As I pulled down my pants, I felt reluctantly exposed. A man wearing rubber gloves was standing close enough behind the toilet door to hear through it, while monitoring a timer.

With my belongings stowed in a locked case, I had a plastic cup I was told to urinate into, then bring it out without flushing the toilet or washing my hands.

If I failed to follow these instructions, I would have one more chance, then my employer would be notified that I had not adhered to the mandatory drug test.

Unlike in Australia, pre-employment drug screening is commonplace in America, and my first encounter with the practice took me by surprise.

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Until now, the only times I have ever provided a urine sample were when I needed information about my health. In other words, when it was my prerogative. If I was going to give anyone part of my body or information about my body, I expected it would be on my terms.

In my excitement for the job opportunity, I suppressed the unsettling feeling I had: the feeling of my privacy being invaded, of being shamed and distrusted, treated like a child even. Then the more I thought about it, I realised my frustrations were not just personal. They were political.

For decades, the fear of drugs has been used for political purposes in America. From Richard Nixon's declaration of a "war on drugs" in the 1970s to Ronald Reagan's demonisation of crack cocaine as "public enemy number one" in the 1980s, drug use has been stigmatised so effectively that few question the rhetoric when it comes to pre-employment drug screening.

Companies justify their drug screening policies by citing the economic cost of drug abuse to the workplace, and by suggesting drug screening encourages a healthy work environment. But there are several problems with this.

The test does not clearly distinguish people who use drugs from people who abuse drugs. The truth is, the vast majority of drug users are employed, high-functioning adults. In fact, almost 10 per cent of fulltime employees in the US use drugs, according to the Substance Abuse and Mental Health Services Administration (SAMHSA).

Only a minority of drug users develop problematic drug abuse. And the biggest predictor for drug abuse? Poverty. Despite common assumptions, drug abuse does not lead to unemployment, drug abuse is what follows unemployment.

Nevertheless, if a company's goal is only to avoid employing candidates who are abusing drugs (not simply using drugs) it would make sense for the test to state clearly the level of drug detection for which it is screening.

For example, do traces of three-week-old THC (cannabis) register a "fail" in a pre-employment drug test? Or are candidates only being screened for significant levels of current drug use, which might suggest dependence?

If employers are concerned only with drug abuse like they say they are, then they have no business screening for low-level drug use. This is simply an invasion of privacy.

But even if employers did make it clear they were only screening for high-level use, the practice would remain arbitrary and wasteful. Here's why.

Pre-employment testing is usually arranged with enough notice for candidates to abstain from using drugs until their urine is clean (48-96 hours). Cannabis - which carries the softest penalties of all illicit drugs - is an exception because it lasts as long as 30 days in urine.

There is no sense to pre-employment screening when the drug that is increasingly being decriminalised across America is the one that could most jeopardise a person's test results. Meanwhile alcohol abuse, which causes significantly more harm to society than any illicit drug, goes unchecked.

Finally, there is no clear evidence base linking drug use with poor performance at work. Employers could be dismissing their best candidates for no reason at all.

So aside from generating revenue for laboratories that specialise in employer drug screening, what purpose does pre-employment drug screening have?

As I left the clinic with my sample sitting under a glowing electronic scanner, I was glad to put the experience behind me.

I farewelled the nurses with as much dignity as I could muster moments after having shared my body's warmth with a stranger, and I determined that if asked for a pre-employment drug test again I would query the invasion of my privacy.

Because whatever my test results, I don't want to be part of a process that politicises my personal life.

Clare Rawlinson is an Australian living and working in America. Follow her on Twitter @clarerawlinson