1) Generally speaking, unless you had an actual written employment for a specified time (e.g. a one-year contract) guarantying you the job, your employment was employment at will and could be rescinded or terminated at any time, for any reason whatsoever.2) Even if you'd had a contract, if the medication could potentially affect your ability to drive, they would be able to legally terminate the contract, because you would not safely be able to do what you'd been hired to do.