Summarize the following:
Employers can administer drug testing before an employee is hired (pre-employment); on an annual basis; if there is reasonable suspicion; after an accident; or at random.  Your right to contest will depend greatly on "when" the test was given. Depending on your state of residency and the time of the drug test, your rights can vary drastically.  For example, random drug testing in California is not considered legally valid because of its discriminatory basis.  Pre-employment testing, however, is entirely valid.  An employer can terminate your employment, or restrict any promotions for "positive" results.  Depending on your location, the state can deny you unemployment benefits, working compensation, and disability benefits. This is the first step to formally contesting your results.  The human resources department will be able to inform you about any re-testing procedures; and will provide you with the paperwork that is required to proceed. Provide them with all of the relevant paperwork, as well as the medical forms you filled out before you took the test.  The process through which this contest is handled will depend greatly on the state of employment.  List of everything you consumed. Any prescriptions. As discussed above, a false positive can be triggered by various household foods, vitamins, over-the-counter medications and work-out supplements.  But, a false positive can also be the result of human error and may be grounds for a retest.  Mix-ups can occur at the laboratory facility where tests are evaluated. The laboratory itself may not be operating up to state standards. Testing methods may be out-of-date or antiquated. Samples could have been contaminated. If you believe that your test results are flawed, or that any of your rights have been violated during the testing process, you should get in direct contact with an employment lawyer in your area.  This individual will be familiar with the local laws and regulations regarding drug testing, and will be able to guide you through any legal action that you may take.

Summary:
Learn the different legal categories of drug testing. Research and know your rights. Meet with the human resources department. Contest the results with your employer. Contest the results with the laboratory. Contact an employment lawyer.