I have to say that becoming more strict and chomping down with drug screens etc. is what pushed my addict loved one closer to his druggie friends and farther from his family. When my parents tried the same regulations with me, I said "okay", obeyed their rules, and got out of there into my own place asap. I never brought drugs into their house, and I respected their rules. But I drank and smoked weed recreationally in other locations like almost every teenager does at least once. I was respectful and not defiant. My parents, upon seeing how unhappy I was, were more sympathetic to my making my own decisions and although they would not budge as far as me breaking the law in my house, they allowed me to go where I wanted when I wanted. Its only because of this that my parents and I are closer than ever, and I am neither a smoker or a drinker. Quit even cigarettes 2 years ago. I'm happily married, we're thinking about having a kid of our own. I pay my own bills, I work 2 jobs, I have my own car, apartment, etc. and I'm healthy. Pot didn't lead to other things for me. Had my parents done to me what was done to my brother, I might not be/have these things.
Point is, your kids are going to do what they are going to do. Period. Especially an 18 year old. That's an adult not a kid. The only thing a parent can control at that point is how much their children are going to hate them/want any kind of a relationship with once they save enough money to get an apartment with their friends. (And its probably not going to be the friends who DON'T get into trouble.)
I'm not saying "allow your kids to smoke weed and drink in your house" I'm just saying that not all young people are the same and sometimes closing your grip tighter around them and making their home more like bootcamp only makes the yellow brick road to moving out and getting on their own look that much more desirable.
I believe my parents doing this very thing was a significant contributor to my brothers addiction accelerating the way it did. He left the night of his 18th birthday in spite of his whole family pleading that we love him and please don't leave. It's been 10 years of him injecting heroin. I hate him for what he's done to my family.
Just my honest 2 cents. I'm still a kid though, depending on how you look at it.