A basic tenet of freedom is whether your body and your mind is your own, or whether indeed it is the government's or your employer or anyone else's that believes that they are the masters of your domain. In order to survive in this material world, there is a necessity to work, and whereas at one time, generations ago, this was an agricultural nation, filled with family farms, sole proprietors, and the like, in the modern age, most people that are employed, are employed by a company that they are not owners or principles of and therein lies the rub.
If we were to presuppose that all drug usage, specifically illicit drug usage, was always pernicious, dangerous, debilitating, and with no redeeming value whatsoever, it certainly would make sense for corporations and government agencies to act as "our brother's keeper" in wanting to see that we did not fall upon evil ways but in fact, that supposition, in of itself, is deeply flawed. However, leaving that aside, if we were to presuppose that illicit drug usage, by its very nature, upon detection of such, means always and without exception that you are either mentally and/or bodily impaired to such a degree at that present time that the performance of your dutiful work would always be negatively impacted and would be in fact, damaging to others within the corporation or would have material adverse effects upon the byproducts of the corporation, that also would be something clearly to avoid, but here too there is an assumption made which is false in its very premises.
The fact of the matter is, your drug usage, may or may not be debilitating to the company, depending upon a multitude of circumstances, to which it can be succinctly stated that rather than drug testing being a means to provide a safety umbrella for the company, its products, and its customers, it is reallya means to discriminate specifically against certain members of the public at large, so as to encourage the employment of certain preferred members of society vis-a-vis other members of society, nothing more, and nothing less.
One of the most common drug test usages, is testing applicants before they are employed or as a condition of being offered a job for employment at a given company. This, on the surface, makes no logical sense, as the person being tested, cannot possibly be able to adversely affect anyone or anything within the company, as they currently are not even employed by said company. This pre-screening, however, does what it is intended to do, which is to chase out those that do not fit the culture of said company.
Then there are companies that reserve the right to drug test employees for reasonable suspicion, which adheres onto these companies police-like powers in which they are, suppose to objectively observe as to who isn't and who is subject to such a reasonable suspicion. What this means in reality, is that the company now has given themselves the power to specifically target employees that are considered to be undesirable, and to use the mechanism of drug testing to either terminate them if they fail, or to serve as a fair warning that they are being watched.
In summary, drug testing is really a means to separate the sheep from the goats, to reward those that adhere to some arbitrary State standard, and to punish those that either are not cognizant of the rules, unable to game the system, or in general, lead a life that is consistent with believing that their body is their own.