You will need your doctor's opinion that the medications you were taking at the time more likely than not caused the false positive. You will need this fast, and must immediately advise the boss/HR or whomever that this action was unfair and discriminates against you based upon your illness/disability. You must demand the right to re-test the same sample at an independent laboratory at your own expense. You will have to bear the cost for this, so be prepared to make that clear in your letter that you know this.
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To add, your doctor should be able to give you this information in a short letter, rather than you having to go in for a full appointment and incurring that additional cost.
This response is meant for general information only and does not constitute legal advice. This response does not create an attorney-client relationship.