If people can use drugs at home and escape detection by their employer, how is it the employer you mentioned is able to enforce his rules?

The United States State department reports "Penalties for possessing, using, or trafficking in illegal drugs in Djibouti are severe, and convicted offenders can expect long jail sentences and heavy fines."

http://travel.state.gov/travel/cis_pa_tw/cis/cis_1101.html

So severe penalties for drug use don't seem to work in your example. And they don't work in prisons in the US, either drug use runs wild and the state has complete control over the lives of the inhabitants.

I don't believe that for a second. Most people I know can get any drugs they want, and yet they're not a bunch of drugged up crazies, just like most adults can buy alcohol whenever they want, and most are not alcoholics.

Let people make their own decisions.