it's in the employment contract.
if you signed it without reading it's your own fault.
you may request an employee handbook from HR at anytime without fear of retribution or punishment.
generally there has to be strict procedures detailed and specified out to the last minute detail on what the drug policy is. for example, many workplaces specify that if you are hurt on the job, you may be required to get a drug test before getting compensation. and random drug tests have to be exactly that, random. they have to explain just how random the drug tests are, and what the selection process is, so the worker may calculate what the odds are of them getting selected for a drug test. they can't just say there's random drug test and 'randomly' select the guy with dreads every time. it is also illegal for a company to say they randomly test for drugs, and to never do it, even though 98% of drug free workplaces are full of drug users who NEVER get tested.
they have to specify what type of test is done, and you may request the sample be taken by a licensed health care professional. your manager at chillis can't ask you to piss in the cup while he watches.
some states have laws prohibiting immediate termination arising from a positive drug test result, and instead require the company to allow the worker to seek substance abuse counseling.
it really depends on where you work and what you do.