc.) or retaliation for engaging in certain forms of legally protected conduct (filing a wage claim, taking FMLA leave, etc.). It doesn't matter whether the basis for termination is fair, reasonable or even true.
This means that an employee can generally be terminated for a positive drug test, even if the drug taken is not an illegal drug and even if the employee was prescribed the drug. The one exception would be if the employee was prescribed the drug for a disability, in which case it would be a form of disability discrimination to terminate the employee for taking a drug that they were treating a disability with.
You can certainly attempt to persuade your employer that termination is unfair and unwarranted, but you would not typically have any basis to sue on the facts you have described. I am very sorry.
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