3.9%? According to this report (click first link) from the Wall Street Journal, that's the percentage of people who test positive for drugs on corporate drugs testing programs. That percentage has been declining for almost 20 years, but this year it went up.
Something to remember: these are the positives from people who knew they would be tested (though they might not know exactly when). You can bet that the actual percentage of drugs users is much higher, because basically everyone who cares about keeping their job would have stayed clean for the test.
Every time I see a test like this, I have to laugh at the impression it leaves. More recently, I also remember a casual conversation I had with an executive at a company I worked for (the company and the executive shall remain unnamed). I asked why the company didn't participate in drugs testing. He said “Because if we did, you’d be the only programmer here!” Hah! I don't think that's actually true – but I'd bet you it was a more accurate assessment than that 3.9% number. Just from incident observations and conversations, I'd guess the actual percentage at the last few companies I worked for was more like 50% to 60% (just counting illegal drugs), and I wouldn't be a bit surprised to find the number was actually 10% to 15% higher.
Since I worked with (primarily) engineers and executives, we're talking about a group of above average intelligence and function – which is enough to make anyone question the alleged brain and motivation destroying properties of casual drug use.
3.9% of American workers use illegal drugs? Don't you believe it! That number is far too low. It's still not as high as the (perfectly legal) use of alcohol, though – despite alcohol use having a much worse impact on workers...