AFAIK some types of drug tests can't measure whether you're high on $drug now or if you're taken it before and you're sober now. If you're driving sober but you've taken $drug yesterday, you might be arrested for DWI.

If these tests can't reliably show you're high at the time the test was taken, don't use them.

For anything other than driving or operating heavy machinery and so on, there's no point in such tests at all. Let people take whatever drugs they want. Just do what we do with legal drugs like alcohol and cigarettes - regulate the quality, require an ID card for purchase and tax them.

This will obviously lower organized crime. If you make prostitution legal, you'll lower it even more. There will still be people trying to sell their shitty home made drugs cheaper than the regulated ones - like we have illegal cigarettes, but that's nothing compared to what we have now.

Make drugs less of a taboo. Educate people on harm reduction and make it easy to admit when you have a problem with something.

As a somewhat-educated person without medical education, I've taken almost everything under the sun and still function well within society. It's really possible to use drugs responsibly. If the image you have is a junkie with ragged clothes lying on some old mattress under a bridge sharing a dirty needle, you're only looking at the uneducated people with no safety net from society. Believe it or not, educated drug users are everywhere. We just don't often talk about it like we don't casually mention our fetishes to others in work or academia.

But drugs and sex are fun, maybe too fun, and we can't let the citizens enjoy themselves too much. :/